RÉSUMÉ

Afin de quantifier les risques d'invasion chez le goujon Asiatique Pseudorasbora parva, un petit cyprinidae d'eau douce, j'ai tout d'abord utilisé une approche corrélative visant à modéliser enveloppe climatique de cette espèce et dans laquelle j'ai intégré un proxy relatif à la probabilité d'introduction. Cette approche s'appuie sur l'hypothèse que les organismes s'adaptent aux conditions environnementales locales ou régionales. J'ai ensuite comparé les conditions climatiques occuppées de deux lignées génétiques majeures à la fois dans les aires natives et non-natives. Puis dans un second temps, par une approche expérimentale j'ai comparé la réponse aux variations de température, des traits d'histoire de vie de populations vivant dans des zones climatiques contrastées. Finalement, je me suis intéressé à la dispersion des goujons Asiatiques afin de tester si les populations situées sur les fronts d'invasions avaient des capacités de colonisation plus importante. J'ai donc quantifié et comparé le long d'un gradient d'invasion, le potentiel de dispersion des individus ainsi que des traits pouvant y être liés (activité et morphologie).

L'étude des risques d'invasions a montré que de nombreuses zones -au delà des zones déjà occupéesétaient climatiquement favorables au goujon Asiatique. C'était le cas en particulier pour certaines zones comme l'Australie, l'Amériques du Sud et du Nord, indiquant que l'invasion de cette espèce pourrait encore s'étendre. Après avoir pris en compte les probabilités d'introduction, cette étude a montré que l'Amérique du Nord était la zone la plus à risques. Par contre, l'enveloppe climatique modélisée entre les différentes lignées dans les zones géographiques non-natives sont très similaire, ce qui laisse à penser qu'il n'y a pas de patrons d'adaptation locale chez cette espèce. Pour autant, enveloppes climatiques de l'espèce modélisée dans la zone européenne (non-native) sont différentes de celle modélisées dans la zone asiatique (native), ce qui suggère une expansion lors processus de colonisation. Les réponses thermiques des traits d'histoire de vie du goujon Asiatique testés expérimentalement n'ont pas variées significativement entre les populations originaires de conditions climatiques continentales et maritime-tempérées. Par exemple, l'effort reproductif global des femelles n'a pas varié entre les températures testées (15-25°C) mais la stratégie temporelle de reproduction a beaucoup varié. L'effort reproductif était plus cours et plus intense à forte température, alors qu'il était plus étalé et avec des pics reproductifs moins forts à faible température. Par ailleurs, il semble qu'il existe un gradient morphologique fort entre les populations situées à différentes distances du front d'invasion ce qui suggère une forte plasticité morphologique mais qui ne serait pas liée à la capacité de dispersion de ces populations. En effet, cette dernière ne variait pas significativement le long du gradient d'invasion. La capacité de dispersion serait principalement liée à la taille du corps de goujon Asiatique, les individus les plus grands ayant une probabilité plus élevée de disperser.

Bien que les prédictions générales du modèle utilise pour caractériser l'enveloppe climatique puissent être affectées par de potentielles adaptations à l'échelle de la population ou de la lignée évolutive, les résultats suggèrent qu'une certaine incertitude liée à ces prédictions persiste puisque la distribution native ne prédisait que très mal la distribution actuelle dans les zones non-natives. Par ailleurs, mes travaux expérimentaux à plus fine échelle suggèrent que cette espèce est extrêmement adaptable et tolère une large gamme environnementale, ce qui pourrait expliquer son caractère invasif. Les connaissances produites au cours de cette thèse constituent donc des ressources extrêmement pertinentes pour développer des stratégies de gestion visant à contrôler les invasions futures du goujon Asiatique.

Mots-clés: Invasion biologique, poisson, niche écologique, modèles spatiaux, Pseudorasbora parva.

ABSTRACT

In this thesis I set out to quantify the risk of invasion from the invasive freshwater fish, Pseudorasbora parva, at a global extent, using traditional correlative ecological niche modelling approaches with the integration of surrogate data representing introduction likelihood (Chapter II). These correlative approaches rely upon key assumptions relating to the presence or absence of local or regional adaptations, and so I subsequently tested for evidence of such adaptations in genetic lineages and in individual populations. This was achieved through analyzing climatic differentiation of key genetic lineages in the native and non-native ranges (Chapter III) and by conducting lab experiments comparing thermal responses of important life history traits in populations from contrasting climates (Chapter IV). The initial risk assessment did not account for a key factor in invasions; namely, natural dispersal.

Natural dispersal has been observed to be subject to selection in vanguard populations of invasive species, and adaptation of dispersal traits can infer additional invasive vigor, allowing the species to spread across the landscape quicker. For this reason, I quantified dispersal, activity and morphological differences, often associated with differential dispersal ability, in populations along a distance-gradient from an invasion front, in order to identify if P. parva is capable of such adaptations.

The initial risk mapping study showed that large areas, beyond the current distribution of the species, are climatically suitable. These areas are mainly in North and South America, Australia and New Zealand, and constitute significant scope for spread and impact of this species. When introduction likelihood was included, N. America appears most at risk. I found no evidence to suggest that native genetic lineages represented local adaptations to their respective native climates -there was little or no differentiation of the lineages' occupied climatic conditions in the European range. It was also apparent, from the comparisons, that the climatic conditions in the European range constituted an expansion, compared to the native range. The thermal responses of P. parva life history traits did not differ significantly between populations from a strongly seasonal continental climate and a mild temperate maritime climate. The overall reproductive output of females did not vary according to breeding season temperature, however, temporal reproductive strategy showed a strong response, with lower temperatures inducing a protracted breeding season and higher temperatures inducing rapid and intense reproductive output. The dispersal and morphology-related study identified a strong gradient of morphological change, corresponding with distance from invasion front. This demonstrates a high degree of plasticity in P. parva's morphology in an invasion context, however this was not linked to either dispersal or activity levels, neither of which was significantly linked to distance from invasion front. Dispersal was best explained by body size, with larger fish more likely to disperse further.

Whilst I found no evidence to suggest that the model predictions (Chapter II) were hampered by differentiation at either lineage or population levels, the findings of Chapter III do highlight the uncertainties surrounding the degree of conservatism in such predictions, mainly owing to the fact that past, native, distribution did not accurately represent the current distribution. The results of Chapters II-IV show broad tolerances and great plasticity in P. parva, which likely underpin this species success as a pan-continental invader. The knowledge produced in this thesis provides a useful new resource for the development of management strategies for P. parva and could be usefully enhanced by the additional of analogous studies on native populations, which could help elucidate the source of the observed plasticity.

Keywords: Biological invasion, fish, ecological niche, spatial modelling, Pseudorasbora parva.

REMERCIEMENTS

First I'd like express my sincere gratitude to Rudy Gozlan, Simon Blanchet, Pippa Gillingham and Rob Britton for their supervision and guidance throughout my studentship. I always left steering meetings with a renewed sense of confidence and clarity about what I next needed to achieve and comments on manuscripts were always helpful and no doubt improved their chances of publication immeasurably. In particular, thanks to Rudy for organising the whole project and the funding in the first place and for helping me to navigate the intricacies of the university administrational system in a country where I have only the weakest grasp of the language. Thanks to Pippa for always being available at short notice for meetings where she typically facilitated my realisation of how to tackle problems with one or two choice questions, for guidance on all things spatial and for her constant encouragement. Thanks to Rob for his guidance on maintaining the fish and the aquaria equipment, for his general guidance on all things fishy. Thanks to Simon for organising the necessary permits and for the inordinate amount of time spent driving us around the South of France, collecting fish, and for his expert guidance on statistical methods and on wine. I must also say a particular thanks to Jean-Francois Martin, for his role in my steering committee and for running around, delivering paperwork and obtaining signatures each year, typically at short notice, for my re-enrolment at the university -I am very, very grateful.

I have been very lucky to have met a number of fantastic, kind, helpful and supportive people during my studentship, both in France and in the UK.

In France, thanks to Julien Cote, who arranged my accommodation whilst I worked in Moulis and to all my other housemates there who made it such an enjoyable experience. A number of fellow PhD students and also some interns helped out on various aspects of the experimental work in Moulis, from helping me build and maintain the experimental systems, to helping catch the fish and run the actual experiments. Thank you to Yoann Buoro, Nico Cant, Yahui Zhao, Lieven Therry, Jerome Prunier, Keoni Saint-Pe and anyone else that helped in any way.

In the UK, the thermal challenge experiments were quite high maintenance; the fish required checking, feeding and cleaning every day. For the times when I was away, thank you to the kind souls who so graciously agreed to help out. Thank you to Danny Sheath, Josie Pegg, Tea Bašić, Adrian Blake, Georgina Busst, for looking after things while I was away. This meant more than you can imagine; I hope the tequila was worth it. Thank you to my office mates and friends at Bournemouth University for providing such an intellectually stimulating environment in which to work -I always looked forward to seeing what interesting new diagram or profound quotation had been added to the white-board.

Thank you also to my family and friends who have been unwavering in their support and encouragement. Last, but by no means least, the most enormous and heartfelt thanks have to go to my wonderful and loving wife, Colette, for supporting me emotionally and financially throughout this endeavour, for being a constant source of inspiration, motivation and encouragementwords cannot adequately express the level of gratitude I feel. (Hulme, 2009;[START_REF] Gurevitch | Emergent insights from the synthesis of conceptual frameworks for biological invasions[END_REF].

LIST OF TABLES

Les impacts écologiques et économiques causés par l'introduction d'espèces peuvent rapidement augmentés avec le temps depuis la première introduction (Norton 2005).

Le plus souvent, il devient très rapidement impossible d'éradiquer les populations non-natives établies sans mettre en oeuvre des politiques extrêmement longues et couteuses (Leung et al., 2002). Cela montre l'importance d'intervenir très tôt au cours du processus d'invasion afin de contrôler les potentielles conséquences négatives [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF]. Etant donné le consensus général sur les risques économiques et écologiques associés aux invasions biologiques, ce n'est pas surprenant qu'un part importante des ressources humaines et financières soient investies vers des outils permettant d'améliore notre compréhension des facteur déterminants la probabilité pour une espèce de devenir invasives [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF]Copp et al., 2005b;2009). (Gozlan et al., 2010;Gozlan, 2012). Cette espèce a étendue sont aire de distribution non-native à un taux d'environ cinq nouveaux pays par décennie pour couvrir à l'heure actuelle une région s'étendant de la Turquie à l'Est de l'Angleterre et le Pays de Galles. On le retrouve même en Afrique du Nord (Algérie et Maroc) et au Moyen Orient (Iran) (Gozlan et al., 2010;Gozlan 2012). Bien que de petite taille corporelle (souvent moins de 120 mm de long), son introduction a causé de très forts impacts du fait de sa capacité à rapidement former des populations de très grandes tailles (fortes densités dans le milieu, Britton & Brazier 2006;Britton et al., 2010;Britton & Gozlan 2013). Ceci est particulièrement le cas dans les milieux dégradés (Rosecchi et al., 2001), ce qui conduit entre autre à une compétition pour les ressources avec les espèces natives [START_REF] Jackson | Variation in the trophic overlap of invasive Pseudorasbora parva and sympatric cyprinid fishes[END_REF]. En plus de ces effets directs, le goujon asiatique est porteur sain d'un agent infectieux émergent (Sphaerothecum destruens) qui est connu pour causer des mortalités importantes chez de nombreuses autres espèces de poissons, parmi lesquelles certains salmonidés à fortes valeurs commerciales (Gozlan et al., 2005;[START_REF] Spikmans | High prevalence of the parasite sphaerothecum destruens in the invasive topmouth gudgeon Pseudorasbora parva in the Netherlands, a potential threat to native freshwater fish[END_REF]Ercan et al., 2015). Malgré le fait que cette espèce pose de sérieux risques écologiques et économiques et qu'elle ait été définie comme l'espèce de poissons d'eau douce la plus invasive en Europe (Gozlan et al., 2010), il n'y a pas eut de quantification de ses long du front d'invasion [START_REF] Phillips | Evolutionarily accelerated invasions: the rate of dispersal evolves upwards during the range advance of cane toads[END_REF]. Au cours du Chapitre V, j'ai donc quantifié et comparé la capacité de dispersion, l'activité et la morphologie générale (deux traits pouvant être liés à la capacité de dispersion) chez six populations de P.

parva le long d'un gradient d'invasion afin de tester si de telles adaptations existaient chez cette espèce.

SYNTHESE DU CHAPITRE II

SYNTHESE CHAPITRE III

SYNTHESE DU CHAPITRE IV

CHAPTER I GENERAL INTRODUCTION BIOLOGICAL INVASIONS

Biological invasions pose a severe threat to biodiversity and ecosystem functioning.

Dispersal of species beyond their native range underpins this pervasive driver of global change. Dispersal is a natural process and is critical to the global distribution of species, to the functioning of demographic processes (i.e. metapopulations theory)

and is even a major driver of species evolution [START_REF] Nentwig | Biological Invasions[END_REF]. The advent of globalisation and the proliferation of international trade and travel have increasingly led to the anthropogenic removal of dispersal barriers with the rapid increase in the translocation of organisms, both intentionally and unintentionally [START_REF] Perrings | How to manage biological invasions under globalization[END_REF]Meyerson & Mooney 2007;Crowl et al., 2008). Species have been intentionally introduced for a variety of reasons, with many seen as assets due to their aesthetic properties (i.e. ornamental fish and plants), their desirability as pets, and their commercial value as a food or material (i.e. crops, livestock, trees for timber) (Pimentel et al., 2005;[START_REF] Pejchar | Invasive species, ecosystem services and human wellbeing[END_REF]. Introduction pathways are manifold and aside from intentional introductions, they may also occur as a result of the contamination of goods, vessels, livestock or people (e.g. parasites or disease), or individuals may themselves disperse naturally, but aided by changes to the environment (e.g. opening up of new waterways, construction of bridges and tunnels Hulme et al., 2009).

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It is not just human facilitated movements of biological organisms that pose the threat of invasions. Climate change has the potential to play a significant role in determining invasion success, both directly, by influencing survivorship of transportation and initial introduction [START_REF] Hellmann | Five potential consequences of climate change for invasive species[END_REF][START_REF] Pyke | Current practices and future opportunities for policy on climate change and invasive species[END_REF], and indirectly, by destabilising or disturbing ecosystems, making them more susceptible to invasion [START_REF] Byers | Impact of non-indigenous species on natives enhanced by anthropogenic alteration of selection regimes[END_REF]. Furthermore, climate change may lead to range expansions and/or range shifts of organisms, particularly at range edges, where conditions were previously of marginal suitability and may now rapidly become suitable [START_REF] Dukes | Does global change increase the success of biological invaders?[END_REF]. Similarly, existing populations of seemingly benign introduced species may outperform native species and become invasive under novel climatic regimes (e.g. [START_REF] Stachowicz | Linking climate change and biological invasions: ocean warming facilitates nonindigenous species invasions[END_REF]. Such changes in species distributions may lead to novel biotic interactions, with unpredictable outcomes, some of which may mean loss of biodiversity or ecosystem services [START_REF] Schweiger | Multiple stressors on biotic interactions: how climate change and alien species interact to affect pollination[END_REF].

Nonetheless, not all introduced non-native species go on to become invasive [START_REF] Blackburn | A proposed unified framework for biological invasions[END_REF]. Many introduced species fail to survive their initial introduction and, of those that do, many do not go on to become invasive [START_REF] Jeschke | Invasion success of vertebrates in Europe and North America[END_REF]). However, a subset of those species introduced go on to establish populations, spread rapidly and cause damage or harm to the native recipient ecosystems, human health or the economy (Williamson & Fitter 1996;Clavero & Garcia-Berthou 2005;Hejda et al., 2009), and these are termed 'Invasive'. Although there is no single universal definition for the term "invasive species", definitions typically include that it is an introduced species that poses a threat, or harm, to the environment, the economy or to human health (but see Copp et al., 2005a).

Biological invasion is a process operating at the global scale (McGeoch et al., 2010).

More than 120,000 non-native plants, animals and microbes have invaded the US, UK, Australia, South Africa, India and Brazil, causing an estimated annual USD $314 billion worth of damage [START_REF] Pimentel | Economic and environmental threats of alien plant, animal, and microbe invasions[END_REF]. The introduction of non-native species is thought to be responsible for the loss of more than 5% of global Gross National Product (GNP) and the loss of more than 20% of global food production [START_REF] Nentwig | Biological Invasions[END_REF]. Clavero & Garcia-Berthou (2005) analysed records of extinction in the IUCN Red List database and found that 54% of the records, for which causes were noted, included the effects of invasive species. Recognition of the importance of the subject of biological invasions is increasing, reflected in the increase in published studies on the subject (Hulme, 2009;[START_REF] Gurevitch | Emergent insights from the synthesis of conceptual frameworks for biological invasions[END_REF].

Both the impact and the associated costs can increase sharply in magnitude with time since initial introduction (Norton 2005). Often, it rapidly becomes unfeasible to eradicate established non-native populations, necessitating expensive long-term management (Leung et al., 2002). For example, since the discovery of the invasive zebra mussel, Dreissena polymorpha, in the North American Great Lakes in 1986 at low abundances [START_REF] Hebert | Ecological and genetic studies on Dreissena polymorpha (Pallas): a new mollusc in the Great Lakes[END_REF], it quickly spread (inhabiting waterways from Minnesota to Louisiana and from Oklahoma to West Virginia by the end of 1993; [START_REF] Johnson | Geographic spread of exotic species: ecological lessons and opportunities from the invasion of the zebra mussel Dreissena polymorpha[END_REF]. This invasion has caused pervasive changes to the Great Lakes ecosystem [START_REF] Mills | Exotic species and the integrity of the Great Lakes[END_REF]. Now the problem is sufficiently severe that there is little prospect for their eradication or control in open waters (Strayer 2009). This exemplifies why it is most practical to intervene early in the invasion process [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF], and is one of many cases that can be cited in support of the call for improved prediction and prevention [START_REF] Gallien | Predicting potential distributions of invasive species: where to go from here?[END_REF].

TACKLING THE PROBLEM

The importance of tackling the problem of harmful non-native species is recognized in a number of international agreements and policies. Specifically, it concerns any measure introduced by member states, to: "protect human or animal life from risks arising from additives, contaminants, toxins or disease-causing organisms in their food"; "protect human life from plant-or animalcarried diseases"; "protect animal or plant life from pests, diseases, or disease-causing organisms"; "prevent or limit other damage to a country from the entry, establishment or spread of pests" (https://www.wto.org/english/Tratop_e/sps_e/spsund_e.htm).

The SPS agreement establishes the use of systematic risk assessment and demands rational justification for any measures that may constitute barriers to trade, thus facilitating the minimisation of arbitrary or protectionist measures that may unfairly discriminate against certain regions or nations (Andersen et al., 2004). Justification should be via scientific risk assessment techniques detailed in the risk analysis standards set by the relevant international organizations, such as the World Health Organization (WHO), the Office International des Epizooties (OIE) and the Food and Agriculture Organization (FAO), for human health, animal health and plant health, respectively [START_REF] Branquart | 3.1 From impact studies to management actions: practicing risk analysis of introduced trees[END_REF][START_REF] Vanderhoeven | Beyond protocols: improving the reliability of expert-based risk analysis underpinning invasive species policies[END_REF].

RISK ANALYSIS

The risk analysis standards share a common general structure, incorporating the elements of hazard identification, risk assessment, risk management and risk communication (IPPC, 2007;CAC, 2011;OIE, 2011) and provide a framework against which to work to identify, quantify and mitigate the risks associated with the introduction of harmful non-native species, by allowing the targeting of often scarce resources towards those species or pathways that constitute the greatest risk, (Andersen et al., 2004;[START_REF] Roy | Developing a framework of minimum standards for the risk assessment of alien species[END_REF].

Hazard identification refers to the identification of organisms capable, or potentially capable of causing harm (OIE, 2004). Risk assessment relates to the characterisation of the likelihood and magnitude of negative impacts associated with exposure to a hazard [START_REF] Vanderhoeven | Beyond protocols: improving the reliability of expert-based risk analysis underpinning invasive species policies[END_REF]. Risk management is a means of identifying, evaluating, selecting and implementing mitigation options, on the basis of risk assessments, whilst accounting for considerations such as efficacy, cost, practicality, acceptability and likelihood of reinvasion (Andersen et al., 2004;[START_REF] Booy | Risk management to prioritise the eradication of new and emerging invasive non-native species[END_REF].

Risk communication is a transverse activity in the process of risk analysis and facilitates the gathering and dissemination of information that will improve both assessment and management of risk, e.g. through the education of the public and/ or industry on hazards associated with the release or escape of non-native species [START_REF] Copp | European Non-native Species in Aquaculture Risk Analysis Scheme-a summary of assessment protocols and decision support tools for use of alien species in aquaculture[END_REF]. Risk assessment is the foundation of the whole process of risk analysis and informs the development of appropriate risk mitigation measures, where necessary. The risk assessment process itself consists of four key evaluation elements, which logically mirror the four well recognised phases of biological invasion: i) introduction, ii) establishment, iii) spread, iv) impact (Andersen et al., 2004).

Introduction

Evaluation of introduction usually consists of quantifying introduction likelihood and is an essential element for the development of effective management plans [START_REF] Hulme | Beyond control: wider implications for the management of biological invasions[END_REF][START_REF] Roy | Developing a framework of minimum standards for the risk assessment of alien species[END_REF]. Where risk of establishment, spread and impact are high, identification and disruption of introduction pathways would be critical to averting the negative impacts. In many regions of the world there is a strong link between GDP and the number of introduced species (Hulme, 2009), and association with a particular trade or commodity can mean that data on the frequency and magnitude of shipments may be used as a proxy for introduction likelihood or, depending on the degree and consistency of the association, even the quantity of individuals introduced (Hulme et al., 2008). However, the means by which a particular species is introduced is not easily identified, unless the organism was intentionally imported directly and legally [START_REF] Minchin | Aquaculture and transport in a changing environment: overlap and links in the spread of alien biota[END_REF]. Even then, the means or pathway of initial introduction into the general region may be documented, but the means of subsequent movements within the region may be missing from the data (Hulme et al., 2008). This may be particularly pertinent in large contiguous areas made up of countries without hard borders/customs checks for trade (i.e. trade blocks). Introduction pathways are also not mutually exclusive for individual species. For instance, in a study of 986 introduced non-indigenous marine species in the Mediterranean, [START_REF] Zenetos | Alien species in the Mediterranean Sea by 2012. A contribution to the application of European Union's Marine Strategy Framework Directive (MSFD)[END_REF] found that 11.6% (114 species) were potentially introduced via two or more pathways. In a more general study of 400 established alien taxa, using data from the Global Invasive Species Database (www.issg.org/database), Hulme et al. (2008) found that less than two thirds (62. 5%) were associated with a single introduction pathway.

Establishment

Evaluation of establishment relates to estimating the likelihood of the introduced organism establishing self-sustaining populations in its new environment. Given the consensus on the magnitude of the problem posed by biological invasions, it is no surprise that identifying predictors of establishment success of introduced species has been of interest to researchers [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF][START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF]Hayes & Barry 2008;[START_REF] Bomford | Predicting establishment success for alien reptiles and amphibians: a role for climate matching[END_REF]. In risk assessments, this can be evaluated in a number of ways, typically based on research-derived indicators of establishment success. These indicators fall into three broad categories: characteristics of introduction events, characteristics of the introduced organism, characteristics of the recipient environment (Hayes& Barry 2008;[START_REF] Rago | Introduction pathway and climate trump ecology and life history as predictors of establishment success in alien frogs and toads[END_REF].

Characteristics of introduction events

One of the most broadly applicable and seemingly universal general principles of establishment success for introduced non-native species is propagule pressure [START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF]Colautti et al., 2006). Propagule pressure, which indicates both the number of individuals introduced and the number of introduction events (see Britton & Gozlan 2013) is a strong predictor of the establishment and spread of introduced species [START_REF] Levine | Species diversity and biological invasions: relating local process to community pattern[END_REF]Colautti et al., 2006;Copp et al., 2007), with greater establishment success associated with increased propagule pressure in a number of studies [START_REF] Levine | Species diversity and biological invasions: relating local process to community pattern[END_REF][START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF]Britton & Gozlan 2013). There are a number of mechanisms by which propagule pressure may affect an introduced population's establishment success. For instance, demographic stochasticity can be a critical factor in the persistence of small populations, as its strength increases as populations get smaller [START_REF] Legendre | Demographic stochasticity and social mating system in the process of extinction of small populations: the case of passerines introduced to New Zealand[END_REF]. Further, introductions of smaller numbers of individuals tend to include lower genetic diversity and thus are more prone to reductions in mean fitness through genetic bottlenecks, genetic drift and inbreeding (i.e. inbreeding depression) [START_REF] Frankham | A primer of conservation genetics[END_REF]. Multiple introductions to the same location may counteract these issues, through the 'rescue effect', particularly where introductions come from multiple source populations [START_REF] Sakai | The population biology of invasive species[END_REF][START_REF] Brook | Australasian bird invasions: accidents of history?[END_REF][START_REF] Tallmon | The alluring simplicity and complex reality of genetic rescue[END_REF]. Such augmentation of genetic diversity through the introduction of individuals from a number of disparate sources may also increase adaptive potential in novel environments [START_REF] Dlugosch | Founding events in species invasions: genetic variation, adaptive evolution, and the role of multiple introductions[END_REF] and has been observed to enhance establishment success [START_REF] Ahlroth | Founder population size and number of source populations enhance colonization success in waterstriders[END_REF], with greater numbers of sources possibly increasing success via the mechanism of heterosis [START_REF] Drake | Heterosis, the catapult effect and establishment success of a colonizing bird[END_REF][START_REF] Verhoeven | Population admixture, biological invasions and the balance between local adaptation and inbreeding depression[END_REF]. Nonetheless, altering abiotic conditions can affect the minimum propagule pressure required for successful establishment [START_REF] Thomsen | Ecological resistance, seed density and their interactions determine patterns of invasion in a California coastal grassland[END_REF], so the role of propagule pressure can be context-dependantregardless of how many individuals are introduced and with what frequency, if the conditions are not suitable, then they will not establish selfsustaining populations. For instance, many agricultural crops are planted in areas with climates that fail to support non-cultivated populations of them, despite high propagule pressure, yet some biological invasions originated from very low propagule pressure [START_REF] Simberloff | The role of propagule pressure in biological invasions[END_REF]. Nonetheless, propagule pressure has been used successfully in predictive models for assessing likelihood of invasion [START_REF] Lambdon | Predicting the invasion success of Mediterranean alien plants from their introduction characteristics[END_REF][START_REF] Herborg | Predicting invasion risk using measures of introduction effort and environmental niche models[END_REF][START_REF] Herborg | Forecasting the potential distribution of the invasive tunicate Didemnum vexillum[END_REF].

Characteristics of the introduced organism

Multiple studies have sought to identify life history traits indicative of invasiveness [START_REF] Sakai | The population biology of invasive species[END_REF], and have observed a number of characteristics common amongst known invaders: the ability to reproduce sexually and asexually, rapid growth and early maturity, association with humans, high fecundity, eurytopy, phenotypic plasticity, rapid dispersal, aggressive behaviour and competitiveness, polyphagy [START_REF] Moyle | Fish introductions into North America: patterns and ecological impact[END_REF][START_REF] Lodge | Biological invasions: lessons for ecology[END_REF][START_REF] Sakai | The population biology of invasive species[END_REF][START_REF] Blackburn | The role of species traits in the establishment success of exotic birds[END_REF]Chapple et al., 2012).

Whilst trait-based studies of invasiveness have found that certain traits that help achieve success at one stage of the invasion process may be a hindrance to success at another stage [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF]Ribeiro et al., 2007), a number of characteristics are generally considered as strong predictors of establishment success in novel environments. Such determinants of establishment success include traits associated with more rapid population growth, e.g. small body size, rapid growth and maturity, shorter life span and high fecundity [START_REF] Duncan | High predictability in introduction outcomes and the geographical range size of introduced Australian birds: a role for climate[END_REF][START_REF] Forsyth | Climatic suitability, life-history traits, introduction effort, and the establishment and spread of introduced mammals in Australia[END_REF], as well as traits relating to ecological generalism, e.g. of habitat [START_REF] Cassey | Life history and ecology influences establishment success of introduced land birds[END_REF], of diet (Vall-Ilosera & Sol, 2009), or of climatic conditions [START_REF] Bomford | Predicting establishment success for alien reptiles and amphibians: a role for climate matching[END_REF]. These traits generally describe r-selected, or opportunistic life modes (Winemiller & Rose 1992).

Characteristics of the recipient environment

In simple terms, the ability of an organism to establish in, or colonize an area is constrained by factors that can be categorized into three broad groups: (i) physical barriers to dispersal; (ii) the spatial distribution of environmental conditions within which the species can survive, reproduce and maintain a population; and (iii) the biotic element of the environment and associated interactions, including competitors, predators including parasites and pathogens, as well as the dynamics and availability of food resources (Guisan & Thuiller 2005;Soberón & Peterson 2005;[START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF][START_REF] Sillero | What does ecological modelling model? A proposed classification of ecological niche models based on their underlying methods[END_REF]. This concept can be illustrated by a heuristic diagram that depicts each of the constraints and visually demonstrating how they can interact, or overlap, to identify the set of conditions and locations that are available and suitable (Fig. 1.1).

The entirety of the area constrained by abiotic factors is referred to as the 'fundamental niche'. The portion in the centre, where all three constraints interact, is typically referred to as the 'realised niche' and the portion constituting the intersection between the abiotic and biotic constraints is referred to as the 'potential niche' (Soberón & Peterson, 2005;[START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF]. In this context the use of the term 'niche' relates to, [START_REF] Hutchinson | Concluding remarks[END_REF][START_REF] Hutchinson | An Introduction to Population Ecology[END_REF] 'ecological niche' -all the "states of the environment" that allow the species to "exist indefinitely" [START_REF] Hutchinson | Concluding remarks[END_REF]. However the partitioning of the biotic and abiotic constraints relate to two contrasting but complementary perspectives on niche theory: The Eltonian perspective [START_REF] Elton | Animal ecology[END_REF], where resource availability and species interactions (i.e. 'bionomic' variablessensu Hutchinson 1978) determine suitability, at a local scale consistent with the movement of individuals within their home range [START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF]; and the Grinnellian perspective [START_REF] Grinnell | The niche-relationships of the California Thrasher[END_REF], where environmental conditions (i.e.

'scenopoetic' variablessensu Hutchinson 1978) determine suitability, at a large scale consistent with species distributions [START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF]. This then suggests a nested hierarchy, based on scale, with climatic factors more important in determining broad areas of suitability and then biotic factors determining suitability of patches within these broad areas. This hierarchical perspective is consistent with how factors affecting the structure of biodiversity operate at varying scales [START_REF] Shmida | Biological determinants of species diversity[END_REF]Mackey & Lindenmayer 2001;[START_REF] Whittaker | Scale and species richness: towards a general, hierarchical theory of species diversity[END_REF]Pearson & Dawson 2003;[START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF][START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF][START_REF] Emery | Niche evolution across spatial scales: climate and habitat specialization in California Lasthenia (Asteraceae)[END_REF].

One fundamental difference of the Grinnellian and Eltonian concepts of niche is that suitable conditions (i.e. Grinnellian niche) may be available to be occupied, whereas an organisms place in a community (i.e. Eltonian niche) is defined by the organism's interactions with other species in order to obtain or utilize resources, survive and reproduce; the latter being determined by the organism's characteristics and those of the surrounding community (Elton 1957;[START_REF] Von Holle | Ecological resistance to biological invasion overwhelmed by propagule pressure[END_REF]. Two species therefore cannot occupy exactly the same Eltonian niche, as they would be competing for the same resources, which would eventually lead to one of the species going extinct, or being excluded (i.e. competitive exclusion [START_REF] Gause | The struggle for existence[END_REF][START_REF] Tilman | Resource competition and community structure[END_REF]. This concept is fundamental to understanding the impact of introduced species, as any overlap in Eltonian niche would result in negative interactions [START_REF] Melbourne | Invasion in a heterogeneous world: resistance, coexistence or hostile takeover?[END_REF].

Invasibility (the intrinsic susceptibility to invasion -Lonsdale 1999) may be regulated by a number of biotic factors, such as predation, competition, or resource availability [START_REF] Tilman | Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly[END_REF]). The implication of the niche concept described above is that a detailed knowledge of resource-consumer dynamics, community structure, competitive interactions and population demographics are required in order to estimate suitability of a given environment at a broad scale, although some generalities have been proposed. For instance, Elton (1958) hypothesised that communities with greater species richness are more stable and therefore provide higher 'biotic resistance' to colonisation by non-natives. However, the inverse has subsequently been proposed, on the basis that high species richness reflects greater resource availability (e.g. [START_REF] Stohlgren | Plant species invasions along the latitudinal gradient in the United States[END_REF]Stohlgren et al., , 2006)). In fact, evidence is equivocal; the correlation between native species richness and non-native species richness has been variously observed to be positive, negative or non-existent (Byers & Noonberg 2003;[START_REF] Levine | A meta-analysis of biotic resistance to exotic plant invasions[END_REF]Leprieur et al., 2008).

The enemy release hypothesis is one of the most cited explanations for the phenomenon of biological invasions worldwide (Maron & Vila 2001;[START_REF] Keane | Exotic plant invasions and the enemy release hypothesis[END_REF]Colautti et al., 2004;[START_REF] Liu | Testing the enemy release hypothesis: a review and metaanalysis[END_REF]. It posits that when introduced into a new environment free from the usual biotic constraints of its native co-evolved enemies (e.g. predators, parasites and diseases) an organism is able to increase in abundance and distribution, as a consequence of the decrease in regulation [START_REF] Keane | Exotic plant invasions and the enemy release hypothesis[END_REF]) -although such a response is also possible as a result of increased resource availability in the recipient environment, relative to the introduced organism's native distribution (Shea & Chesson 2002). Nonetheless, as biotic interactions occur within complex food webs that include multiple trophic levels [START_REF] Van Der Putten | Predicting species distribution and abundance responses to climate change: why it is essential to include biotic interactions across trophic levels[END_REF], predicting invasibility of communities in natural environments is particularly difficult because so many of the interactions between species and ecosystem processes are poorly understood (Stohlgren & Schnase 2006).

Furthermore, as environmental conditions can influence outcomes of biotic interactions (e.g. interspecific competition - [START_REF] Davis | Individualistic species responses invalidate simple physiological models of community dynamics under global environmental change[END_REF], this means that such interaction outcomes can vary radically across entire distributions [START_REF] Sagarin | Moving beyond assumptions to understand abundance distributions across the ranges of species[END_REF], particularly for species with large geographical distributions that encompass a wide range of environmental conditions. Thus, quantitative or descriptive data that characterize such interactions and their variability typically require lengthy detailed studies and, as a consequence, are often lacking [START_REF] Soberón | Niche and area of distribution modeling: a population ecology perspective[END_REF]. Often if a species has a history of being invasive elsewhere, then it may be appropriate to assume that it possesses traits that can effectively ameliorate biotic resistance and allow it to successfully establish (Bomford et al., 2010), unless the contrary is known for a particular situation (e.g. the presence of an effective native predator -DeRiviera et

al., 2005).

Whilst datasets representing relevant biotic factors are often lacking, other types of data covering large, sometimes global spatial extents have become increasingly available (see Table 1 in Turner 2003 for a list of data-types and sources). At the broad continental scales synonymous with the definition of species distributions [START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF], climate is the single most important in determining suitability for organisms' survival [START_REF] Willis | Species diversity--scale matters[END_REF]Pearson & Dawson, 2003). A number of analyses have found that climatic similarity between the native and introduced range is the most consistent and accurate predictor of establishment likelihood (e.g. [START_REF] Thuiller | Niche-based modelling as a tool for predicting the risk of alien plant invasions at a global scale[END_REF]Hayes & Barry 2008;[START_REF] Bomford | Predicting establishment success for alien reptiles and amphibians: a role for climate matching[END_REF]. With the availability of high resolution temperature and precipitation-derived datasets, at a near global extent (e.g. Hijmans et al., 2005;[START_REF] Karger | Climatologies at high resolution for the earth's land surface areas[END_REF] and based on the lack of other suitable biological data on a large scale basis, climate comparisons are often the only feasible means of predicting environmental suitability [START_REF] Baker | The role of climatic mapping in predicting the potential geographical distribution of non-indigenous pests under current and future climates[END_REF]. There is a variety of methods for estimating environmental suitability using coarse-scale abiotic datasets, from simplistic distance-based metrics of similarity, to complex statistical models, which are underpinned by niche theory (see below section, on Ecological Niche Modelling).

Spread

Whilst the distribution of suitable conditions in the recipient environment may determine the ultimate scope for spread, the rate at which this spread is realized is determined by dispersal ability. Two classes of dispersal may occur subsequent to establishment in a new location: (i) aided dispersal, via vectors such as people, vehicles and equipment, but also other species (e.g. the spread of zebra mussels by birds -Johnson & Carlton 1996); (ii) unaided dispersal, either passively (i.e. via water or wind currents) or actively, determined by the organism's physical ability to move through the landscape [START_REF] Lodge | Risk analysis and bioeconomics of invasive species to inform policy and management[END_REF]. Local spread may be determined predominantly by the organism's physical dispersal ability and behaviour (or prevalent conditions, in the case of passive dispersal), although these factors often vary according to life-stages (e.g. planktonic larval stages), populations density, size, gender, which makes predicting rate of spread complicated as it may require in-depth knowledge of the species ecology and demographics in order to make accurate predictions [START_REF] Holway | Animal behavior: an essential component of invasion biology[END_REF][START_REF] Van Den Bosch | Analysing the velocity of animal range expansion[END_REF]. Such unaided dispersal may be limited by the extent of suitable environmental conditions, but further dispersal may be possible with the presence of corridors linking suitable patches (e.g. [START_REF] Brown | Toad on the road: use of roads as dispersal corridors by cane toads (Bufo marinus) at an invasion front in tropical Australia[END_REF]. Unaided dispersal such as this has historically been modelled using a diffusion principle, which has proved accurate in a number of circumstances [START_REF] Holway | Animal behavior: an essential component of invasion biology[END_REF], but inaccurate in a number of other cases (e.g. [START_REF] Allen | A diffusion model for dispersal of Opuntia imbricata (cholla) on rangeland[END_REF][START_REF] Dwyer | Spatial scale and the spread of a fungal pathogen of gypsy moth[END_REF], which has been attributed to instances of long-distance dispersal [START_REF] Hastings | The spatial spread of invasions: new developments in theory and evidence[END_REF]. Aided dispersal can facilitate medium and long distance dispersal between discrete patches of suitable habitat, even if they are separated by large areas of unsuitable habitat. For instance the spread of the invasive zebra mussel (Dreissena polymorpha) in North American between lakes was facilitated, in part, by the transport of adult and juveniles on macrophytes entangled on recreational boat trailers, which were driven overland between lakes [START_REF] Johnson | Overland dispersal of aquatic invasive species: a risk assessment of transient recreational boating[END_REF]. Known association with particular vectors may allow modelling of dispersal rates and patterns. For example, [START_REF] Herborg | Forecasting the potential distribution of the invasive tunicate Didemnum vexillum[END_REF] used data on the spatial distribution of important transport vectors to accurately describe the occurrence patterns of the invasive tunicate, Didemnum vexillum, in British Columbia.

Impact

The negative ecological impacts generated by the introduction of non-native species can manifest in a number of ways. An introduced non-native species may compete directly with native species for finite resources (e.g. food, habitat) and territory, leading to displacement of the natives and even local or regional extinctions. For instance, several species of Erythrean penaeid prawn, including Marsupenaeus japonicus, were introduced into the Mediterranean through the colonisation of the Suez Canal and increased numerically to the point where they outcompeted native prawn species, Melicertus kerathurus for habitat and resources, leading to the extirpation of this species [START_REF] Galil | Loss or gain? Invasive aliens and biodiversity in the Mediterranean Sea[END_REF]. Some ecosystems may have particular vulnerabilities, having evolved without the presence of certain taxonomic groups that are present in other systems. If members of these taxonomic groups are then artificially introduced, some native species may have no suitable defence mechanism to prevent being predated, or may be outcompeted for food resources by the introduced species. For example, the terrestrial faunal assemblages of New Zealand evolved in the absence of terrestrial mammalian predators. Since human colonisation and the associated introductions of cats, rats and mustelids, the avifauna of these naïve systems has been severely depleted through predation [START_REF] Dowding | The impact of predation by introduced mammals on endemic shorebirds in New Zealand: a conservation perspective[END_REF]. In addition to direct competition and predation, non-native species act as novel vectors for existing diseases or as carriers of novel pathogens. In fact, one of the earliest documented biological invasions, of unintentionally introduced Asian rats, is thought to be responsible for the spread of the bubonic plague throughout Europe in the mid-14th century [START_REF] Morand | Global parasite and Rattus rodent invasions: The consequences for rodent-borne diseases[END_REF]. Biological invasions can have impacts at the level of individual taxa, but also at the whole ecosystem level, with, for example, some invasive plants affecting the above and below ground carbon and nitrogen pool sizes [START_REF] Liao | Altered ecosystem carbon and nitrogen cycles by plant invasion: A meta-analysis[END_REF] and various invasive species impacting ecosystems by means of physical habitat alteration [START_REF] Davis | Invasion Biology[END_REF]. This diversity of possible mechanisms for ecological impact, along with similarly diverse means of causing economic impact, mean that both qualitative and quantitative prediction of impact is notoriously difficult (Hulme 2012), particularly because impacts are context dependant and so can vary to a great degree across regions [START_REF] Ricciardi | Predicting the impacts of an introduced species from its invasion history: an empirical approach applied to zebra mussel invasions[END_REF]. Even projections of monetary costs and benefits are subject to great variability, based on currency values and returns on alternative investments, forgone in order to fund management [START_REF] Cook | Predicting the economic impact of an invasive species on an ecosystem service[END_REF]. It may be appropriate and sufficient to assume that an introduced species will have significant negative impacts in a particular regions, based on a record of having had similar impacts in other regions [START_REF] Ricciardi | Predicting the identity and impact of future biological invaders: a priority for aquatic resource management[END_REF], particularly where the novel region is climatically similar to those where historical impacts have occurred (Venette, et al., 2010). Potential area affected may then be used as a surrogate for the quantification of impacts, based on the historic impacts elsewhere.

RAPID RISK ASSESSMENTS AND SCREENING TOOLS

Fully comprehensive impact assessments may be relatively time-consuming and expensive, which is why streamlined rapid means of assessing potential invasiveness can be useful to determine whether or not this expenditure of time and money is warranted [START_REF] Copp | Risk identification and assessment of non-native freshwater fishes: concepts and perspectives on protocols for the UK[END_REF]. Pheloung et al. (1999) developed the Weed Risk Assessment (WRA) as a means for assessing plants prior to their potential introduction to Australia and New Zealand on the basis of information concerning their 'weed status in other parts of the world', 'climate and environmental preferences' and 'biological attributes', under the principle that a history of being a weed elsewhere in the world implies that a plant is more likely to be weedy (or invasive) in areas with similar environmental conditions (Pheloung et al., 1999). The WRA has since been adapted and developed for use in a number of other regions (Hawaii -Daehler & Carino 2000;the Czech Republic -Krivánek & Pysek, 2006;Florida, USA -Gordon et al., 2008) and to assess a number of different groups of taxa, including freshwater fishes, amphibia, marine fish and invertebrates [START_REF] Copp | Risk identification and assessment of non-native freshwater fishes: concepts and perspectives on protocols for the UK[END_REF](Copp et al., , 2009;;[START_REF] Tricarico | Calibration of FI-ISK, an invasiveness screening tool for nonnative freshwater invertebrates[END_REF][START_REF] Copp | The Fish Invasiveness Screening Kit (FISK) for non-native freshwater fishes-a summary of current applications[END_REF]. Even in their modified, or developed, forms these methods retain a number of the original principles of the WRA, in particular, that invasiveness elsewhere in the world confers high likelihood of invasiveness in similar environmental conditions. Similarity of the 'conditions' in question, typically climatic conditions, can be assessed by a variety of means, including simple distance-based metrics (e.g. mahalanobis, euclidean, etc.) and more complex statistical models, commonly referred to as Ecological Niche Models (ENM) [START_REF] Sillero | What does ecological modelling model? A proposed classification of ecological niche models based on their underlying methods[END_REF].

ECOLOGICAL NICHE MODELS

As already alluded to above, often the only feasible means of estimating invasibility at broad scales is by comparison of coarse-scale environmental conditions between an organism's current distribution and that of potential recipient regions. An effective means of carrying this out is by use of statistical methods commonly referred to as Ecological Niche Models (ENM) [START_REF] Sillero | What does ecological modelling model? A proposed classification of ecological niche models based on their underlying methods[END_REF].The approach involves the estimation of the ecological niche), using coarse-resolution environmental datasets, based on locations of known species occurrences. The use of the term 'niche' in the context of the use of these methods refers to the Grinnellian concept, whereby the niche is approximated using broad scale abiotic, or scenopoetic (sensu [START_REF] Hutchinson | An Introduction to Population Ecology[END_REF] variables effectively representing environmental conditions as habitat [START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF]. Whilst the biotic element is undeniably important in determining suitability, it is generally accepted that these determinants of suitability tend to operate at a local scale, and so are difficult to account for or measure at broad geographical scales [START_REF] Austin | Spatial prediction of species distribution: an interface between ecological theory and statistical modelling[END_REF][START_REF] Araujo | Five (or so) challenges for species distribution modelling[END_REF][START_REF] Soberón | Grinnellian and Eltonian niches and geographic distributions of species[END_REF][START_REF] Emery | Niche evolution across spatial scales: climate and habitat specialization in California Lasthenia (Asteraceae)[END_REF]. Although inclusion of such factors in geographical predictions of suitability may improve accuracy, it may also restrict applicability to broader areas (Hirzel & Le Lay 2008).

One way of incorporating such data is to use it to construct a separate fine-scale suitability model for the extent of the available data and overlay the output on the broad-scale ENM output, in order to identify suitable areas common to both (Hopkins, 2009). Nonetheless, even without data describing biotic interactions directly, many broad-scale models, based on the Grinnellian principle, achieve high degrees of accuracy (Pearson et al., 2002;[START_REF] Vetaas | Realized and potential climate niches: a comparison of four Rhododendron tree species[END_REF], underpinned by the principle that a species large-scale distributional patterns typically correspond to accessible areas of the projected Grinnellian niche in geographical space, in accordance with Hutchinson's niche-biotope duality concept [START_REF] Hutchinson | An Introduction to Population Ecology[END_REF][START_REF] Colwell | Hutchinson's duality: the once and future niche[END_REF]. Although correlational, there is significant utility in the application of these ENM methods for the prediction of biological invasions, as, once characterised, the Grinnellian niche can be projected in multi-dimensional climate-space, representing large geographical areas [START_REF] Araújo | Uses and misuses of bioclimatic envelope modeling[END_REF]. This, in theory, allows the assessment of environmental suitability of a site for a particular species at any location for which these climatic data are available.

In the absence of explicit definitions, with reference to original source literature, interpretations of the concept of ecological niche can be somewhat subjective and inconsistent with one another. Whilst these ENM methods enjoy increasingly widespread use (Qiao et al., 2015), the definition of the term niche is not always clarified in the context of its use. Unfortunately it seems commonplace in the literature to assume that the reader is familiar with the adoption of the Grinnellian perspective of niche by such biogeographical-type studies (e.g. [START_REF] Soberón | Niche and area of distribution modeling: a population ecology perspective[END_REF][START_REF] Peterson | Ecological niches and geographic distributions (MPB-49)[END_REF][START_REF] Peterson | Ecological niche conservatism: a time-structured review of evidence[END_REF]. This is perhaps best highlited by the use of neologisms, without any clarification on what is meant or reference to historic literature concerning the niche concept. For instance, there have been a number of journal articles published over the last decade (e.g. [START_REF] Kozak | Accelerated rates of climatic-niche evolution underlie rapid species diversification[END_REF], 2016;Petitpierre et al., 2012;Mairal et al., 2017, although many other examples exist) where the term "climatic niche" is used, without any definition being given, or indeed any reference to the published works of the three main proponent authors of the development and synthesis of the niche concepts (i.e. those of Grinnell, Elton and Hutchinson). Even in the absence of these neologisms, when 'niche' or 'ecological niche' is used, definitions are often implied, rather than explicitly stated. For instance, [START_REF] Peterson | Transferability and model evaluation in ecological niche modeling: a comparison of GARP and Maxent[END_REF] compare two methods of Ecological Niche Modelling, training models on presence location data for three bird species and using environmental descriptors of climate and topography. At no point in the article do the authors provide a definition of the term 'niche' or indeed discuss the lack or omission of data concerning biotic interactions. In another example, [START_REF] Warren | Ecological niche modeling in Maxent: the importance of model complexity and the performance of model selection criteria[END_REF] discuss the importance of parameterisation in the use of a particular modelling method (MaxEnt) for ecological niche modelling, however they fail to provide a definition of what is meant by the term 'niche' in the context of the study. Although it is implicit, by the nature of the descriptive data used (scenopoetic), that a Grinnellian perspective of niche is adopted, a clear statement should be used, to avoid uncertainty and aid accurate interpretation of results. For this reason, henceforth, for the duration of this thesis, where the terms 'niche', 'climatic niche' and 'ecological niche' are used, they refer to the broad-scale Grinnellian niche unless stated otherwise.

SPECIES' ADAPTATION TO NEW ENVIRONMENTS

ENM methods for estimating environmental suitability rely on the key assumption of niche conservatism, which is "the tendency of a species to retain ancestral ecological characteristics" [START_REF] Wiens | Niche conservatism: integrating evolution, ecology, and conservation biology[END_REF] essentially maintaining the characteristics of the niche unchanged over time. Studies of pairs of congeneric species between geographically separated ranges have found that they tend to occupy similar conditions, implying conservatism [START_REF] Peterson | Sensitivity of distributional prediction algorithms to geographic data completeness[END_REF]Prinzig et al., 2001).

However, other studies have found that, for some species, the niche can shift, or evolve, even over relatively short time periods, such as decadal (Moran & Alexander, 2014). This exemplifies that this important assumption of ENM is not always met, although this could ultimately depend on the demographic level at which the niche is defined (i.e. genus, species, lineage, population).

Many species exhibit local adaptations, particularly across large and varied geographical ranges [START_REF] Leimu | A meta-analysis of local adaptation in plants[END_REF]Hereford 2009). For instance, many species of insects [START_REF] Hoffmann | Opposing clines for high and low temperature resistance in Drosophila melanogaster[END_REF][START_REF] Aardema | Considering local adaptation in issues of lepidopteran conservation-a review and recommendations[END_REF] and plants [START_REF] Mckay | Local adaptation across a climatic gradient despite small effective population size in the rare sapphire rockcress[END_REF]Savolainen et al., 2007;[START_REF] Fournier-Level | A map of local adaptation in Arabidopsis thaliana[END_REF] display intraspecific local adaptations to temperature. Such adaptations, where a new phenotype is produced through a shift in allele frequencies, due to natural selection, tend to involve trade-offs and so increased performance at local conditions often comes at the detriment of wider tolerances (Hereford 2009). Where such adaptations occur, high levels of gene flow from outside the local area tends to dilute them; hence their tendency to be concordant with intraspecific genetic structure (Aitken & Whitlock, 2013). The validity of the assumption of niche conservatism will tend to rely upon the identification of the correct demographic level at which to characterise the niche, i.e. to account for any local or regional adaptations. This means that, for accurate and representative modelling, the correct demographic unit should ideally be used as the basis for niche characterisation and, if necessary, more than one model may be required in order to account for identified local adaptations, or particular ecotypes, within the species. Further, where this is the case, in the context of predicting biological invasions, this means that there may exist differential invasion risk, based on the location or genetic identity of the introduced propagule.

Whilst evolutionary adaptations are classically associated with extended periods of time, i.e. thousands or even millions of years, contemporary micro-evolutionary adaptation is increasingly being observed to have occurred over much shorter periods.

Whilst typical examples of this micro-evolutionary adaptation occur over 50-150 generations, it can happen over fewer than 25 (Moran & Alexander, 2014). There are a number of studies where this contemporary adaptation has been observed to occur and some of the best-documented examples involve biological invasions (Quinn et al., 2000;[START_REF] Ellstrand | Hybridization as a stimulus for the evolution of invasiveness in plants?[END_REF][START_REF] Colautti | Contemporary evolution during invasion: evidence for differentiation, natural selection, and local adaptation[END_REF]. Introduced species can be subject to very different and rapid selective regimes, from biotic and abiotic conditions, in the introduced ecosystems, which can lead to these rapid adaptive evolutionary changes taking place [START_REF] Mooney | The evolutionary impact of invasive species[END_REF][START_REF] Sakai | The population biology of invasive species[END_REF]Savolainen et al., 2007).

Such adaptations can also sometimes be facilitated by propagule pressure, as a low or moderate inflow of genetic diversity from the native range could increase the genetic variability within the introduced population, providing greater chance of recombination producing new genotypes upon which the novel selective pressures can act to produce locally adapted individuals (e.g. [START_REF] Lavergne | Increased genetic variation and evolutionary potential drive the success of an invasive grass[END_REF]. Lag phase, commonly observed in invasions, could sometimes be a result of this phenomenon [START_REF] Sakai | The population biology of invasive species[END_REF]. For instance, the European green crab (Carcinus maenas) was introduced in the US, around New York, in the early 19 th century and initially spread slowly northward to the Gulf of Maine over a period of approximately 100 years [START_REF] Audet | Geographic expansion of a nonindigenous crab,Carcinus maenas (L.), along the Nova Scotian shore into the southeastern Gulf of St Lawrence, Canada[END_REF]. A subsequent explosive range expansion since the 1950s has been linked to the augmentation of genetic diversity by multiple propagules, seemingly from a number of different genetic lineages [START_REF] Roman | Diluting the founder effect: cryptic invasions expand a marine invader's range[END_REF]).

Adaptations may not always have a clearly heritable genetic basis (i.e. altered genotype). Instead, they can be the product of epigenetic effects, i.e. altered gene expression induced by developmental or environmental stimuli [START_REF] Chown | Biological invasions, climate change and genomics[END_REF].

Such mechanisms, where environmentally induced epigenetic variation has shifted phenotypic means and environmental tolerances, have been implicated in biological invasion success (e.g. [START_REF] Herrera | Jack of all nectars, master of most: DNA methylation and the epigenetic basis of niche width in a flower-living yeast[END_REF][START_REF] Zhang | Epigenetic variation creates potential for evolution of plant phenotypic plasticity[END_REF][START_REF] Schrieber | The genetic paradox of invasions revisited: the potential role of inbreeding× environment interactions in invasion success[END_REF]. However, they can be difficult to identify and quantify without strictly controlled experimental designs [START_REF] Verhoeven | Epigenetics in ecology and evolution: what we know and what we need to know[END_REF]. Further, whether or notand to what extent, if soenvironmentally induced epigenetic variants are actually heritable is a source of much debate [START_REF] Heard | Transgenerational epigenetic inheritance: myths and mechanisms[END_REF]. This all means that predicting responses in novel environments, under strong selective regimes (i.e. in the context of a typical invasion scenario), is unlikely to be feasible at present. Nonetheless, identification and quantification of phenotypic plasticity, regardless of its basis, has merit, as phenotypic plasticity in itself is a documented characteristic of successful invaders [START_REF] Sakai | The population biology of invasive species[END_REF]Chapple et al., 2012).

Failure to account for adaptations, or adaptability in invasion predictions could lead to significant under-predictions of the risk associated with introducing a particular species, or individuals from a particular population (if that population is locally adapted, or shows increased adaptability) [START_REF] Parker | An evolutionary approach to understanding the biology of invasions: local adaptation and general-purpose genotypes in the weed Verbascum thapsus[END_REF]. Such adaptations could influence transportation survival and establishment success, as well as determining the overall geographical extent of the area within which any introduction may spread (Broennimann et al., 2007;Fitzpatrick et al., 2007).

Although genetic adaptation to climatic conditions may influence the size of the geographic area suitable for the introduced population, the speed with which that spread-potential is realised is largely determined by individuals' dispersal propensity and dispersal ability, which may be facilitated by the presence of dispersal corridors (i.e. suitable habitat linking habitable areas) and/or by association with human activities (e.g. horticulture, aquaculture, sport). As already mentioned, efficient dispersal is a common characteristic of successful invaders (Chapple et al., 2012).

Whilst it directly impacts the rate of spread, it is also a pivotal mechanism in a metapopulation context, providing an asymmetric flow of individuals between populations, allowing the persistence of populations in conditions that would otherwise not support net positive growth rates and the colonisation of new locations (Hanski & Gyllenberg, 1993;Hanski, 1998). This sort of asymmetric gene flow, typical of range expansions, can also provide ideal opportunities for contemporary adaptations, due to local novel selective forces and the supply of new genetic material (Savolainen et al., 2007;Aitken & Whitlock, 2013). These types of contemporary adaptations can even concern the species dispersal ability, particularly due to spatial sorting, which would tend to mean that individuals more prone to dispersal, or with greater dispersal ability, would show higher prevalence in populations located on the dispersal frontline (Shine et al., 2011). Evidence in the literature supports this assertion, with examples of recently colonized populations phenotypically different from more established populationsparticularly with reference to traits linked to dispersal (Olivieri et al., 1995;Hanski et al., 2004;Duckworth & Badyaev, 2007).

As adaptability and dispersal are both critical to invasion success, an improved understanding of these aspects in introduced species could be key to predicting both likelihood and potential scope of invasion success (Fitzpatrick et al., 2007;[START_REF] Yonekura | A peculiar relationship between genetic diversity and adaptability in invasive exotic species: bluegill sunfish as a model species[END_REF][START_REF] Wilson | Something in the way you move: dispersal pathways affect invasion success[END_REF][START_REF] Gallien | Predicting potential distributions of invasive species: where to go from here?[END_REF]. During the initial introduction phase, understanding the demographic scale at which niche is conserved (e.g. population, lineage, whole species) is critical for correct predictions of environmental suitability, whereas scope for spread may be strongly influenced by an organism's ability to adapt quickly and strength of introduction/dispersal pathways (With 2002;[START_REF] Wilson | Something in the way you move: dispersal pathways affect invasion success[END_REF][START_REF] Gallien | Predicting potential distributions of invasive species: where to go from here?[END_REF]. This not only highlights the importance of preventing initial introductions of know invasive species, but it also highlights that the extent of the spread in such scenarios can be unpredictable. Further, knowledge of whether an invasive species is prone to such adaptations may influence the prioritisation of management and policy relative to that species (e.g. as suggested by Dlugosh & Parker 2008). For instance, existing invasion risk assessment methods may use or suggest use of climate matching as part of overall evaluation (e.g. the Fish Invasiveness Screening Kit -FISK, [START_REF] Copp | Risk identification and assessment of non-native freshwater fishes: concepts and perspectives on protocols for the UK[END_REF], however, these methods do not typically account for potential adaptability, whether it is heritable, or not.

FOCAL SPECIES

My thesis focuses on the topmouth gudgeon, Pseudorasbora parva (Temminck & Schlegel), a small freshwater cyprinid fish (Fig. 1.2), native to East Asia, where it spans an enormous geographical and climatic range from Siberia, in the north of its range, to Southern China and Taiwan in the South [START_REF] Kottelat | Handbook of European freshwater fishes[END_REF]Gozlan et al., 2010;Gozlan 2012). The maximum length of P. parva is 120 mm, with females rarely exceeding 50 mm (Gozlan et al., 2010). This sexual size dimorphism is thought to reflect their reproductive strategies, whereby large dominant males build nests and guard eggs against predation, and females allocate more energy toward egg production, as they are batch-spawners, spawning 1-14 times per season [START_REF] Maekawa | Reproductive success in male Japanese minnows, Pseudorasbora parva: observations under experimental conditions[END_REF]Katano & Maekawa, 1997;[START_REF] Kottelat | Handbook of European freshwater fishes[END_REF]. Although P. parva can produce populations in lotic conditions [START_REF] Sunardi | Foraging of a small planktivore (Pseudorasbora parva: Cyprinidae) and its behavioural flexibility in an artificial stream[END_REF](Sunardi et al., , 2007;;Asaeda et al., 2007), they usually breed in areas of low or no flow [START_REF] Kottelat | Handbook of European freshwater fishes[END_REF]. Individuals can reach sexual maturity in their first year of life and have been recorded to live up to 5 years, although typical maximum ages are 3-4 years (Gozlan et al., 2010).

The diet of P. parva is varied, usually consisting of assorted small zooplankton, including cladocerans, chyronimids, copepods and aquatic insects [START_REF] Xie | Distribution, feeding and body condition of four small fish species in the near-shore and central areas of Liangzi Lake, China[END_REF][START_REF] Didenko | Trophic interaction between topmouth gudgeon (Pseudorasbora parva) and the co-occurring species during summer in the Dniprodzerzhynsk reservoir[END_REF], but it is also known to eat the eggs of other fish [START_REF] Xie | Seasonal patterns in feeding ecology of three small fishes in the Biandantang Lake, China[END_REF] and macrophytes [START_REF] Xie | Distribution, feeding and body condition of four small fish species in the near-shore and central areas of Liangzi Lake, China[END_REF]. It occurs in a wide range of habitats, from reservoirs, lakes, ponds and irrigation ditches to the faster flowing waters of streams and rivers [START_REF] Pinder | Sunbleak and topmouth gudgeon: two new additions to Britain's freshwater fishes[END_REF]Ekmekçi & Kirankaya, 2006;[START_REF] Beyer | Microhabitat use and interspecific associations of introduced topmouth gudgeon Pseudorasbora parva and native fishes in a small stream[END_REF]Onikura & Nakajima, 2011). Individuals usually inhabit the low to mid water column [START_REF] Xie | Distribution, feeding and body condition of four small fish species in the near-shore and central areas of Liangzi Lake, China[END_REF] and have been observed to show preference for areas containing submerged vegetation [START_REF] Kapusta | The significance of stone moroko, Pseudorasbora parva (Temminck and Schlegel), in the small-sized fish assemblages in the littoral zone of the heated Lake Licheńskie[END_REF]. Since its introduction to Europe, in the 1960s (Gozlan et al., 2010;Gozlan, 2012), P.

parva has spread at a rate of approximately five new countries per decade and its introduced range (see Fig. 1.3) now spans Europe, from Turkey in the east to England and Wales in the west and even occurs in North Africa (i.e. Algeria, Morroco) and parts of the Middle East (i.e. Iran) (Gozlan et al., 2010;Gozlan 2012).

Although physically small in form, its introduction can cause severe impacts through its ability to rapidly form high-density populations (Britton & Brazier, 2006;Britton et al., 2010;Britton & Gozlan, 2013), which thrive in degraded habitats (Rosecchi et al., 2001) and compete with native fish for resources due to trophic niche overlap [START_REF] Jackson | Variation in the trophic overlap of invasive Pseudorasbora parva and sympatric cyprinid fishes[END_REF]. In addition to its direct impacts, it is also a healthy carrier of an emergent infectious disease (Sphaerothecum destruens) known to cause mortality in a number of other fish species, including commercially valuable salmonids (Gozlan et al., 2005;[START_REF] Spikmans | High prevalence of the parasite sphaerothecum destruens in the invasive topmouth gudgeon Pseudorasbora parva in the Netherlands, a potential threat to native freshwater fish[END_REF]Ercan et al., 2015). Although it poses significant risk and has been referred to as the Europe's most invasive freshwater fish (Gozlan et al., 2010), there has been no global assessment of invasion risk to date. This constitutes an important knowledge gap, which could hamper efforts to prevent further spread and impact.

INTRODUCTION

The introduction and establishment of species into novel ecosystems can result in rapid dispersal rates (Stohlgren & Schnase, 2006;[START_REF] Vitousek | Human domination of Earth's ecosystems. Urban Ecology: An International Perspective on the Interaction Between Humans and Nature[END_REF], with the magnitude of subsequent impacts and associated management costs increasing sharply with the time since the initial introduction (Norton, 2005). When eradication is no longer a feasible management option, long-term strategies aimed at control and containment are generally adopted [START_REF] Fernald | Changes in cyanobacterial dominance following the invasion of the zebra mussel Dreissena polymorpha: Long-term results from the Hudson River estuary[END_REF]Britton et al., 2011a). Such approaches tend to be expensive and only prevent further damage rather than restoring the invaded systems to their former status (Myers et al., 2000).

Consequently, more effective management strategies focus more on preventing introductions rather than managing subsequent invasions [START_REF] Vander Zanden | A management framework for preventing the secondary spread of aquatic invasive species[END_REF].

Through increased global connectivity and trade, opportunities for the introduction of non-native species have increased (Crowl et al., 2008), with resources at border control stretched and ineffective at preventing the entry of non-native species into new regions [START_REF] Keith | Revisiting green data species lists[END_REF]. Risk analysis frameworks can be a strong mechanism to help allocate finite resources between preventing, detecting and managing introductions of non-native species (Leung et al., 2002;Copp et al., 2009).

As an integral component of risk analysis, risk assessment provides the evidencebased foundation for such decisions to be made and involves estimating both the likelihood of invasion and the magnitude of the consequences (Andersen et al., 2004;EFSA, 2012;[START_REF] Vanderhoeven | Beyond protocols: improving the reliability of expert-based risk analysis underpinning invasive species policies[END_REF]. An important part of this process is estimating likelihood of introduction and establishment, as species with a history of being invasive elsewhere are generally more likely to become invasive if introduced to a location with similar climatic conditions to its native or current range [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF]Bomford et al., 2008;Hayes & Barry, 2008). Therefore geographical predictions of how the likelihood of introduction and of establishment varies can be an important tool for the development of policies, particularly those relating to prevention and early detection of potential biological invasions (Venette et al., 2010).

For species known to have a history of being invasive, two major determinants of introduction and establishment are propagule pressure and climatic suitability [START_REF] Kolar | Progress in invasion biology: predicting invaders[END_REF]Hayes & Barry, 2008;Bomford et al., 2010). Risk maps summarize landscape suitability for non-native species within a region by incorporating multiple factors, such as climatic or habitat suitability and potential introduction pathways, and thus can highlight areas within the region, or continent, where the risk is greatest (Hulme, 2009).

Freshwater ecosystems provide societies with valuable goods and services, yet are disproportionately vulnerable to biological invasions, compared to terrestrial systems, due to their high degree of isolation and endemism (Gozlan et al., 2010a;Moorhouse & McDonald, 2015). Introductions of non-native fishes represent a considerable threat to global freshwater fish conservation [START_REF] Villéger | From current distinctiveness to future homogenization of the world's freshwater fish faunas[END_REF] and can cause considerable economic damage (Gozlan et al., 2010a). For example, in North America the annual cost of invasive non-native fish species is estimated as at least US$ 5.4 billion (Pimentel et al., 2005). In Europe, at least 650 fish species have been introduced into aquatic ecosystems, yet only a relatively small proportion of these are considered invasive and cause detrimental impacts [START_REF] Savini | The top 27 animal alien species introduced into Europe for aquaculture and related activities[END_REF], such as the topmouth gudgeon Pseudorasbora parva (Temminck & Schlegel). This Asian cyprinid fish has been described as Europe's most invasive freshwater fish (e.g. [START_REF] Gozlan | Occurrence of the Asiatic cyprinid Pseudorasbora parva in England[END_REF], having achieved a pan-continental distribution in less than forty years, following its initial accidental introduction into Romania in the 1960s (Gozlan et al., 2010b). Its ability to invade new waters stems from traits that include high tolerance to degraded ecosystems, high reproductive effort and early sexual maturity alongside batch spawning and paternal nest guarding [START_REF] Gozlan | Evidence of female preference for hidden sex signals in distant fish species[END_REF]. Moreover, it can rapidly colonize new waters (Britton & Gozlan, 2013), with their larval stages particularly likely to disperse into the wider environment (Pinder et al., 2005). It is also a healthy carrier of an emergent infectious fungal disease Sphaerothecum destruens that has been implicated in declines in native European fishes (Andreou et al., 2012;[START_REF] Gozlan | Current ecological understanding of fungal-like pathogens of fish: what lies beneath[END_REF]. Given its rapid rate of invasion in Europe and its disease risk, it is thus important to develop tools to predict additional locations at risk, outside of its current range. Such tools would enable countries that contain potentially suitable habitats but have not yet been invaded to set up the necessary infrastructure to prevent an introduction and subsequent invasion.

Integral parts of invasion risk are the likelihood of the species being introduced (e.g. the presence of an introduction pathway) and the suitability of the environment in the new region (e.g. climate suitability for the species - Bomford et al., 2010).

Consequently, the aim of this study was to predict P. parva's likelihood of introduction and establishment at a global scale in relation to its climate suitability and likely introduction pathways. Objectives were to (i) use climatic and other relevant abiotic data in both P. parva's native and introduced ranges to determine its realized niche; (ii) assess its potential global introduction pathways in relation to its known introduction pathways in Europe; and (iii) develop a niche model that characterizes its realized niche and combine this with data on introduction pathways to develop a global risk map of P. parva, based on likelihood of introduction and establishment. The implications of the risk map are then discussed in a global context.

MATERIALS & METHODS

Data on presence and absence of P. parva

Previous Ecological Niche Modelling (ENM) studies suggest that presence location data from the introduced range can sometimes provide a significantly better approximation of the actual species niche breadth, thus producing more accurate suitability predictions beyond the native range (Fitzpatrick et al., 2007;Guisan et al., 2014). Consequently, the presence dataset incorporated data from the European range, compiled from European national monitoring schemes with consistent recording over the last 30+ years, as well as data from its native Asian range. When the European national monitoring data were combined with literature review and native and nonnative presence records from the Global Biodiversity Information Facility (GBIF, www.gbif.org), this provided 2,882 confirmed P. parva observations from unique locations, across both native and introduced ranges, the latest of which was recorded in 2014. In order to minimize bias caused by multiple presence points in grid cells, i.e. spatial clustering of records, I applied a form of spatial filtering similar to that used by [START_REF] Boria | Spatial filtering to reduce sampling bias can improve the performance of ecological niche models[END_REF], whereby I removed all but one record from grid cells, essentially leaving 2,048 "presence cells" in the dataset.

Absence data were generated on the principle of implied absence, as P. parva's European distribution, after 40 years since introduction, suggested the species had already reached saturation point (cf. Gozlan et al., 2010b) and I also assumed equilibrium in the native range. To avoid spatial autocorrelation with presence cells, random points were generated outside buffers of 50 km around presence cell centroids. These random points were limited to a maximum distance of 100 km from presence cell centroids to prevent the characterization of areas far outside the species' core areas, which can result in poor model performance [START_REF] Mainali | Projecting future expansion of invasive species: comparing and improving methodologies for species distribution modeling[END_REF]. These methods also maintained a degree of parity with regard to the ratio of presence to absence points across the two distributional aggregations (Asian and European). A total of 10,000 absence points were generated in 10 absence datasets, and weighted to give equal prevalence for the overall response variable dataset, which was coded in a binary (1/0, presence/absence) fashion. Due to the relatively high recording effort achieved in this monitoring, it was assumed these locations represented genuine absences, rather than just characterizing the background environmental variation present within the introduced range.

Predictor variable data

I used a suite of abiotic variables as a basis for the description of P. parva's niche: 19 Bioclimatic datasets (Bio1 -Bio19, Hijmans et al., 2005, see Wetness Index (TWIS); and Topsoil pH (TpH) as the initial suite of variables (see Table 2.1 for details of datasets). Climate is a primary determinant of broad-scale species distributions and is often used as the foundation of Ecological Niche Modelling (ENM) approaches (e.g. Rodder et al., 2009;[START_REF] Mainali | Projecting future expansion of invasive species: comparing and improving methodologies for species distribution modeling[END_REF]Qiao et al., 2015). Further, Bomford et al. (2010) found climatic similarity between native and introduced ranges was a significant predictor of establishment success in freshwater fishes. Hijmans et al. (2005) developed a suite of "bioclimatic" variables, describing annual trends in temperature and precipitation, specifically for such uses. Incoming solar radiation is known to effect temperature in aquatic environments [START_REF] Rudi | Using geomorphological variables to predict the spatial distribution of plant species in agricultural drainage networks[END_REF] and 'static variables', such as TWI typically improve niche modelling predictions, particularly when making predictions outside the training areas [START_REF] Stanton | Combining static and dynamic variables in species distribution models under climate change[END_REF][START_REF] Title | ENVIREM: An expanded set of bioclimatic and topographic variables increases flexibility and improves performance of ecological niche modeling[END_REF]. Water salinity is a known limiting factor for P.

So as to promote parsimony and minimize over-fitting in models, I refined the list of variables used in the final modelling procedure, using Variance Inflation Factor to identify and remove collinearity in the final suite of variables used. Stepwise selection of variables was carried out using the r package 'VIF' (Lin et al., 2012) and left a refined suite of ten abiotic variables (see variables marked with asterisk in Table 2.1) to use in the models. All datasets were used at a spatial resolution of 10 km and a spatial reference of GCS WGS 1984. Conforming to accepted protocols in niche modelling (Guisan & Zimmerman, 2000), all of the remaining ten variables were ecologically relevant to P. parva.

Additional data

In order to produce a risk map accurately representing both the likelihood of introduction and climatic suitability, I incorporated a dataset representing the known major introduction pathways. Given that the spread of P. parva throughout Europe was strongly related to aquaculture-related introductions (Gozlan et al., 2010b), and because there is a strong correlation between non-native introductions and international trade [START_REF] Bradley | Global change, global trade, and the next wave of plant invasions[END_REF], the "Aquaculture Pressure" dataset from the "Global Threats to Human Water Security and River Biodiversity" database (Vörösmarty et al., 2010) was used as a surrogate for introduction likelihood. This dataset was derived using national annual average aquaculture harvests from inland and diadromous fishes from 1997-2006 from the UN Food and Agriculture Organization's FishStat Plus database (www.fao.org/fishery), which were then distributed proportionately to the grid-cell specific discharge for each nation. The original dataset was re-sampled using bilinear interpolation to match the resolution of all other datasets (10 km). All spatial datasets shared the same spatial reference (GCS

WGS 1984).

Modelling methods

There exists a great variety of modelling techniques available and recommended for the purpose of ENM, making it difficult to decide which to use [START_REF] Thuiller | BIOMOD-a platform for ensemble forecasting of species distributions[END_REF]. Spatial predictions of suitability may vary substantially between these modelling techniques, even amongst models with uniformly high evaluation scores and particularly for predictions in regions outside those containing training data [START_REF] Pearson | Model-based uncertainty in species range prediction[END_REF]. One way of accounting for the idiosyncrasies of individual modelling techniques is to use a multiple techniques and then produce ensemble forecasts, combining predictions from individual models, using consensus or averaging methods [START_REF] Thuiller | BIOMOD-a platform for ensemble forecasting of species distributions[END_REF].

The first phase of modelling used nine modelling techniques: Artificial Neural Networks (ANN), Classification Tree Analysis (CTA), Flexible Discriminant Analysis (FDA, [START_REF] Hastie | Flexible discriminant analysis by optimal scoring[END_REF], Generalized Additive Models (GAM), Generalized Boosted Models (GBM), Generalized Linear Models (GLM), Multivariate Adaptive Regression Splines (MARS), Random Forests (RF) and Surface Range Envelope (SRE, [START_REF] Busby | Nature Conservation: Cost Effective Biological Surveys and Data Analysis. BIOCLIM -A Bioclimatic Analysis and Prediction System[END_REF]. The use of GLM, GAM, CTA and ANN is described and discussed in [START_REF] Thuiller | Generalized models vs. classification tree analysis: predicting spatial distributions of plant species at different scales[END_REF]. GBM incorporates interactive relationships between predictor variables, as well as being able to discern complex response curves (Wisz et al., 2008). MARS and ANN are good at deciphering complex relationships (Moisen & Frescino, 2002) and RF models have been found to perform consistently well when evaluated against various other well established methods (Prasad et al., 2006). The models were implemented in the 'R' software [START_REF] Ihaka | R: a language for data analysis and graphics[END_REF], using the 'biomod2' R package [START_REF] Thuiller | biomod2: Ensemble platform for species distribution modeling[END_REF].

Modelling parameters were kept at default values, in the interest of repeatability.

Model predictions were output as probability of presence (continuous values between 0 and 1).

Evaluation of model accuracy (predictive power) was carried out using the Area Under Curve (AUC) of the Receiver Operating Characteristic (ROC) plot [START_REF] Fielding | A review of methods for the assessment of prediction errors in conservation presence/absence models[END_REF][START_REF] Pearce | Evaluating the predictive performance of habitat models developed using logistic regression[END_REF]. A random 70% selection of the data was used to train each model before the remaining 30% of data were used to evaluate accuracy.

The final ensemble forecast output was produced in 'biomod2' using a weighted average technique, based on the AUC values of each model. Models with higher AUC values were more influential in deciding final cell values. Evaluation of final ensemble model accuracy was conducted using the same cross-validation approach as that used for individual models. Variable importance for each individual model was assessed through the use of biomod2's randomization function, where each variable's values are randomized in turn. Pearson's correlations of model predictions before and after randomization were used to infer relative importance of the variable being randomized [START_REF] Thuiller | BIOMOD-a platform for ensemble forecasting of species distributions[END_REF]. Where the correlation is high, the relative importance of the variable to the prediction outcome is low. Final Ensemble Model (EMmw) variable importance was calculated by applying the weightings used for the ensemble forecasting to the variable importance scores for individual models, summing values by variable then dividing by the number of models used.

Creation of the P. parva risk map

The final risk map was created by multiplying the values of the EMmw layer, which represents suitability of conditions according to modelled realized niche, by those of the "Aquaculture pressure" layer, which represents likelihood of introduction. Both layers' values range from 0-1, so the final Risk Map represents equal weightings of the two factors in assessment of the risk of successful invasion, with potential values ranging from 0 (no risk) to 1 (high risk).

RESULTS

Predictive accuracy in models

Predictive accuracy across the nine individual modelling methods was consistently high and the mean of each modelling method always exceeded 0.847 in crossvalidation runs, except for SRE (Fig. 2.1). SRE models were dropped from the final EM, as they were consistently lower scoring than the others. AUC scores indicated that all of the retained individual models were acceptable [START_REF] Hosmer | Applied Logistic Regression[END_REF] for predicting the climatic suitability of locations for P. parva. However, the weighted method of ensemble forecasting provided a significant increase in model accuracy (Welch's t-test p ≤ 0.001, testing results of all retained individual models against ensemble model). Variability of predictive accuracy across the different absence datasets and the randomized subsets of data (i.e. partitioning for training and testing) was very similar across all modelling methods, indicating that the data for training were representative of the total dataset in each instance (i.e. for each absence dataset and for each randomization run, of which there were 100 in total for each modelling method; 900 in total).

Predictor variable importance

The three most important predictor variables, Bio14, Bio3 and TWIS (in that order) accounted for 66% of the sum of all importance values (Table 2.2). The density plot curve for Bio14, precipitation of driest month, (Fig. 2.2) suggested that P. parva prefer areas with values between 25 and 60 mm. The density plot for Bio3, isothermality, suggested that P. parva prefers areas with a mean monthly temperature range that is approximately 33% of the total annual range. Topographic wetness index was the most important of the non-'bioclim' variables used in the final modelling, ranking 3rd with a mean score of 11.64% in the randomization tests.

Suitability predictions

Current known ranges, both non-native and native, were generally well represented by the model output (Fig. 2.3a). However there were some areas within the native range where the suitability score was lower than expected, such as Taiwan. In addition to the known current range, a number of other areas displayed medium to high suitability values. These areas included parts of central and southern Africa, Malaysia, Indonesia, New Guinea, New Zealand, Australia, North and South America. The highest values outside the current known range were in the USA (mainly in the eastern and southern states), South America (focused around southern Brazil, Eastern

Paraguay and Argentina, and Uruguay), in Eastern Australia and in New Zealand.

Risk predictions

The process of incorporating introduction likelihood to the map refined the suitability map by producing a prediction of relative likelihood of a successful invasion (Fig.

2.3b

). Areas with predicted high suitability and high levels of aquaculture pressure, a surrogate for introduction likelihood, were fairly widespread. The most significant areas of high and medium-high risk values included the USA, Brazil, Paraguay, Uruguay, Argentina, coastal South Africa, Malaysia, Indonesia and Australia.

Although New Zealand possessed areas with high suitability values, the low values of aquaculture pressure meant that the risk levels were very low throughout the country.

Similarly, the Island of New Guinea possessed medium-high suitability values, but aquaculture values in the country of Papua New Guinea meant that the eastern part of the island received generally low risk values, whilst the western part of the island, a region of Indonesia with higher levels of aquaculture, received a medium-high risk of successful invasion.

DISCUSSION

The risk maps indicated that P. parva has a substantial scope to invade areas and regions beyond its current range, and these cover all continents. There were several hotspots for climatic suitability and risk of introduction in each continent that could represent a potential future point of entrance. In North America for example, the key states at risk are those of lower Mississippi River and the eastern Atlantic south of New York. In South America, two geographically separated hotspots were identified, which included medium-high values in upper parts of the Amazon basin in Peru and Columbia and Brazil. Further south, high values were observed in the south of Brazil, Uruguay and north-east parts of Argentina. In Africa there were narrow fringes of medium-high risk in coastal areas of South Africa, Madagascar and in Liberia.

However, these were surrounded by more diffuse areas of medium risk, generally following the coast but also present in central countries such as the Democratic Republic of the Congo and Uganda, where the upper part of the Congo River is most at risk. Although already present in East Asia, the outputs indicated that areas in northern India would be suitable for P. parva, particularly the foot hills of the Himalayas, including the Indus River basin as far west as Srinagar in the Jammu and Kashmir state of India. Of particular note is the recent discovery of P. parva in the Brahmaputra River in India (Makhrov et al., 2013), an area predicted to be climatically suitable and at medium-high risk by the analysis, thus providing confidence in the performance of the risk maps, particularly given that the training dataset contained only one presence cell in the entirety of India. Finally, in Oceania, predicted areas at medium-high to high risk were in Indonesia, Malaysia and parts of eastern Australia.

The suite of predictor variables used to estimate variation in environmental suitability mainly consisted of descriptions of global variation in temperature and precipitation, but did not include the potentially limiting factor of salinity. Scott et al. (2007) found in an experimental study that P. parva showed a lack of ability to hypo-osmoregulate and is consequently limited in its potential to disperse into river systems through brackish water conditions (i.e. > 13.7% salinity). Freshwaters are becoming increasingly impacted by salinization, particularly in urban and arid areas [START_REF] Carpenter | State of the world's freshwater ecosystems: physical, chemical, and biological changes[END_REF]. [START_REF] Wicke | The global technical and economic potential of bioenergy from salt-affected soils[END_REF] produced at global map of 'salt-affected areas" based on soil attributes. Their map is a categorical dataset, classified relative to plant growth and agronomics [START_REF] Nachtergaele | Harmonized world soil database (version 1.1)[END_REF] and does not include values for areas covering the majority of the P. parva occurrence dataset, or indeed much of the area predicted to have high environmental suitability by the ENM.

Comparing the map of [START_REF] Wicke | The global technical and economic potential of bioenergy from salt-affected soils[END_REF] to a map identifying a number of salinization-affected freshwater wetlands [START_REF] Herbert | A global perspective on wetland salinization: ecological consequences of a growing threat to freshwater wetlands[END_REF], these 'salt-affected areas' (i.e. soil) appear to provide a poor surrogate for freshwater salinity, which may be a result of not accounting for important sources of salt inputs, such as point pollution from industry, saltwater intrusion and road salting [START_REF] Kaushal | Increased salinization of fresh water in the northeastern United States[END_REF].

Combined, the issues concerning the data type, format, coverage and apparent lack of applicability to freshwater salinization mean that it is unlikely that inclusion of the 'salt-affected area' data in the present analysis would have rendered improved predictions. However, future modelling studies on P. parva should reassess the situation with regard to availability of data and account for salinity in their models where feasible.

The modelling in the present study was carried out at a resolution of 10 km. There is typically a trade-off between detail and extent when it comes to selecting data for such analyses (Hirzel & Le Lay 2008), with more detailed (i.e. higher resolution) datasets much larger (in terms of physical memory required) than a less detailed dataset for the same extent. Whilst broad-scale patterns are often accurately depicted in outputs of ENMs based on coarse resolution data [START_REF] Peterson | Prediction of bird community composition based on point-occurrence data and inferential algorithms: a valuable tool in biodiversity assessments[END_REF][START_REF] Raxworthy | Predicting distributions of known and unknown reptile species in Madagascar[END_REF], finer-scale patterns resulting from local processes are not accounted for.

In the case of freshwater fishes, physical factors such as hydrological characteristics, water quality (including known limiting factors such as water salinity, in the case of P. parva -Scott et al., 2007), microclimate effects, as well as composition of recipient biological communities are all likely determinants of establishment success that act at local scales [START_REF] Jackson | What controls who is where in freshwater fish communities the roles of biotic, abiotic, and spatial factors[END_REF]Mackey & Lindenmayer 2001;[START_REF] Soberón | Niches and distributional areas: concepts, methods, and assumptions[END_REF][START_REF] Mims | Life history theory predicts fish assemblage response to hydrologic regimes[END_REF]. In order to incorporate such factors, some advocate a multi-tiered approach (Hopkins, 2009;Lauzeral et al., 2011) , where broad-scale determinants are first modelled at coarse resolutions, then determinants that act at more local scales are modelled at higher resolution for specific areas (i.e.

areas of high predicted suitability). In application, whilst high resolution data pertaining to hydrological properties of individual rivers may not be available in consistent formats, or may be too computationally intensive to analyze at a global extent, such data often exists within national databases (e.g. in the US, [START_REF] Falcone | GAGES: A stream gage database for evaluating natural and altered flow conditions in the conterminous United States[END_REF] and could feasibly be used in analyses over smaller extents (e.g. countries, or regions). However, often complimentary suitable fine-scale presence location data is often lacking for many freshwater fish, as well as for other taxa more generally (Pyšek et la., 2008;Leprieur et al., 2009b). Increased availability of more detailed distribution (and preferably abundance) data for both native and introduced species would undoubtedly provide considerable opportunities for developing more refined local-scale models.

The lack of records of failed introductions also hampers efforts to investigate such local-scale determinants of establishment likelihood and more resources should be allocated to focusing research on such instances, particularly where they have occurred in regions where climatic suitability is high, e.g. an instance in Belgium (Copp et al., 2007). More information on the physical and hydrological conditions, local assemblages present in such locations, as well as characteristics of the introduction (i.e. number of individuals and source) could render important insights that could help identify reasons for failure to establish and help inform management of further introductions. In this vein, predictive models based on course-scale determinants, such as the climatic suitability output of the present study, could provide a useful basis for the identification of important areas for further study of such local scale processes.

Evaluation of the model via cross-validation techniques indicated that the modelling methods used to predict climatic suitability achieved a high degree of accuracy.

However, there were areas where high suitability values were expected, due to known presence of P. parva, but were not predicted by the model. For instance, there were locations in Belarus and southern Turkey that received low suitability scores, but contained P. parva recordsalthough they are only present in relatively low abundances (Ekmekçi & Kirankaya, 2006;[START_REF] Semenchenko | Nonnative fish in Belarusian and Polish areas of the European central invasion corridor[END_REF]. Equally, in the native range, Taiwan received generally low suitability scores, despite being known to host widespread P. parva populations (Gozlan et al., 2010b). This disparity between predicted and observed suitability in Taiwan may have been due to underrepresentation of distinct environmental conditions present there, owing to low numbers of presence points in this location. Such an issue can lead to underrepresentation of these conditions in the modelled niche. Because niche conservatism is a fundamental assumption of the ecological modelling techniques, the projected niche may not have included the full range of suitable conditions present in Taiwan.

In time, introduced species will adapt and evolve within their new range, potentially shifting their climatic preferences and their ecological niche. Simon et al. (2011Simon et al. ( , 2015) ) revealed P. parva populations of Japan and Taiwan have been isolated from those found in continental China for some time (5-6 MYA and 1-1.5 MYA, respectively), allowing the adaptation of the Taiwanese populations to the more tropical climate there. Neither the Japanese, nor the Taiwanese populations have been introduced to Europe, which could explain why the climatic niche of the introduced range may have not yet included the distinct part of the niche occupied only by the Taiwanese populations, which would compound the underrepresentation of this part of the niche in the data used to train the models. One implication of this is that the risk of invasion may be heavily dependent upon the source population for the introduction, particularly when the population inhabits environmental conditions distinct compared to that of other populations. In the case of the Taiwanese populations, this may mean that scope for successful invasion into areas with more tropical climates is greater, and for the species as a whole, this may mean that current risk predictions are underestimated for tropical regions, however this would be reliant upon the assumption that the Taiwanese populations were better adapted for more tropical climatic conditions than already introduced populations, which has yet to be tested.

Despite some of the establishment predictions in India already being realized, the risk level of individual countries will also be determined by extant policies on managing non-native species regarding aspects such as import prohibition, customs screening and surveillance. Assessing introduction and establishment risks of invasive species at a large scale (i.e. regional, continental, or global) can be particularly relevant where suitable areas potentially straddle borders (Hulme et al., 2008), as control measures taken in one area can be rendered ineffective by lack of controls in a neighboring area across a national border (Hulme et al., 2008;[START_REF] Dahlstrom | A review of international, regional and national biosecurity risk assessment frameworks[END_REF]. In order to effectively manage invasive species in continental areas made up of numerous countries with shared land borders, it is important that countries with shared borders collaborate and communicate effectively [START_REF] Caffrey | Tackling invasive alien species in Europe: the top 20 issues[END_REF], as effective biosecurity considerations should start pre-border (Hulme et al., 2008;[START_REF] Caffrey | Tackling invasive alien species in Europe: the top 20 issues[END_REF]. The risk map from the present study could be used as a shared resource, to highlight relevant areas where cross-border collaboration could help develop consistent application of policies, such as import controls (e.g. shared blacklists).

Failing to prevent future invasions of P. parva would greatly increase the associated threat to fish biodiversity (Gozlan et al., 2005;Britton et al., 2010). The combination of P. parva's its ability to form dense populations rapidly, across a broad range of environmental conditions, and its status as a healthy carrier (i.e. infection does not cause mortalities) of S. destruens, means that it constitutes a significant transmission threat (Gozan et al., 2005;Gozlan et al., 2010b). Dense populations of healthy hosts can act as persistent disease reservoirs for an outbreak. A recent study has highlighted the particular significance of the threat posed by such species [START_REF] Al-Shorbaji | The consequences of reservoir host eradication on disease epidemiology in animal communities[END_REF]. Given the long distance nature of natural salmon migrations, the potential impact of an outbreak of S. destruens among North American salmonid populations, which are highly sensitive to the disease [START_REF] Arkush | Pathology associated with the rosette agent, a systemic protist infecting salmonid fishes[END_REF]Arkush et al., 2003), could have far reaching consequences. P. parva's invasion could thus be the potential stepping stone to a much wider transmission of the disease. We now know that the rate of spread after initial introduction in Europe was approximately 0.5 countries per year (Gozlan et al., 2010b) and that invasion in a region comprising largely suitable areas is likely to result in an array of negative impacts. Given this, there exist compelling reasons to prevent further introductions of this species outside its current distribution. Whilst simple climate-matching approaches can be useful, more representative models such as ours, accounting for critical factors such as introduction likelihood/ propagule pressure (cf. Colautti et al., 2006;Britton & Gozlan, 2013) should be used in order to provide more accurate risk representations. Consequently, the outputs in this paper can be used to formulate stronger preventative measures and ensure the invasion of P. parva is limited to its current extent and does not develop more globally.

INTRODUCTION

Understanding how species and ecosystems react to climatic shifts is critical to protecting biodiversity and has become a major focus of ecology and conservation biology [START_REF] Mcmahon | Improving assessment and modelling of climate change impacts on global terrestrial biodiversity[END_REF]. Rapid, sustained climate change is expected to impose strong selective pressures on life history traits likely to be important for fitness (Gienapp et al., 2008). For those organisms unable to track their preferred environment in space or time (through an inability to migrate or other constraints) the only option is to adapt to the changing environmental regime in situ.

The main source of adaptation on earth is evolution through natural selection [START_REF] Darwin | On the Origin of Species by Means of Natural Selection (J. Murray, London). JD Stilwell/Palaeogeography[END_REF]. Although evolution is a generally considered to occur over considerable periods of time, rapid evolutionary adjustments have been documented to occur over relatively short time-spans, e.g. <20 generations [START_REF] Kremer | Microevolution of European temperate oaks in response to environmental changes[END_REF], with some studies purporting to have identified these 'micro-evolutionary' adaptations in response to the currently observed directional climate change (Bradshaw & Holzapfel, 2001;[START_REF] Réale | Genetic and plastic responses of a northern mammal to climate change[END_REF]. Nonetheless, despite the seeming ubiquity of microevolutionary adaptations in the literature (see [START_REF] Hendry | An introduction to microevolution: rate, pattern, process[END_REF], there is still very little empirical genetic evidence to test the view that the observed adaptations are directly attributable to changes in gene frequencies, as opposed to being a product of phenotypic plasticity (Gienapp et al., 2008).

Phenotypic Plasticity (PP) is commonly implicated in the success of species in novel climatic conditions [START_REF] Przybylo | Climatic effects on breeding and morphology: evidence for phenotypic plasticity[END_REF]. Some assert that PP acts to shield populations from the selective pressures of environmental change, essentially acting as a buffer to evolutionary adaptations [START_REF] Futuyma | Evolution 2ed[END_REF]. However, others assert that PP can play a role in driving diversification and speciation [START_REF] Pfennig | Phenotypic plasticity's impacts on diversification and speciation[END_REF].

Indeed, it is increasingly accepted that various forms of phenotypic plasticity can help facilitate evolutionary adaptation in populations subjected to novel environments (Chevin et al., 2010). For example, Lepomis gibbosus has been observed to display morphological plasticity in response to habitat type (Yavno & Fox 2014), presence of competitors (Yavno et al., 2014) and water flow velocities (Yavno & Fox 2013).

Nonetheless, PP has limitations, which in the long-term can only be overcome by evolutionary, genetic adaptations (De Witt, 1998;[START_REF] Pigliucci | Evolution of phenotypic plasticity: where are we going now?[END_REF][START_REF] Auld | Re-evaluating the costs and limits of adaptive phenotypic plasticity[END_REF].

Understanding the dynamics and limits of evolutionary adaptation and adaptation by The use of Ecological Niche Models (ENM) is well established as a means of predicting distributions of species and they are regularly used to predict biological invasions (e.g. Soberón et al., 2001;Peterson, 2003;[START_REF] Thuiller | Niche-based modelling as a tool for predicting the risk of alien plant invasions at a global scale[END_REF]Ficetola et al., 2007;Soberón & Peterson, 2011), however these methods are underpinned by a number of assumptions which are not always validated. Two relevant critical distributions of introduced populations of disparate provenance (i.e. northern and southern lineages). Whilst patterns of genetic differentiation highlight important population processes, they also echo historical refuges and re-colonization events [START_REF] Hewitt | Post-glacial re-colonization of European biota[END_REF][START_REF] Habel | The fourth paradigm pattern of post glacial range expansion of European terrestrial species: the phylogeography of the Marbled White butterfly (Satyrinae, Lepidoptera)[END_REF][START_REF] Michaux | Taxonomy, evolutionary history and biogeography of the broad-toothed field mouse (Apodemus mystacinus) in the eastern Mediterranean area based on mitochondrial and nuclear genes[END_REF][START_REF] Reimann | Palaeogeography and palaeoecology of Carabus auronites (co-leoptera): Characterisation and localisation of glacial refugia in southern France and reconstruction of postglacial expansion routes by means of allozyme polymorphisms[END_REF]. For the development of ENMs, it is important to fully understand whether the clines observed across a species' native range are the result of adaptation to environmental conditions [START_REF] Sezgin | Single-locus latitudinal clines and their relationship to temperate adaptation in metabolic genes and derived alleles in Drosophila melanogaster[END_REF][START_REF] Huestis | Interaction between maternal effects and temperature affects diapause occurrence in the cricket allonemobius socius[END_REF] or of neutral evolutionary processes [START_REF] Kontula | Molecular and morphological analysis of secondary contact zones of Cottus gobio in Fennoscandia: geographical discordance of character transitions[END_REF][START_REF] Sotka | Strong genetic clines and geographical variation in gene flow in the rocky intertidal barnacle Balanus glandula[END_REF][START_REF] Macholán | Genetic analysis of autosomal and X-linked markers across a mouse hybrid zone[END_REF].

In order to investigate the possible implications of the genetic population structure of a species on potential non-native distributions, I focussed on the geographical distribution of populations of the topmouth gudgeon Pseudorasbora parva, a small freshwater fish species widely introduced to Europe from Asia (Gozlan, 2012b). This species is a major vector of the rosette agent Sphaereothecum destruens, a growing fungal threat to freshwater biodiversity in Europe (Gozlan et al., 2005;[START_REF] Andreou | Temperature influence on production and longevity of Sphaerothecum destruens zoospores[END_REF][START_REF] Gozlan | Identification of a rosette-like agent as Sphaerothecum destruens, a multi-host fish pathogen[END_REF][START_REF] Andreou | Sphaerothecum destruens pathology in cyprinids[END_REF]Andreou et al., 2012;[START_REF] Rowley | Impacts of mesomycetozoean parasites on amphibian and freshwater fish populations[END_REF] and thus predictions of future establishment risks would be useful to conservationists. Its native distribution spans Japan, Korea and in China from the Amur to the Pearl river basin, covering a wide range of climatic conditions from harsh continental in the north to tropical in the south. As a species of no commercial value, its introduction in Europe was accidental, along with the intentional introductions of various Chinese carp species (Bănărescu, 1964). It has become one of the most rapidly invasive freshwater fish in the world and represents a major environmental risk for the spread of fungal S. destruens, similar to that observed in amphibians with chytrid fungus (Gozlan, 2012a).

Systematic sampling across both the native and introduced ranges has identified genetically distinct mitochondrial lineages across the native range of P. parva, of which two have been introduced into the European range (Simon et al., 2001;Simon et al., 2015;Hardouin et al., 2018)

MATERIALS & METHODS

Occurrence data

Data on the locations of 2557 populations across both the native and introduced ranges were collected through a combination of annual national stock-assessments, as per Gozlan et al. (2010a), supplemented with data from the Global Biodiversity Information Facility (GBIF, 2013). Any points that were obviously incorrect (e.g. at sea) were removed from the final dataset. Finally, data were spatially harmonised to the same spatial resolution as the predictor variable layers (i.e. 10 km), leaving cell centre coordinates for cells within which presence records occurred. A total of 1262 10 x 10 km cells contained records of P. parva presence.

Lineage data

An expedition was undertaken to sample genetic material for 30 populations spread across China, Taiwan and Japan, covering the major catchments of the Amur River, Yellow River, Yangtze River, Pearl River and the majority of smaller intermediary catchments. Similarly, genetic material for 27 populations spread across the introduced range, from Iran to England, was obtained (see Gozlan et al., 2010a). The relative proportions of lineages at each site (see Fig. 3.1) are based on mitochondrial DNA analysis conducted on the tissue sample collection (Simon et al., 2001;Simon et al., 2015;Hardouin et al., 2018). As only two of the native Asian mainland lineages were found to have been introduced to Europe these were the only two lineages used in these analyses and are referred to as "Northern Lineage" (NL) and "Southern Lineage" (SL). These names allude to the North/South distribution of the two lineages observed specifically in the native range on both sides of the Yangtze River (see Fig. wetness index (TWIS) as well as gross domestic product (GDP, [START_REF] Ghosh | Shedding light on the global distribution of economic activity[END_REF], in the initial predictive variables dataset (see Table 3.1). These variable datasets were chosen for their balance of biological relevance, coverage (coincident with occurrence records), and for a measure of likely sampling bias (i.e. GDP as a surrogate of human population density). I then used Variance Inflation Factor (VIF) analysis [START_REF] Graham | Confronting multicollinearity in ecological multiple regression[END_REF] to refine the final dataset, removing those variables displaying multi-

Ordination techniquesniche overlap, equivalency and similarity tests

Lineage data (i.e. NL or SL), along with range data (i.e. Native or Introduced) were used to define the groups: i) Native (Nat), ii) Introduced (Int), iii) Native Northern Lineage (NNL), iv) Native Southern Lineage (NSL), v) Introduced Northern Lineage (INL), vi) Introduced Southern Lineage (ISL). These groups, henceforth referred to as entities, were analysed in a pair-wise manner (see Table 3.2 for list of entity pairs and respective PCA calibration data). from PCA-Env), were calculated by applying a kernel density function, which corrected for availability of environmental conditions in the study regions (Broennimann et al., 2012;Early & Sax, 2014) and occurrence densities were weighted, where applicable (i.e. lineage data), according to relative abundance of the lineage in question (e.g. at a location where NNL makes up 90% of the sampled population, the NNL occurrence is weighted at 0.9). All three niche comparison metrics were calculated on the 'kernel smoothed' entities, so as to give results that were unbiased by the arbitrarily selected resolution of the gridded environment-space (see Warren et al., 2008). Equivalency and similarity were calculated using the same methodology and parameters as Broennimann et al. (2012), including the 100 randomization iterations, allowing the production of p-values for hypothesis testing.

All statistical analysis was conducted using R (R Core Team, 2014).

RESULTS

Niche comparisons of lineages in native range

Analysis of the 'smoothed' niche entities in the native range (Fig. 3.2) showed that the niches of the two lineages within the native range (NNL and NSL), exhibit relatively low overlap (Schoener's D = 0.196). 63.8% of the variation in environmental variables was captured in the first two principal components (PCs).

The first PC captured 42.5% and was also the axis on which the most marked separation of the lineages occurred. PC1 is dominated by three key variables; X5mean temperature of driest quarter, X1 -annual mean temperature, X6 -annual precipitation, in order of diminishing contribution. The NSL's niche is characterized by higher values of these three variables (i.e. hotter and wetter). Randomization tests of niche equivalency and similarity showed that the niches of the NNL and NSL are not equivalent (p < 0.05), but they are significantly more similar to one another than randomly selected locations from within the native range (p < 0.05). Randomization tests of niche equivalency showed that none of the niches were equivalent (p = 0.02 for all comparisons). Test of niche similarity showed that the niche of NNL was not significantly more similar to that of INL than random, nor INL to that of NNL, whilst NSL was not more significantly similar to ISL than random, but ISL was significantly more similar to NSL (p = 0.02) than random. Similarly, for the smoothed niche entities of the undifferentiated whole species data, native vs introduced similarity was not significantly greater than by random, however introduced vs native was significantly (p = 0.04) more similar than random. Looking at the PCA outputs, there appears to be a region in environment-space that is not occupied in the native range and falls between the two lineages' smoothed niche entities. This region corresponds, broadly, to the combined smoothed niche entities in the introduced range.

Niche comparisons of lineages in the introduced range

Analysis of the 'smoothed' niche entities in the introduced range (Fig. 3.4) showed that the niches of the two lineages within the introduced range (INL and ISL), exhibit relatively high overlap (Schoener's D = 0.645). 50.72% of the variation in environmental variables was captured in the first two principal components (PCs).

The first PC captured 30.08% and is dominated by five key variables; X5 -mean temperature of driest quarter, X2 -mean diurnal temperature range; X3isothermality; X8 -Topsoil pH, X1 -annual mean temperature, in order of diminishing contribution. Randomization tests of niche equivalency and similarity showed that the niches of the INL and ISL are not equivalent (p < 0.05), but they are significantly more similar to one another than to randomly selected locations from within the introduced range (p < 0.05 for both directional comparisons).

DISCUSSION

This study identified that the geographical distributions of the two lineages in the native range correspond to differentiated sets of climatic conditions, as evidenced by the lack of overlap in environment -space (D = 0.196). However, when introduced to Europe the two lineages occupy very similar environmental conditions to one another, as evidenced by the high degree of overlap in environment-space (D = 0.645). When considering the undifferentiated (whole species) data, the conditions occupied in the European range constitute a different region in environment-space to that of the native range (D = 0.09). However this region of climate-space is available in the native range, but not occupied, falling between the two key aggregations occupied by the native populations, on the first axis in environment-space (PC1).

The defining characteristics of the differing conditions occupied by the two lineages in the native range are warmer, wetter conditions for the southern lineage and cooler, drier conditions for the northern lineage, whereas both lineages occupy similar conditions, characterised by intermediate values for these two variables, in the European range. This implies that the cause of the separation of the lineages in the native range is not present in the introduced range. The cause could be the result of a number of possible factors and may relate to dispersal constraints, abiotic conditions not accounted for by the environmental variables in this analysis, biotic interactions, or a combination of all three (Soberón & Peterson 2005).

As alluded to in Chapter II, a potential limiting abiotic factor for P. parva is its lack of tolerance to brackish water conditions (Scott et al., 2007). This factor was not accounted for in the suite of variables used to characterise the environmental conditions in the PCA-based analysis. A number of regions in China, both in the north and the south, do suffer from freshwater salinization issues [START_REF] Liu | China's environment in a globalizing world[END_REF], however it is unclear whether the affected regions share a pattern of severity with the north-south geographical distributions of the two P. parva lineages. Furthermore, in the absence of evidence to suggest differential in the two lineages' tolerances to salinity it is reasonable to assume that both will be similarly affected by the issue, which suggests it is unlikely to be responsible for the geographical separation of the two lineages in the native range.

Competitive exclusion, based on resource availability and differences in growth efficiency has been observed between fish species, leading to differences in relative fitness along a thermal gradient [START_REF] Finstad | Competitive exclusion along climate gradients: energy efficiency influences the distribution of two salmonid fishes[END_REF]. However I am unaware of any documented observations of intraspecific (i.e. between genetic lineages) competitive exclusion occurring in fish, so there may be little to suggest that this mechanism is be responsible for the relative distributional patterns of the two P. parva lineages in the native range. Further, as a species P. parva displays plasticity of diet [START_REF] Adamek | Reproduction parameters in a natural population of topmouth gudgeon, Pseudorasbora parva, and its condition and food characteristics with respect to sex dissimilarities[END_REF]) and low micro-habitat electivity (Asaeda & Manatunge 2005), so it seems unlikely that differences in availability of these factors would impact one lineage over the other, although data on these aspects of P. parva's ecology in the native range is limited (Gozlan et al., 2010).

The Qinling Mountains, marking the north-south divide in China [START_REF] Yan | The divergence of two independent lineages of an endemic Chinese gecko, Gekko swinhonis, launched by the Qinling orogenic belt[END_REF][START_REF] Wu | The biogeography of fungal communities in wetland sediments along the Changjiang River and other sites in China[END_REF], correspond with the north-south geographical pattern of the two P.

parva lineages and could represent a significant and longstanding biogeographical barrier to dispersal between the northern and southern lineages. In fact the uprising of these mountains has been implicated in the divergence of lineages in other species, such as the Chinese gecko, Gekko swiinhonis [START_REF] Yan | The divergence of two independent lineages of an endemic Chinese gecko, Gekko swinhonis, launched by the Qinling orogenic belt[END_REF]. A similar north-south pattern of divergence between two genetic sub-lineages of another native Chinese small freshwater fish (Rhynchocypris oxycephalus) was identified by [START_REF] Yu | Geological events and Pliocene climate fluctuations explain the phylogeographical pattern of the cold water fish Rhynchocypris oxycephalus (Cypriniformes: Cyprinidae) in China[END_REF], which they attributed to historic geological processes dating around 1.98-3.99 MYA.

This offers compelling support for the theory that the northern and southern lineages of P. parva are a result of similar processes, centred on the uplift of the Qinling Mountains, as the divergence of the two lineages was dated by Hardouin et al. (2018) at around 2.5 MYA. The functional absence of such large-scale biogeographical barriers in the introduced range, through multiple human mediated long-distance introductions (Gozlan et al., 2010), could also explain the overlap in occupied environmental conditions seen in the introduced range.

Whilst the separation of the two lineages in the native range is not exhibited in the European range, there is little overlap of the occupied conditions of the native and European ranges in environment-space (D = 0.09). Although analogous conditions to those occupied in the European range are present in the native range (i.e. they fall within the boundaries of the background data for the native range in panel 3a of Fig. 3.3), they are not occupied, suggesting that locations corresponding to these conditions are unsuitable for another reason.

A potentially important abiotic factor not accounted for in the present analysis could explain the absence of native populations from the native region of environmentspace concordant with that occupied by European populations. As already mentioned, salinity has been shown to be one aspect of water conditions to which P. parva does not possess a particularly wide tolerance, with salinities of > 13.7 % shown to be unsuitable (Scott et al., 2007). Soil salinization in China is an issue for some provinces in the east of the country, principally Hebei, Shandong and Jiangsu, (Vörösmarty et al., 2010). This coincides with the North China plain, an area of intense agricultural activity, where overexploitation of water resources is understood to be primarily responsible for the increases in the salinization of freshwaters (Changming et al., 2001). This area corresponds with the central part of the latitudinal range of P. parva in China and so could correspond to the conditions of the uninhabited region in environment-space in the native range.

Whilst biotic factors have traditionally been considered to have minimal effect of the distributional patterns of species at broad scales [START_REF] Whittaker | Communities and ecosystems. Communities and ecosystems[END_REF]Pearson & Dawson 2003), it is increasingly recognised that these factors may play a larger part than previously thought [START_REF] Wisz | The role of biotic interactions in shaping distributions and realised assemblages of species: implications for species distribution modelling[END_REF]. Beyer et al. (2008) found no significant indication that predation controlled P. parva numbers in a lotic context in the UK, however the virtual absence of any obligate predators in the study area may have been a factor. Nonetheless, biotic interactions have been observed to have an impact on P.

parva abundance in both Europe (Belgium -Verhelst et al., 2015) and in parts of Asia (Japan -Yuma et al. 1997), with both studies showing that other fish species can exert a significant effect on populations. The presence of an effective competitor or predator of P. parva in the native range could explain its absence from areas with certain environmental conditions, particularly if the distribution of the predator coincides specifically with these areas, although Verhelst et al. (2015) suggest that P.

parva's lack of strong habitat electivity may allow it to avoid such predation effects by moving into habitats unsuitable for the predator species. Notwithstanding this, the addition of data relating to such biotic interactions has been shown to improve the accuracy of species distribution predictions in a number of cases (Guisan & Thuiller 2005;Heikkenen et al., 2007;Gonza´lez-Salazar et al., 2013) and the presence of competitors or predators has been shown to exclude species from areas with otherwise suitable environmental conditions (e.g. [START_REF] Hayes | Experimental reduction of wolves in the Yukon: ungulate responses and management implications[END_REF][START_REF] Finstad | Competitive exclusion along climate gradients: energy efficiency influences the distribution of two salmonid fishes[END_REF].

Subsequent analyses should focus on the isolation of this region of environment-space and the geographical projection of the conditions therein, as this could help to elucidate whether it comprises a contiguous geographical region, or a collection of disjunct locations possessing similar attributes. Whilst the latter is possible, according to Tobler's first law of geography distance between locations is a general surrogate for dissimilarity (Tobler 1970) and so would suggest the former is more likely. If this is the case, then identification of the geographical region in question would allow further investigation of the cause of its lack of occupancy by P. parva, including the potential explanations mentioned above.

The results of the present study highlight that observed spatially and climatically distinct lineages are not necessarily a product of climate-based selection but could instead be the product of physical segregation, which through neutral genetic drift has led to allopatric divergence. In such instances, it could be the case that current climatic conditions under-represent the potential of the species in question, which could in fact include in its fundamental niche conditions to which it has not been subjected for many generations. Here, the fact that both lineages, when introduced to the European range, occupied essentially the same climatic niche space shows that despite being separated 2.5 MYA (see Hardouin et al., 2018) and subsequently occupying relatively distinct conditions in the native range, both lineages now apparently possess the same capability to survive in conditions outside those found in their native distributions. It could be that the invasive potential of P. parva, as a species, is a result of retained ancestral physiological tolerances to conditions that were inhabited prior to the separation of the two key lineages 2.5 MYA (see Hardouin et al., 2018).

Whilst long-term retention of physiological tolerances could be a characteristic of some species, others exhibit contrasting qualities, where adaptation to local or regional conditions is firmly conserved, to the detriment of broader tolerances. For instance, a number of experimental studies have investigated intraspecific adaptations of animal and plant populations to localised climatic conditions (see Hereford, 2009) and some reciprocal transplant studies have observed marked local adaptations to climatic conditions in studied tree species [START_REF] Rehfeldt | Genetic responses to climate in Pinus contorta: niche breadth, climate change, and reforestation[END_REF]Savolainen et al., 2007). For instance, Savolainen et al. (2007) observed that survival in northern populations of Pinus sylvestris transplanted > 4° northwards dropped to zero, even though populations of local provenance persisted in the same conditions. These kinds of adaptation are not restricted to plants, and have also been observed in pacific pink salmon, Oncorhynchus gorbuscha [START_REF] Gordeeva | Genetic differentiation of Pacific pink salmon during colonization of a new area[END_REF][START_REF] Gordeeva | Experimental microevolution: transplantation of pink salmon into the European North[END_REF], as described in the introduction of this chapter, and are implied in other fishes by the presence of interspecific variation in thermal tolerances in common killifish, Fundulus heteroclitus [START_REF] Fangue | Intraspecific variation in thermal tolerance and heat shock protein gene expression in common killifish, Fundulus heteroclitus[END_REF] and also in brook trout, Salvelinus fontinalis [START_REF] Stitt | Intraspecific variation in thermal tolerance and acclimation capacity in presence of native fish species: evidence based on species distribution modelling[END_REF]. For such species displaying intraspecific differences in conserved climatic niche, using ENMs trained on data for the whole species, rather than smaller demographic units, might give a distorted picture in the context of invasion potential, as this potential would depend strongly upon the source of the introduction. Similarly, modelling such an organism at a whole-species level in order to predict its fate under climate change could considerably under-predict resultant mortality.

For species displaying population-level, discrete and conserved climatic niches, the relevant biological units upon which to base predictive ENMs are the populations. It is thus important to be able to differentiate between those species with populations exhibiting discrete, tightly conserved narrow climatic niches and those species that display high phenotypic plasticity and broad climate niche flexibility, as it dictates the relevant demographic scale at which to base predictive ENMs (i.e. the relevant demographic/biological unit) and also because high phenotypic plasticity is thought to be a likely indicator of invasiveness (Gozlan et al., 2010b). A major step in support of this differentiation is to be able to test whether the observed genetic differentiation at a population level is a product of climatic selective forces, or merely a by-product of allopatric genetic drift. Developments in quantitative genetics and molecular biology have made it possible but some of these techniques [START_REF] Whitehead | Neutral and adaptive variation in gene expression[END_REF] can be restrictive in terms of the data required and are often not robust to demographic factors [START_REF] Nielsen | Molecular signatures of natural selection[END_REF]. Nonetheless, there are still challenges beyond that of identifying the appropriate biological units for niche modelling, as this study has highlighted, (i.e. barriers to dispersal, potential biotic constraints.

A number of confounding factors may prevent us from obtaining accurate predictions of species current potential climatic niche through traditional correlative ENM techniques. In particular: i) The existence of physical barriers to dispersal which are unaccounted for. ii) Biotic interactions preventing occupation of otherwise suitable locations iii) Tightly conserved localised adaptation to climatic conditions. In the context of physical barriers to dispersal, all too often the importance of the absence component of these modelling techniques is downplayed, with conventional wisdom suggesting that restricting pseudo-absence data to outside the species known climatic envelope is sufficient to obtain realistic predictions [START_REF] Austin | Current approaches to modelling the environmental niche of eucalypts: implication for management of forest biodiversity[END_REF][START_REF] Le Maitre | Developing an approach to defining the potential distributions of invasive plant species: a case study of Hakea species in South Africa[END_REF]. However, for those species whose distributions are severely restricted by their local geography (for instance, those occupying islands) we could never hope to obtain a realistic representation of the true potential of the species to survive in novel climates using these methods. The inclusion of mechanistic models based on physiological attributes is showing significant promise in increasing the realism and accuracy of methods providing predictions relating to species' potential to persist in novel climatic conditions [START_REF] Guisan | Making better biogeographical predictions of species' distributions[END_REF][START_REF] Morin | Comparing niche and process based models to reduce prediction uncertainty in species range shifts under climate change[END_REF], as is the inclusion of data relating to biotic interactions. These developments may provide a significant contribution to addressing the three above confounding factors, as well as complementing studies of niche evolution (including REA). However, these developments bring with them the need for more data and better understanding of physiological requirements, limitations and biotic interactions. Verhelst, P., Boets, P., Van Thuyne, G., Verreycken, H., Goethals, P. L., & Mouton, A.

M. ( 2016). The distribution of an invasive fish species is highly affected by the

INTRODUCTION

Biological invasions are one of the most pervasive contemporary threats to global biodiversity (McGeoch et al., 2010). Increased global connectivity and trading has led to the systematic introduction of species into naïve systems and communities (Hulme, 2009;[START_REF] Levine | Forecasting biological invasions with increasing international trade[END_REF], often with profound implications (e.g. [START_REF] Edelist | Restructuring the Sea: profound shifts in the world's most invaded marine ecosystem[END_REF]Ricciardi & MacIsaac, 2000). Negative impacts can manifest through a variety of mechanisms (Pimental et al., 2005;Gozlan et al., 2010a), including competition and predation [START_REF] Gallardo | Global ecological impacts of invasive species in aquatic ecosystems[END_REF], hybridization [START_REF] Brennan | Hybridization due to changing species distributions: adding problems or solutions to conservation of biodiversity during global change?[END_REF], and novel pathogen transmission (Gozlan et al., 2005;[START_REF] Jussila | Invasive crayfish and their invasive diseases in Europe with the focus on the virulence evolution of the crayfish plague. Biological invasions in changing ecosystems: vectors, ecological impacts, management and predictions[END_REF]. The isolated nature and high endemism of freshwaters result in these ecosystems being at disproportionately high risk from invasions [START_REF] Dudgeon | Freshwater biodiversity: importance, threats, status and conservation challenges[END_REF]Moorhouse & Macdonald, 2015). Whilst invasions in freshwaters can develop from introductions of non-native fish from a range of pathways, the global aquaculture trade is recognised as the principal pathway [START_REF] Gozlan | Interference of non-native species with fisheries and aquaculture[END_REF].

The risk-based management of non-native fishes is reliant on understanding the ability of particular species to establish, impact and disperse in the novel environment, i.e. its invasiveness [START_REF] Britton | Towards the successful control of the invasive Pseudorasbora parva in the UK[END_REF][START_REF] Britton | Non-native fish in aquaculture and sport fishing in Brazil: economic benefits versus risks to fish diversity in the upper River Paraná Basin[END_REF].

Fundamental to such understandings are analyses of life-history traits (LHTs), as these tend to be strong predictors of establishment success [START_REF] Vila-Gispert | Life-history traits of invasive fish in small Mediterranean streams[END_REF][START_REF] Olden | Life-history strategies predict fish invasions and extirpations in the Colorado River Basin[END_REF], especially for species exhibiting high plasticity (Davies & Britton, 2015a). Thus, understanding how the LHTs of introduced fishes vary over time and space, and across a range of environmental conditions, enables us to predict the invasion risk [START_REF] Cucherousset | Life-history traits and potential invasiveness of introduced pumpkinseed Lepomis gibbosus populations in northwestern Europe[END_REF]Gozlan et al., 2010a). Some studies have compared LHTs of introduced non-native fishes within regions (e.g. Iberia; Garcia-Berthou, 2007), of species between their introduced and native ranges (e.g.

Micropterus salmoides, Britton et al., 2010c;Cyprinus carpio, Oyugi et al., 2010), and of non-native fishes across their introduced range (e.g. Gambusia holbrooki, [START_REF] Benejam | Life history and parasites of the invasive mosquitofish (Gambusia holbrooki) along a latitudinal gradient[END_REF]Lepomis gibbosus, Copp & Fox, 2007). However, most of these studies have been based on wild populations in field conditions, limiting precise inferences into the response of introduced founders from one region to another of similar or contrasting climate due to the lack of control and measurement of the environmental conditions.

Following an introduction, a pre-requisite for the founders to survive and then establish is the presence of suitable environmental conditions that enables LHT expression to facilitate establishment [START_REF] Moyle | Fish invasions in California: do abiotic factors determine success[END_REF]. For freshwater fishes, these conditions are strongly influenced by climate and, in lentic habitats, this will be primarily temperature related [START_REF] Crozier | Plastic and evolutionary responses to climate change in fish[END_REF]. General patterns are of faster growth and earlier maturity in more southerly latitudes where water temperatures tend to be higher [START_REF] Benejam | Life history and parasites of the invasive mosquitofish (Gambusia holbrooki) along a latitudinal gradient[END_REF][START_REF] Cucherousset | Life-history traits and potential invasiveness of introduced pumpkinseed Lepomis gibbosus populations in northwestern Europe[END_REF][START_REF] Oyugi | The introduced Micropterus salmoides in an equatorial lake: a paradoxical loser in an invasion meltdown scenario?[END_REF]. Whilst most studies on the invasive potential of non-native species consider the climate pattern in their native range and transpose them on to non-native regions (e.g. [START_REF] Gallardo | Priority setting for invasive species management: risk assessment of Ponto-Caspian invasive species into Great Britain[END_REF], there has been less focus on secondary introductions within the non-native range. Yet, for many invasive species, especially those that are moved within global trade (such as global aquaculture and ornamental trades), new introductions often involve species being moved within their non-native range (Gozlan et al., 2010b). These populations will already have adapted to new conditions following initial introductions, in an 'invasive bridgehead effect' (see [START_REF] Lombaert | Bridgehead effect in the worldwide invasion of the biocontrol harlequin ladybird[END_REF] with, for example, shifts in their reaction norms that can represent climatic niche shifts (e.g. Broennimann et al., 2007;[START_REF] Tingley | Realized niche shift during a global biological invasion[END_REF]Medley, 2010). Thus, where numerous populations of a non-native species have adapted to the local novel climatic conditions of their recipient systems (e.g. temperature regime; Kavanagh et al., 2010), these local adaptations would mean that provenance could strongly influence their invasive potential in any given new, secondary location of introduction/translocation (e.g. [START_REF] Rey | Where do adaptive shifts occur during invasion? A multidisciplinary approach to unravelling cold adaptation in a tropical ant species invading the Mediterranean area[END_REF].

The topmouth gudgeon Pseudorasbora parva is a highly invasive freshwater cyprinid fish in Europe, having gained a pan-continental distribution since the 1960s (Gozlan et al., 2010b;Gozlan, 2012). Its Asian native range spans much of China, parts of Russia, Japan, Taiwan and the Korean peninsula, with movement to and within Europe being strongly associated with the aquaculture trade (Gozlan et al., 2010a). It now inhabits a wide range of climatic conditions across its introduced range, from highly seasonal continental European climates (e.g. Poland) to warmer Mediterranean regions (e.g. Spain and Morocco) and temperate maritime climates (e.g. Great Britain). These climates generally represent novel conditions when compared to those occupied in the Asian native range, where the species encounter tropical climatic conditions in south China and harsh continental ones in the North (see Chapter III). If these non-native populations of P. parva have adapted for optimum performance in their respective climates, particularly during the warmer months that cover their reproductive period and growth season, it would be expected that there will be differences in their performance response across temperature gradients. For example, fish adapted to more continental climates (e.g. Poland) would be predicted to be poorly adapted to cooler temperate climates (e.g. Great Britain).

The aim of this study was to test, under common garden conditions, the plastic responses and adaptive potential of P. parva over a temperature gradient. The responses measured were the reproductive output (as fecundity) of adults and the growth performance of their progeny using a British population (temperate climate), and two populations from Poland (continental climate). It was predicted that reproductive outputs and early growth would be optimal at higher temperatures, with fish from continental climates showing low adaptation to temperatures that represent temperate climates (i.e. cooler breeding season).

MATERIALS & METHODS

Sources of adult fish

Adult fish were sourced from three geographical locations. Two populations were sampled from a continental climate in Poland: populations 'PL1' and 'PL2', and one population from a temperate climate in England, 'UK' -see Table 4.1 for details of locations and climatic descriptors. Sampling of the Polish populations was undertaken in June and the UK population in late summer of 2013, using a combination of electro-fishing and baited fish traps. Following their capture, adult P. parva (identified as fish > 35 mm) were transported from sampling locations in sealed large plastic bags, containing approximately 25 L of water, with 15 L of oxygen added, kept cool and transported to the aquarium facility. Thus, following initial conditions of 18 °C for 14 days when they were brought into the aquarium facility, their water temperature was continuously decreased at a rate of 0.5 °C every two days until reaching a consistent 5 °C. This temperature was then maintained for 100 days, before commencing the experimental procedures. These winter conditions were similar to those experienced by these populations in the wild.

A reduced photoperiod (10:14 light:dark) was also implemented during this period, as changing photoperiod has also been implicated in reproductive cycling in teleosts (e.g. [START_REF] Baggerman | Photoperiod and endogenous control of the annual reproductive cycle in teleost fishes[END_REF][START_REF] Fraile | Effects of photoperiod on spermatogenesis in Gambusia affinis holbrooki (Teleostei: Poecilidae) during the period of testicular quiescence[END_REF][START_REF] Shimizu | Effect of photoperiod and temperature on gonadal activity and plasma steroid levels in a reared strain of the mummichog (Fundulus heteroclitus) during different phases of its annual reproductive cycle[END_REF][START_REF] Singh | Effect of photoperiod and temperature on testicular regression in Channa punctatus[END_REF][START_REF] Vlaming | Environmental control of teleost reproductive cycles: a brief review[END_REF]. The photoperiod was however kept constant at this level so as not to confound the focal effects of temperature.

Experimental design

Each experimental system comprised a stack of three 90 L tanks in a recirculation configuration, with in-line filtration and heater/chillers, to control temperature. Each 90 L tank was separated into five equal compartments, using Corex™ plastic sheeting, which defined reproductive units of two females and one male. Each of the compartments were kept bare, but included an upturned 120 mm section of plastic guttering, to act as a refuge/nest (Fig. 4.2). The top two tanks in each system (i.e. ten compartments) were available for reproductive output assessments and the bottom tanks (i.e. five compartments) were used for growth assessments. In total, nine systems were prepared, one at each of the three temperatures (15 °C, 20 °C and 25 °C) for each of the three populations. Due to limitations in the number of either males or females, depending on population, it was not possible to have a full ten replicates for each population at each temperature treatment (see Table 4.2 for details of replicates). Fish were sexed, counted and placed into their experimental compartments at 5 °C, to match their holding conditions, and maintained for a minimum of 10 days to acclimate to the new surroundings. In order to trigger spawning behaviour, the photoperiod was increased, moving the lighting timer to a 12:12 light:dark cycle (after Katano & Maekawa 1997) and temperatures were increased by 1 °C per day, until the target temperatures were reached. Both photoperiod and temperature have been implicated in reproductive cycling in cyprinid fishes [START_REF] Wang | Photothermal control of the reproductive cycle in temperate fishes[END_REF]. Temperature increases were staggered so that target temperature attainment was synchronised for all systems and photoperiod was kept constant throughout monitoring period so as not to confound potentially the effects of temperature by having differing photoperiod-temperature gradients between treatments. Fish were fed ad libitum on a daily basis, on 'Skretting Nutra Plus Fry Feed', and any uneaten food removed.

Data collection

Laid eggs were counted daily, and recorded for each of the active experimental units, for a total duration of 125 days from target temperature attainment. This was completed using digital photography (Nikon AW100 16.0 MP) and manual counts of individual eggs from the captured images. These egg count data were used to quantify the reproductive output of the fish through 'clutch size' (number of eggs within a clutch), 'number of clutches' (the number of clutches produced) and 'total eggs laid' (i.e. total fecundity). A 'clutch' was defined as a temporally and spatially discrete set of laid eggs. Correspondingly, if two separated spatial clusters were laid in an area on the same day, they were interpreted as two clutches. Similarly, if eggs were laid over the top of existing eggs, but on another day, then this was also considered a separate clutch. At the end of the 125-day period, standard length was recorded for the female parent fish.

Monitored progeny were removed, in a single clutch, from parental compartments prior to hatchingfertilised eggs were identified by checking for developed eyes (black dots), then selectedand placed in net fish hatcheries (165 mm x 135 mm x 120 mm) in the bottom aquaria of the corresponding system, where they were maintained at the same temperature at which they were conceived. This procedure was carried out once for each population at each temperature treatment. Progeny, once hatched, were fed ad libitum on the same fry feed as the adult fish, but it was first crushed to a powdered form. As before, uneaten food was removed to prevent water quality issues.

Growth rates of progeny were recorded by measuring lengths of a sample of approximately 25 individuals from each population and each temperature treatment, using digital photography, with a Canon 600D DSLR camera attached to an AmScope© 7-45x dissecting stereo microscope. Individuals were placed in a petri dish containing a reference scale bar, with 0.1 mm increments in the frame, and Image-J software version 1.48 (Schneider et al., 2012) was used to measure lengths from captured images. Individuals were returned to their corresponding hatcheries immediately after photographs were captured. The start of the spawning season for P. parva, as observed by [START_REF] Yan | Variations in reproductive strategies between one invasive population and two native populations of Pseudorasbora parva[END_REF], typically coincided with water temperatures reaching 15 °C, around April/May and ceased approximately 10 weeks later, in July/August. In Britton et al. (2010a), it is noted that the growth season is typically from April to September, meaning a nominal growth period of approximately 10 weeks and a maximal growth period of approximately 20 weeks for young-of-the-year. An initial sample measurement was conducted on the day of hatching, and then subsequent weekly measurements were taken for a period of 10 weeks, with a final measurement at 20 weeks, each time taking a sample of approximately 25 individuals to measure. As the fish were very small and were measured live, it was not possible to tag or identify individuals within each subset.

Length measurements were made on the basis that analyses would be on mean growth of the sample, rather than a mean value of the growth rates of multiple individuals. and all p-values were adjusted using the Bonferroni method.

The growth of the progeny was assessed using two-way ANCOVAs that tested differences in growth responses between temperature treatments and populations. SL was used as the dependant variable, with Temperature and Population as factors, and

Age (in days) as the covariate. Post hoc tests were carried out, where slopes of individual regression lines were compared in pairwise fashion, within-population groups and within-temperature groups, by testing the significance of the interaction terms in an ANOVA framework (i.e. F-tests of model with interaction term vs. model without interaction term). These tests were performed on corresponding pairwise subsets of the data (e.g. PL1 15 °C and PL1 20 °C for between-temperature test, or PL1 15 °C and UK 15°C for between-population test) and all p-values were adjusted using Bonferroni corrections. All statistical tests were completed using R (R Core

Team, 2016).

influenced by the size of the female fish (ANCOVA: F 1, 49 = 5.07, p < 0.01), with females of 60 mm typically producing more than 1500 additional eggs compared to females of 50 mm (see Fig. 4.4). Within the populations, there were no significant differences in 'number of clutches' between temperature treatments (ANCOVA: F 2, 49 = 1.24, p = 0.30). There were, however, significant differences between populations (ANCOVA: F 2, 49 = 10.65, p < 0.001), linked to corresponding differences in SL (F 1, 49 = 11.33, p = 0.001), with a mean 0.64 ± 0.18 additional clutches laid per 1 mm increase in body length. Pairwise post hoc tests showed a significant difference in 'number of clutches' (depicted in Fig. 4.5) between PL1 and UK (ANOVA: F 1, 47 = 26.50, p < 0.001), a marginally significant difference between PL1 and PL2 (ANOVA: F 1, 26 = 3.09, p < 0.09), but no significant difference between PL2 and UK (ANOVA: F 1, 34 = 1.48, p < 0.232).

Temporal pattern in reproductive effort

There was a significant, negative relationship between proportion of total eggs laid and time (GLM: F 1,288 = 361.65, p < 0.001), with significant interactions between Period and Temperature (GLM: F 2, 279 = 32.66, p < 0.001), and Period and Population

Number of clutches

PL1 PL2 UK

There was a significant negative relationship between proportion of total clutches laid and time (GLM: F 1, 288 = 234.66, p < 0.001) and significant interactions between Period and Temperature (GLM: F 2, 281 = 28.92, p < 0.001) and Period and Population (GLM: F 2, 279 = 5.88, p = 0.003). There was also a significant three-way interaction between Period, Temperature and Population (GLM: F 3, 273 = 3.78, p = 0.011). For all populations, the proportion of total clutches laid declined over time, with the steepness of these declines increasing with temperature (Fig. 4.6). Post hoc tests showed that increases in the steepness of these declines were not significant between 15 °C and 20 °C for either UK or PL1 populations (GLM: F 1, 91 = 2.23, p = 0.14 and F 1, 66 = 2.60, p = 0.11, respectively), however they were significant between 15 °C and 25 °C (GLM: F 1, 91 = 51.26, p < 0.001 and F 1, 66 = 16.33, p < 0.001, for UK and PL1 respectively) and between 20 °C and 25 °C (GLM: F 1, 96 = 34.03, p < 0.001 and F 1, 66 = 7.01, p = 0.01, for UK and PL1 respectively) for these same populations. The increase in steepness between 15 °C and 25 °C for PL2 was not significant (GLM: F 1, 36 = 0.78, p = 0.38). For PL2 fish, the proportion of total clutches per 25-day period declined significantly more rapidly at 15 °C than either of the other two populations (GLM: F 1, 61 = 9.12, p = 0.004 and F 1, 51 = 7.95, p < 0.001, for UK and PL1 respectively). It also declined significantly more rapidly in UK fish, compared to PL1 fish (GLM: F 1, 81 = 14.15, p < 0.001). There was a significant negative relationship between mean clutch size and time

Growth of progeny

Growth rates of offspring increased significantly with temperature (ANCOVA: F 2, 1299 = 40.56, p < 0.001). Whilst growth rates were not significantly different between populations (ANCOVA: F 2, 1299 = 1.60, p > 0.05), there was a significant Temperature:Population interaction, affecting growth (ANCOVA: F 3, 1299 = 4.43, p = 0.004). For all populations, growth rates increased as a function of increasing temperature (Fig. 4.12), with a significant increase in growth rate between 15 °C and 25 °C for all populations (PL1: F 1, 390 = 8.29, p = 0.029, PL2: F 1, 263 = 40.65, p < 0.001, UK: F 1, 279 = 29.67, p < 0.001). Generally, the thermal responses were not significantly different between populations, but at 15 °C, PL1 larval growth was significantly higher than PL2 and UK (p = 0.002 for both comparisons). For combined data (i.e. without differentiating by population), growth ratesincreased significantly with each increment in temperature (p < 0.005 for all comparisons).

DISCUSSION

The study tested for the first time the plastic responses and adaptive potential of P.

parva over a temperature gradient, using metrics of female fecundity and the growth performance of their progeny. Contrary to prediction, the results indicated there was no significant variability in total reproductive output across a temperature range of 15 °C to 25 °C. Although there were differences in numbers of clutches laid by individuals from different populations, these were associated with the variation in the size of individuals from those populations. Temperature did significantly impact the duration and intensity of reproductive output, with higher temperatures inducing rapid intensive output and lower temperatures inducing more protracted and lower intensity output, with this consistent with the prediction. Growth rates of progeny increased with temperature and were similar across populations, with the exception of fish from PL1, which had a significantly higher growth rate at 15 °C than those of the other two populations studied. Observed growth rates of all populations at all temperatures in the present study were consistent with offspring reaching body sizes that could enable reproduction in their first year of life (Zahorska et al., 2013).

Although no clear differences in reproductive outputs were observed across temperatures or between populations in this study, such single generation studies based on F0 generation fish from the wild may be subject to maternal effects [START_REF] Whitney | Rapid evolution in introduced species,'invasive traits' and recipient communities: challenges for predicting invasive potential[END_REF]. Life history has been observed to impact reproductive parameters such as fecundity and egg size in other studies on a variety of fishes [START_REF] Heyer | Maternal effects as a recruitment mechanism in Lake Michigan yellow perch (Perca flavescens)[END_REF][START_REF] Bunnell | Testing for evidence of maternal effects among individuals and populations of white crappie[END_REF][START_REF] Lauer | Yellow perch length-fecundity and length-egg size relationships in Indiana waters of Lake Michigan[END_REF][START_REF] Kamler | Parent-egg-progeny relationships in teleost fishes: an energetics perspective[END_REF], with these parameters varying due to differential reproductive investment by parents (Bernado 1996). Such maternal effects on egg size and quality can influence offspring growth and survival, thus parental condition could be responsible for the observed differential in observed growth rates across the populations at 15 °C. Whilst it was not possible to account fully for such effects in the present study, efforts were made to minimise effects of parental condition by holding the adults under common conditions and feeding ad libitum prior to monitoring reproductive outputs. However, to more effectively control for such effects, ideally studies should be conducted on F1, or F2 generations of fish reared under common conditions (e.g. [START_REF] Shama | Bet hedging in a warming ocean: predictability of maternal environment shapes offspring size variation in marine sticklebacks[END_REF]. Continuation of the study across concurrent generations of the fish could have potentially led to differences in response becoming more apparent once any maternal effects were reduced.

The assessment of early growth of progeny in the present study was relatively simplistic, accounting only for length of individuals. Other studies of early life history in fishes have focussed on ontogenetic developmental steps [START_REF] Copp | When do fish with indirect development become juveniles[END_REF]Gozlan et al., 1999aGozlan et al., , 1999bGozlan et al., , 2005;;[START_REF] Kováč | Morphometry of the stone loach Barbatula barbatula (L.): do mensural characters reflect the species' life history thresholds?[END_REF]. The saltatory theory of ontogeny concerns the development of some fish via a sequence of stabilised states, punctuated by periods of rapid change [START_REF] Balon | Epigenesis of an epigeneticist: the development of some alternative concepts on the early ontogeny and evolution of fishes[END_REF]). Each of these periods of rapid change relate to the development or shift in the configuration of anatomical features [START_REF] Balon | Saltatory processes and altricial to precocial forms in the ontogeny of fishes[END_REF]) and impact the ability (e.g. locomotive ability for swimming) and requirements (e.g. gape size determines usable food item size) of the individual in relation to its environment. Recording these developmental steps would have provided an increased resolution perspective on the early life of the progeny in the present study and potentially allowed identification of subtle differences between populations and temperatures.

Water temperatures above physiological optima can act as a stressor in fishes, resulting in a suppressive influence on reproductive output through elevated cortisol levels (Pankhurst et al., 2011). In temperate fishes, inhibitory effects on reproduction are typically observed above 24 °C [START_REF] Pankhurst | Effects of climate change on fish reproduction and early life history stages[END_REF] and although reproductive output was not assessed, [START_REF] Davies | Influences of population density, temperature and latitude on the growth of invasive topmouth gudgeon P seudorasbora parva[END_REF] observed a decrease in adult growth rates above 23 °C in P. parva. In the present study, however, there was no indication of reproductive output being suppressed at 25 °C when compared to the lower temperatures. This emphasizes the broad tolerances of P. parva to thermal conditions. Indeed, it can be argued that their invasion success at global levels, with colonisation of Europe and spread into North Africa and the Middle East (Gozlan et al., 2010b), is at least in part related to their very broad temperature tolerances, whereby they are able to form sustainable populations in all climates encountered between North Africa and Northern Europe (Gozlan et al., 2010b). This then has considerable implications for their potential to invade other areas of the world, should introductions manifest, given that outputs of Chapter III predicted a number of high-risk areas for P. parva invasion, including southern states in the USA where summer water temperatures are likely to be in the range of 25 °C.

The lack of significant thermal response of total reproductive output across the experiment may be partly a function of the relatively moderate and controlled overwinter conditions (stable 5 °C water temperature) of all treatments. In the wild, overwinter conditions and associated mortality are considered to be a strong selective force [START_REF] Hurst | Causes and consequences of winter mortality in fishes[END_REF], with survival positively related to fish body size [START_REF] Wootton | Ecology of Teleost Fishes[END_REF]. Further, wild populations are present at latitudes in Europe, where harsh winter conditions are experienced (e.g. PL2 near Warsaw, Poland), implying that 5 °C is within comfortable lower limits for this species. Future studies of the thermal responses of reproductive output and early growth in invasive fish could therefore be significantly enhanced by including factors such as body condition and variation in simulated winter conditions in order to assess their relative importance versus the conditions in the breeding season.

In a study of another freshwater fish species known to be invasive in parts of its introduced European range (Lepomis gibbosus), [START_REF] Masson | Thermal Influences on Life-History Traits and Reproductive Effort of Introduced Pumpkinseed Sunfish Lepomis Gibbosus in the River Moselle Basin (Northeastern France)[END_REF] observed that juvenile growth and reproductive effort were positively related and age at maturity negatively related to increased temperatures, but the majority of the effect was centered on age-1 individuals, with older fish not showing a significant relationship between reproductive effort and temperature [START_REF] Masson | Thermal Influences on Life-History Traits and Reproductive Effort of Introduced Pumpkinseed Sunfish Lepomis Gibbosus in the River Moselle Basin (Northeastern France)[END_REF]. In the present study, fish ages were not established for the adult fish, although all females were > 46 mm in length, which would give them an estimated age of 2+ years according to other studies (Boltachev et al., 2006;[START_REF] Britton | A case study on the population ecology of a topmouth gudgeon (Pseudorasbora parva) population in the UK and the implications for native fish communities[END_REF][START_REF] Britton | Contrasting life history traits of invasive topmouth gudgeon (Pseudorasbora parva) in adjacent ponds in England[END_REF]. If P. parva show a similar lack of significant relationship between temperature and reproductive effort in fish older than age-1, then this could account for the absence of this relationship being observed.

Despite the larger clutch sizes from the UK fish, they then laid a reduced number of clutches compared to the other populations and thus the overall reproductive outputs were similar across the populations. In addition, temperature also affected how the reproductive output was distributed over time, with distinct strategies detected between lowest and highest temperatures. At the highest temperature, there was rapid production of eggs over a relatively short period, whereas lower temperatures induced a more gradual and protracted production of eggs over a longer time period. [START_REF] Zięba | The effect of elevated temperature on spawning frequency and spawning behaviour of introduced pumpkinseed Lepomis gibbosus in Europe[END_REF] observed a similar response pattern in an experimental assessment of Lepomis gibbosus reproductive behaviour between heated and ambient artificial ponds, with fish in heated ponds displaying increased reproduction early in the season compared to ambient temperature ponds. A subsequent study found that although more offspring were produced in ambient temperature ponds than heated ponds, they were of smaller size and consequerntly displayed significantly increased overwinter mortality by comparison -66% vs 7% [START_REF] Zięba | How Will Climate Warming Affect Non-Native Pumpkinseed Lepomis gibbosus Populations in the UK?[END_REF].

In many fish species, protracted breeding seasons are thought to be a response to variable conditions, with the production of progeny over extended periods increasing the chance of survival of the cohort, such as from stochastic weather events like floods that can be deleterious to larval fishes [START_REF] Harvey | Susceptibility of young-of-the-year fishes to downstream displacement by flooding[END_REF][START_REF] Bolland | Lifetime consequences of variable 0 year group length in riverine populations of chub Leuciscus cephalus (L.)[END_REF][START_REF] Nunn | Recruitment patterns of six species of cyprinid fishes in the lower River Trent, England[END_REF][START_REF] Nunn | Variations in the spawning periodicity of eight fish species in three English lowland rivers over a 6 year period, inferred from 0+ year fish length distributions[END_REF]. Examples of protracted spawners include L. macrochirus (Garvey et al., 2002), L. gibbosus and Squalius cephalus [START_REF] Nunn | Recruitment patterns of six species of cyprinid fishes in the lower River Trent, England[END_REF][START_REF] Nunn | Variations in the spawning periodicity of eight fish species in three English lowland rivers over a 6 year period, inferred from 0+ year fish length distributions[END_REF]. However, as the water temperatures were tightly controlled in the experiment then these results suggest that the reduced temperature difference between the winter and spawning temperature induced this protracted reproductive strategy.

The general plasticity in reproductive strategy (i.e. shape of output over time) relative to temperature has been observed in the wild, by [START_REF] Yan | Variations in reproductive strategies between one invasive population and two native populations of Pseudorasbora parva[END_REF], who also associated this with environmental factors, including temperature. They observed a more "protracted", less pronounced (lower peak GSI) breeding season in a lake with relatively cooler, more stable thermal regime. This corroborates these ex situ observations and potentially lends credence to the applicability of such observations in real world situations (i.e. management of wild populations).

In P. parva, sexual maturity in females has been observed at lengths as low as 24 mm SL (Zahorska et al., 2013). Given a mean size at hatching of 6mm across temperatures of 15 °C, 20 °C and 25 °C and growth rates of 0.11 ± 0.004, 0.14 ± 0.005 and 0.17 ± 0.007 mm d -1 respectively, individuals could reach sexual maturity between 121 and 187 days (assuming a linear relationship and consistent temperature, with non-limiting food resources). This is consistent with field observations of individuals reaching sexual maturity within their first year (Rosecchi et al., 1993;Rosecchi et al., 2001;Zahorska et al., 2013;[START_REF] Yan | Variations in reproductive strategies between one invasive population and two native populations of Pseudorasbora parva[END_REF], although actual reproduction might not actually occur until the following spring [START_REF] Britton | A case study on the population ecology of a topmouth gudgeon (Pseudorasbora parva) population in the UK and the implications for native fish communities[END_REF]. This highlights the considerable influence that water temperatures have on P. parva growth, with individuals taking 1.5 times as long to reach potential maturity at 15 °C as at 25 °C.

Where food is not limiting, the present study has identified that total reproductive output is a function of body size, whereas reproductive strategy and early growth are directly linked with water temperature. The combined effects of both adult breeding strategy and early growth of progeny, determined by environmental factors, could have implications on the demographics of populations over time. Over multiple generations the combined effects of these two temperature-dependant life history traits could constitute divergent ontogenies leading to morphologically differentiated populations, as predicted by temperature-size theories. Temperature-size relationships in the natural world have long been of interest to biologists [START_REF] Angilletta | The temperature-size rule in ectotherms: simple evolutionary explanations may not be general[END_REF] and a number of theories or rules have been formulated over the years. For instance, James' rule [START_REF] James | Geographic size variation in birds and its relationship to climate[END_REF], concerning intraspecific variation of body size, states that populations in warmer environments tend to have smaller bodies at age than their conspecifics in cooler environments. Further, the temperature-size rule (TSR) asserts that the size of individuals' bodies, in ectotherms, is negatively related to temperature [START_REF] Atkinson | Temperature and organism size: a biological law for ectotherms?[END_REF]. A more recent synthesis, of these two rules and others, in relation to the effects of climate change on aquatic systems [START_REF] Daufresne | Global warming benefits the small in aquatic ecosystems[END_REF], suggests that size-at-ages should decrease over time with increased temperatures, as the TSR predicts that a more rapid growth leads to lower final size in these conditions. Applying this to the case of P. parva, this would mean: northerly populations of less abundant, slower growing, later maturing individuals of larger body size and greater age, and southerly populations of more abundant, faster growing, early maturing individuals of smaller body size and lower age. However mean size of individuals could be significantly impacted by other factors, such as size-dependant predation, population density and resource availability. Furthermore, selective pressure on size, related to reproductive success being associated with larger size in males, could potentially lead to reverse temperature-size relationships [START_REF] Ohlberger | Climate warming and ectotherm body size-from individual physiology to community ecology[END_REF]. if dispersal were random. However other factors may also significantly influence dispersal, including sociability and potential density-dependent effects.

INTRODUCTION

Species are increasingly introduced to areas outside their historical native range, often facilitated by transportation and commerce (Meyerson et al., 2007). Whilst numerous introduced species fail to survive [START_REF] Blackburn | A proposed unified framework for biological invasions[END_REF], a proportion of those introduced successfully establish, spread and have a negative impact on the recipient system (Williamson & Fitter, 1996;Clavero & Garcia-Berthou, 2005;Hejda et al., 2009). These invasive species can have severe ecological consequences for the native ecosystem, in terms of biodiversity, as well as for economic interests [START_REF] Mack | Biotic invasions: causes, epidemiology, global consequences, and control[END_REF][START_REF] Bax | Marine invasive alien species: a threat to global biodiversity[END_REF]. Eradication is not always feasible, either due to possible collateral damage [START_REF] Thresher | Options for managing invasive marine species[END_REF], or because of the sheer scale of the problem and the associated cost (e.g. Feral goats in Australia, see [START_REF] Bomford | Eradication or control for vertebrate pests?[END_REF] the zebra mussel in North America, see Strayer 2009). Prevention is the most effective form of biological invasion management [START_REF] Britton | Towards the successful control of the invasive Pseudorasbora parva in the UK[END_REF], but this involves better understanding of how invasions happen and what characterises successful invaders, relative to unsuccessful introduced species.

The process of biological invasion can be conceptualised as comprising several distinct stages [START_REF] Richardson | Naturalization and invasion of alien plants: concepts and definitions[END_REF]Colautti & MacIsaac 2004), each containing filters or barriers to invasion success [START_REF] Richardson | Naturalization and invasion of alien plants: concepts and definitions[END_REF][START_REF] Mitchell | Biotic interactions and plant invasions[END_REF].

Various studies suggest that dispersal characteristics may be important in determining the success, or otherwise, of organisms negotiating one or more of these barriers and ultimately becoming invasive (e.g. [START_REF] Weis | The role of behavior in the success of invasive crustaceans[END_REF]Cote et al., 2010;Chapple et al., 2012). Dispersal is also pivotal in a metapopulation context, as it determines the flow of individuals between local populations, enabling the persistence of populations experiencing otherwise net negative growth as well as the colonisation of new patches of suitable habitat (Hanski & Gyllenberg, 1993;Hanski, 1998). Dispersal can also facilitate local adaptations and mitigate maladaptations in novel conditions, where gene flow is asymmetric due to non-random patterns in dispersal (Savolainen et al., 2007;Aitken & Whitlock, 2013). Intraspecific variability in dispersal may have direct impacts on population dynamics and certainly will impact a species' ability to expand its range, as the ability of a species to expand or invade will be determined by the demographic structure of the populations at the edge of the current range and the presence of dispersal phenotypes [START_REF] Clobert | Informed dispersal, heterogeneity in animal dispersal syndromes and the dynamics of spatially structured populations[END_REF]. Dispersal has often been linked to physiology, particularly morphological traits that aid the dispersal process [START_REF] Thomas | Ecological and evolutionary processes at expanding range margins[END_REF][START_REF] Berwaerts | On the sex-specific mechanisms of butterfly flight: flight performance relative to flight morphology, wing kinematics, and sex in Pararge aegeria[END_REF]. However, variability in dispersal may be reflective of morphological and behavioural adaptations together (Hanski et al., 2004) and so a more holistic approach considers a host of behavioural traits combined with physical ability, in the form of dispersal syndromes [START_REF] Arjona | Long-distance dispersal syndromes matter: diaspore-trait effect on shaping plant distribution across the Canary Islands[END_REF][START_REF] Cote | Evolution of dispersal strategies and dispersal syndromes in fragmented landscapes[END_REF]. A number of specific behaviours are thought to influence the success of establishment and spread of introduced species, including boldness, activity, exploratory behaviour and dispersal tendency (Chapple et al., 2012). Despite the importance of holistic approaches to dispersal, suggested by the literature, there are still very few such studies, particularly concerning freshwater vertebrates (with the exception of Gambusia affinis, e.g. Cote et al., 2010). A number of studies have investigated morphological differences, including in traits related to locomotion (Yavno et al., 2013), between native and introduced populations of invasive freshwater fish, relative to occupied habitat type (Yavno et al., 2013;Yavno & Fox 2014), water velocities (Yavno & Fox 2013) and presence of competitors (Yavno et al., 2014). A more recent study has even investigated differences in boldness and dispersal tendancy between native and introduced populations of the same species, Lepomis gibbosus (Ashenden et al., 2017). However none of these studies have focussed specifically on multiple naturally dispersing populations along a distancegradient at an expanding range front, where spatial sorting theory predicts the accumulation of dispersal-types will occur (Shine et al., 2011).

There is increasing evidence in the literature to suggest that recently colonised populations differ phenotypically from older populations, particularly with respect to morphological traits linked to dispersal (Olivieri et al., 1995;Hanski et al., 2004;Duckworth & Badyaev, 2007), with persistence in an area sometimes leading to selection for reduced mobility [START_REF] Zera | Physiology and ecology of dispersal polymorphism in insects[END_REF]. Therefore, I might expect to see morphologically distinct populations at an active invasion front, but I may also see that this morphologically distinct phenotype is also more prone to disperse and more active (Shine et al., 2011).

Pseudorasbora parva is a highly invasive freshwater cyprinid fish, native to East Asia. In less than 40 years, it has achieved pan-continental invasion success, with an average of 5 new countries succumbing per decade (Gozlan et al., 2010). P. parva possesses a number of life-history traits that facilitate its invasion success, including: high reproductive effort, early sexual maturity, batch spawning and paternal nest guarding (see Chapter IV) as well as a high tolerance for degraded ecosystems (Gozlan, 2012). Aside from the negative impacts associated with its observed trophic niche overlap with native fishes (Britton et al., 2010a), it is also known to be a healthy carrier of Sphaerothecum destruens, an emergent infectious disease known to cause mortality in a number of species (Gozlan et al., 2005;Andreou & Gozlan, 2016). Specific information on dispersal in this species is largely restricted to humanmediated dispersal (Gozlan et al., 2010), with a dearth of information quantifying natural dispersal behaviour. As dispersal characteristics can influence rate of spread, in an invasion context (e.g. Shine et al., 2011), differential in these characteristics between populations could mean that level of invasiveness also varies accordingly.

The ability to identify populations according to their dispersal characteristics could then be useful in estimating their relative potential for spread, or for identifying vanguard populations and thus the invasion front.

Here I take six P. parva populations in the River Garonne in the south of France and investigate the relationship between dispersal propensity, activity levels and morphological characteristics, with distance from invasion front. Whilst downstream drift of individuals was not quantified in this study, it is assumed that downstream drift occurs between these populations, based on the observation of this phenomenon in P. parva at other watercourses (e.g. Beyer 2008). Whilst this process may act broadly to homogenise genetically the populations, if the likelihood of individuals to flow downstream is non-random and is negatively related to their propensity to disperse upstream (i.e. the direction of the invasion front movement), then this process will potentially magnify the spatial sorting effect, by selectively moving less capable dispersers downstream.I tested the leading-edge effect along a distancegradient with regards to population morphology and dispersal-related behaviour.

Specifically, I expected that: (i) there would be morphological differences between populations, related to distance from the invasion front; (ii) there would be individual variation in activity levels, related to both morphological variation and to distance from the invasion front; (iii) there would be variation in propensity to disperse and this would be related to morphological characteristics, activity levels and to distance from the invasion front. Individuals were captured from locations in the shallow river margins and adjacent backwaters, amongst emergent vegetation outside the main flow of the river.

According to the semi-quantitative methodology as described in [START_REF] Carter | Seasonal abundance and microhabitat use of bullhead Cottus gobio and accompanying fish species in the River Avon (Hampshire), and implications for conservation[END_REF], all of these locations possessed flow conditions of 'zero water velocity', however they are not cut off from the flowing part of the river. Sampled individuals (between 50 and 100 for each location) were placed into polythene sacks with about 20 L of river

Activity

In open-field experiments, level of activity (i.e. total distance travelled) in a novel environment has been observed to be positively associated with boldness in a number of fish species (e.g. guppies -Budaev 1997;three-spined stickleback -Bell 2005;[START_REF] Harcourt | Personality counts: the effect of boldness on shoal choice in three-spined sticklebacks[END_REF]and zebrafish -Moretz et al., 2007). Boldness has been linked to dispersal distance in both natural and experimental contexts [START_REF] Fraser | Explaining leptokurtic movement distributions: intrapopulation variation in boldness and exploration[END_REF], therefore activity levels in an experimental arena could be indicative of propensity to disperse. Activity assays were carried out under controlled laboratory conditions, in white plastic buckets of 20 cm diameter. Each bucket contained 4 cm depth of dechlorinated water and one fishfresh, clean buckets and water were used for each experiment, to avoid behavioural reactions to scent of previous fish. A total of eight individual buckets could be monitored simultaneously by the lab setup. Activity was filmed on four 12 MP webcams, mounted at a height of 40 cm above the buckets, and footage captured using iSpy™ software (https://www.ispyconnect.com) at a rate of 10 frames per second. Activity was recorded for a total of 20 minutes, but only the final 15 minutes was analysed, to allow 5 minutes at the beginning for fish to settle after the lids of the buckets were removed and the room vacated. Footage was analysed using idTracker™ software [START_REF] Pérez-Escudero | idTracker: tracking individuals in a group by automatic identification of unmarked animals[END_REF] to track the position of individuals and quantify the total distance travelled. Distance travelled was converted, from pixels into km, using individual calibrations for each of the cameras used, to allow for any minor differences in field/angle of view and optical aberrations.

Morphology

Following the activity quantification assays, each group of eight fish was photographed on a gridded background (see Fig. A total of 12 systems, arranged in a two by six array, were used. Systems were filled with a 45 cm depth of tap water and left for a minimum of 72 hours prior to use, to allow for dechlorination. Each system was drained, cleaned and refilled with clean water after each experimental trial. Although dispersal in the natural environment would involve flowing water conditions, flume systems were unavailable for use in this study due to a number of constraints. Dispersal assays were instead conducted in static water conditions, after a pilot study in a single system confirmed that differentiation in distance dispersed was displayed in a group of eight individuals.

The dispersal assays were conducted on the fish in groups of eight individuals (from the same population), as P. parva are a gregarious species that typically aggregate in groups and has observed doing so in lotic environments (e.g. Boltachev et al., 2006).

To isolate individuals might have led to abnormal behaviour, yet the group setting could influence dispersal, so the issue of pseudoreplication was addressed in the statistical methods used to analyse the collected data. Fish were introduced to the first tank (location 0 in Fig. 5.3) of randomly assigned systems, in their groups of eight, at 5 pm and left overnight to acclimate to the system. Connecting pipes were closed for the duration of the acclimation period, to confine fish to start tanks until the beginning of the dispersal period. Connections were opened at 9 am the following day and then the fish were left to disperse for 72 hours, after which time links were closed, the fish were identified and their final locations recorded (see Fig. 5.3). In some instances the elastomer tags were rejected by the fish during the trial. Where this happened and definitive identification was not possible, individuals were removed from the analysis (this was the case for a total of 12 individuals). In total data were collected for 188 fish, over five trial runs, giving between three and five replicates for each population (specific numbers of replicates per population were: POR = 5, GRE = 4, BOU = 5, AUV = 5, TON = 3, COU = 3).

Morphology analysis

Linear discriminant analysis (LDA) was performed, using the implementation in the 'Momocs' R package [START_REF] Bonhomme | Momocs: outline analysis using R[END_REF], on the 11 measured morphometric characteristics, using source populations as the grouping factor, to produce dimensionally reduced measures of fish shape (five linear discriminants). Only the first two linear discriminants (LDs) were used for subsequent statistical analysis (For bi-plot, see Fig. 5.4), as these captured the majority of the between group variance -a combined 69.9%. The relationship between shape (i.e. LD1 and LD2) and distance from invasion front was tested by means of analysis of variance, using each of the LDs as the dependant variable in turn and using river channel distance, rather than straight-line distance, between population sample locations. I also tested for underlying patterns in SL between populations by using the same method, with SL as the dependant variable. Subsequent pairwise post hoc tests were carried out on relevant subsets of data, where appropriate, using population as the independent variable in an ANOVA framework.

Activity analysis

Because the activity data showed signs of overdispersion and because these data were non-integer, I used GLMs with a quasi-Poisson error term to estimate the relationship between activity (km travelled in 15 minutes), the first two LDs, and standard length (SL) along with possible interactions of each of these variables with distance from invasion front.

RESULTS

Morphometric analysis

LD1 and LD2 together explained 69.9% of the between-group variance (see Table 5.1), with LD1 dominated by traits relating to relative proportional size of head and other head-related metrics (eye diameter, head length, postorbital distance and preorbital distance cumulatively accounted for 94% of contributions to coefficients of linear discriminants) and LD2 was similarly dominated by head-related measures (see Table 5.2). Analysis of variance showed that LD1 was not significantly associated with SL (ANOVA: F 1, 186 = 3.3853, P > 0.05), however LD2 was (ANOVA: F 1, 186 = 12.203, P < 0.001), meaning that LD2 likely represents characteristics of shape that are associated with overall size (i.e. allometry). LD1 and LD2 were both significantly associated with distance from invasion front (ANOVA: F 1, 186 = 40.471, P < 0.01 and F 1, 186 = 4.0604, P < 0.05, respectively), with LD1 more consistent in this trend (see Fig. 5.5), however there was no significant association between SL and distance from invasion front (ANOVA: F 1, 186 = 2.4396, P = 0.12).

Dispersal

Of the 367 models fitted, 29 had a ∆ AICc of < 2. Model averaging, based on these 29 models, showed that three of the 11 parameters had high relative importance ('log(SL)', 'LD1' and 'PropFront'), with the rest receiving medium to low values (see Fig. 5.7). Final parameter estimates (Fig. 5.8) showed that 'log(SL)' had a positive relationship with dispersal distance (i.e. larger individuals dispersed further) and the lower 95% confidence interval remained above zero. 'LD1' had a generally negative relationship with dispersal distance, however the upper 95% confidence interval crossed zero, meaning that the sign of the relationship could not be predicted with confidence. 'PropFront' (proportion of total distance from invasion front) had a slight positive relationship with dispersal distance, but contained a high degree of variability across individual models and the lower confidence interval crossed zero by some margin, meaning that the sign of the relationship could not be predicted with certainty. All other parameters, other than intercept, had 95% confidence intervals straddling zero, meaning that the sign of the relationships could not be predicted with confidence. Relative Importance

DISCUSSION

This study showed that: (i) there were significant morphological differences between populations, synonymous with distance from invasion front; (ii) variation in individuals' activity levels was not significantly related to either morphology or distance from invasion front; (iii) Propensity to disperse was not significantly related to activity levels, or distance from invasion front, however it was significantly related to morphology, but only with respect to individuals' size (SL) and not shape (LD1 & LD2).

LD1 and LD2 were significantly related to distance from invasion front, with LD1 highly significant and LD2 marginally significant. This supported the hypothesis that there would be a gradient of morphological change, relative to distance from the invasion front. Whilst LD2 was considered to capture typical shifts in morphology as size increases (i.e. scaling in growth), because of its association with SL, LD1 was considered to represent a phenotypic morphological plasticity associated with the 'leading edge' effect I predicted would be present. LD1 was dominated by 'eye diameter' and 'head length', with the former positively related to distance from invasion front and the latter negatively related to distance from invasion front. This means that individuals at the invasion front typically had smaller eyes and larger heads, and individuals further from the invasion front had larger eyes and smaller heads.

The observed relationship between eye diameter and head length corresponds with known trade-offs between eye size and mandibular arch musculature, seen in other fish [START_REF] Von Scheven | Neural tube derived signals and Fgf8 act antagonistically to specify eye versus mandibular arch muscles[END_REF], including some cyprinids [START_REF] Hulsey | Do constructional constraints influence cyprinid (Cyprinidae: Leuciscinae) craniofacial coevolution?[END_REF], however it is unclear whether the driver of this morphological variation is directly related to the eye, or to the head and jaw, as both have been observed to be plastic in response to various factors. For instance, [START_REF] Franssen | Morphological divergence and flow-induced phenotypic plasticity in a native fish from anthropogenically altered stream habitats[END_REF] found that individuals from low flow environments had shallower bodies and smaller heads than those from higher flow environments (in Cyprinella venusta) and variation in head size has been found to relate to gill size (i.e. larger heads have larger gills) and be influenced by water flow and dissolved oxygen levels in other cyprinid species (Barbus neumayeri [START_REF] Schaack | Interdemic variation in the African cyprinid Barbus neumayeri: correlations among hypoxia, morphology, and feeding performance[END_REF][START_REF] Langerhans | Complex phenotype-environment associations revealed in an East African cyprinid[END_REF][START_REF] Baltazar | Fitness Consequences of Divergent Oxygen Environments in a Widespread African Cyprinid[END_REF]. In the context of a leading edge effect at the invasion front, larger heads and consequently larger gills could result in enhanced swimming endurance [START_REF] Domenici | Habitat, body design and the swimming performance of fish[END_REF], a phenomenon similar to that seen in cane toads, where individuals at the invasion front have greater endurance [START_REF] Llewelyn | Locomotor performance in an invasive species: cane toads from the invasion front have greater endurance, but not speed, compared to conspecifics from a long-colonised area[END_REF]. However further studies relating head size to gill size and swimming endurance would be necessary in P. parva, as no current studies quantify these possible relationships for this species.

Although Sunardi et al. (2007) have studied respiration rates in relation to current velocities, they did not account for relative differences in size of morphology between test individuals.

Intraspecific diet-related plasticity in jaw and head morphology has been observed in numerous fish, with divergent morphotypes apparent in individuals, based on the predominant food-type consumed [START_REF] Grudzien | Direct evidence that the Ilyodon morphs are a single biological species[END_REF][START_REF] Wimberger | Plasticity of jaw and skull morphology in the neotropical cichlids Geophagus brasiliensis and G. steindachneri[END_REF]Hagrenes 2001). In P. parva there could be a shift in diet at the invasion front leading to larger jaw muscles and the trade-offs already mentioned leading to smaller eyes.

Such a shift in dietary preference has been observed in the European catfish, between recently introduced populations (i.e. invasion front) and earlier introductions [START_REF] Carol | Growth and diet of European catfish (Silurus glanis) in early and late invasion stages[END_REF]. Future studies of the P. parva populations used in the present study, could investigate possible differences in diet, through gut-content analysis (e.g. [START_REF] Almeida | Dietary traits of invasive bleak Alburnus alburnus (Actinopterygii, Cyprinidae) between contrasting habitats in Iberian fresh waters[END_REF] and relationships with morpholometric measures used in this study, but also supplemented with those of trophic anatomy (e.g. [START_REF] Vila-Gispert | Morphological variation in pumpkinseed Lepomis gibbosus introduced into Iberian lakes and reservoirs; adaptations to habitat type and diet?[END_REF].

Whilst greater eye size is typically synonymous with greater focal length and resolving power [START_REF] Fernald | Aquatic adaptations in fish eyes[END_REF], and has been linked with importance of vision in feeding [START_REF] Evans | Structural modifications correlated with feeding habits in four species of cyprinid fishes[END_REF][START_REF] Otten | Vision during growth of a generalized Haplochromis species: H. elegans Trewavas 1933 (Pisces, Cichlidae)[END_REF], large eyes can make individuals more conspicuous to predators and so increase predation risk [START_REF] Wickler | Mimicry in plants and animals[END_REF]. [START_REF] Kim | Phenotypic integration between antipredator behavior and camouflage pattern in juvenile sticklebacks[END_REF] observed, in three-spined sticklebacks, individuals with larger eyes were more fearful when foraging under predation. If this were applied to P. parva, then individuals at the invasion front, with smaller eyes, may suffer less from predator related stress, a mechanism which has been shown to increase respiration rate significantly in this species (Sunardi et al., 2007). This could be a particular benefit to individuals who are less social and more prone to disperse, as they would likely be subject to greater predation risk.

Activity is thought to be an important determinant of invasion success (see Chapple et al., 2012) and is thought to be indicative of exploratory behaviour [START_REF] Walsh | The open-field test: A critical review[END_REF]. However, contrary to the hypothesis, I found no link between activity and distance from invasion front, or morphological characteristics, which may mean that this is independent of body shape and size (i.e. a personality trait, such as that seen by Cote et al., 2010 in Gambusia affinis) and does not appear to be significantly more prevalent at the invasion front, even though activity in individuals at the invasion front are higher than all populations apart from Tonniens (seen Fig.

5.6)

The only clear predictor of dispersal propensity found in this study was fish size (SL), in that larger individuals dispersed further in the experimental systems. Whilst relative importance values of other parameters were moderate or high, in the fullaveraged model (i.e. 'LD1', 0.82 and 'PropFront', 0.64 in Fig. 5.7), confidence intervals straddled zero, so the sign of the relationship was not consistently predicted.

So, contrary to the hypothesis, dispersal was not significantly related to the measured morphological traits (beyond that of SL) and did not vary corresponding to distance from invasion front.

Movement and range size in animals typically increases with body size (Peters, 1986;[START_REF] Knight | Allometric Scaling of Intraspecific Space Use[END_REF] and this is certainly the case for a number of freshwater fish (Minns, 1995;Radinger & Wolter, 2014). My results seem to support this general rule, and other studies too have similarly found a lack of differentiation in dispersal between populations from the invasion front and from areas where they are well established (i.e. Hemichromis bimaculatus [START_REF] Lopez | Nonnative African jewelfish are more fit but not bolder at the invasion front: a trait comparison across an Everglades range expansion[END_REF]. However, it is also possible that the experimental dispersal assays did not measure dispersal in a fully realistic way. Whilst the P. parva populations in the River Garonne are subjected to flowing water when dispersing, my experimental dispersal systems (Fig. 5.3) contained static water. This could mean that instead of quantifying dispersal, the assays merely quantified exploratory behaviour, which along with boldness, Cote et al., (2010) found to be unrelated to dispersal in another invasive freshwater fish (Gambusia affinis). Studies of dispersal in P. parva relative to flow velocity, specifically the willingness of individuals to move into flowing water from still waters, could certainly help to provide a means of calibration for future studies of dispersal, to ensure that what is being measured is in fact a fair representation of dispersal, relevant to the natural environment. However, due to the complexity of the natural environement and also because dispersal behaviour may relate to a number of context-specific factors (e.g. population density, resource availability, predation risk, abiotic conditions) it may be difficult to quantify natural dispersal behaviours ex situ.

Although Cote et al. (2010) found exploratory behaviour and boldness unrelated to dispersal, sociability was strongly linked to dispersal. In fact, sociability has been linked to dispersal in other taxa too (e.g. [START_REF] Cote | Social personalities influence natal dispersal in a lizard[END_REF][START_REF] Quinn | Scale and state dependence of the relationship between personality and dispersal in a great tit population[END_REF][START_REF] Thorlacius | Behavioral dependent dispersal in the invasive round goby Neogobius melanostomus depends on population age[END_REF], suggesting that dispersal can be context dependant, relating to factors linked to social interactions. P. parva is a gregarious species and tend to form groups, which is why I conducted the dispersal assays on groups, rather than individuals. However, if dispersal is linked to social interactions, then population density, or group size, could be critical in triggering the expression of variation in this behaviour. For example, Broderson et al. (2008) found that the decision to make partial-migrations in Rutilus rutilus was context-dependent, determined by body condition and food availability. Competition and hence food availability will vary according to the population density [START_REF] Bertram | Living in groups: predators and prey[END_REF], potentially leading to densitydependent dispersal behaviour, which could also be dictated by body size (a determinant of competitive ability, [START_REF] Hoare | Body size and shoaling in fish[END_REF]. For instance Brownscombe & Fox (2012) found, in Neogobius melanostomas, that individuals in newly occupied areas (i.e. range front) tended to be smaller, suggesting that perhaps larger aggressive males in the core longer-established areas are forcing out smaller, less competitive individuals. Increasing the group size of fish in the dispersal assays may have had a significant impact and future studies should consider the potential impact of population density on dispersal outcomes. It is possible that differences in dispersal may only manifest at higher population densities, particularly if social interactions are triggers for such behaviour.

CONCLUSIONS

The results of the present study suggest that at low densities the dispersal propensity of P. parva in still waters relates to fish size, with larger fish dispersing further.

Introductions of larger individuals may be more prone to actively spread and to do so more rapidly than smaller individuals. However, this pattern of dispersal may differ at higher population densities and could include a shift in diet, manifested physically in craniofacial morphological changes. Whilst in my study morphological traits measured were not implicated in dispersal, or activity levels, there was a strong gradient of change relative to distance from invasion front. This morphological shift

shows that these P. parva possess the potential to be highly phenotypically plastic, even along a c. 210 km stretch of the same river; a relatively short distance. Such plasticity is thought to be a key indicator of invasive species (Davidson et al., 2011) and the results of the present study provide evidence to reinforce this view. Whether this plasticity is related, directly or indirectly to dispersal, further research on social behaviour and possible density-dependent effects could be invaluable in elucidating mechanisms that determine successful spread and population dynamics in this species, including how it maintains a seemingly high potential for phenotypic plasticity in invasion scenarios, where we would often expect low genetic diversity.

parva is capable of such adaptations (which could influence management and containment protocols to prevent secondary translocation of individuals from expansion front locations).

The projection of the niche model in the risk mapping study (Chapter II) showed that large areas, beyond the current distribution of the species, are climatically suitable.

These areas are mainly in North and South America as well as Australia and New

Zealand and constitute significant scope for spread and impact in this species. When The US Great Lakes are already subject to a considerable burden from invasive species and there is concern that this may be facilitating further invasions [START_REF] Ricciardi | Facilitative interactions among aquatic invaders: is an" invasional meltdown" occurring in the Great Lakes?[END_REF]. However, the US Fish and Wildlife Service (USFWS) have already identified P. parva as a high risk invasive species with potential for high negative impact and in September 2016 it was added to their list of "injurious species", effectively prohibiting its import into and movement within the US (USFWS, 2016). Whilst this is positive and will prevent intentional introductions, given that most introductions of this species beyond its native range were unintentional, through contaminated consignments of other species, these measures may not necessarily prevent such introductions in the US (Britton et al., 2011). Although fines resulting from even accidental breaking of laws may give added incentive to those importing other species to more thoroughly check consignments for P. parva, a high level of vigilance is required in order to detect possible introductions early and deal with them before they escalate beyond the invasion-event-horizon for feasible eradication (Davies et al., 2013).

In means that all 16 of these variables are given equal weighting and are treated as noninteracting in determining suitability for P. parva. Such a method will tend to consistently over-predict, as the Euclidean CLIMATCH metric is based on comparison of target sites with a mean value of each climatic variable. The authors of the software do acknowledge that this is the case [START_REF] Crombie | Climatch user manual[END_REF] and this also fits with precautionary principal approach, which enjoys widespread support from conservationists (e.g. [START_REF] Simberloff | How much information on population biology is needed to manage introduced species?[END_REF]. In the case of species such as P. parva, which are known to have strongly negative impacts and little or no commercial value, this seems eminently sensible. However, care should be taken to not adopt too conservative an approach for all taxa, as inappropriate rejection of species with considerable commercial value, due to overestimation of invasion risks, could have net negative effects (see [START_REF] Smith | When to ignore advice: invasion predictions and decision theory[END_REF]. As an example of the difficulty that can be faced when trying to a priori differentiate potentially invasive species, from those that are benign, Pseudorasbora elongata, as a close relative of P. parva [START_REF] Yang | The phylogenetic relationships of the Gobioninae (Teleostei: Cyprinidae) inferred from mitochondrial cytochrome b gene sequences[END_REF], is similar in both biology and ecology, however is in decline in its native range and despite being associated with ornamental fish trade, is not recorded to be invasive [START_REF] Chen | Pseudorasbora elongata. The IUCN Red List of Threatened Species 2011[END_REF]. Pseudorasbora pumila is also very closely related to P. parva, but is endemic to Japan and, in some instances, may be threatened by introductions of P.

parva (see [START_REF] Koga | Genetic structures of allopatric and sympatric populations in Pseudorasbora pumila pumila and Pseudorasbora parva[END_REF].

Parts of South America are identified as other possible locations at risk from P. parva invasion. The fact that South America is not typically closely linked with the topic of biological invasions perhaps owes much to the relative dearth of research and general interest on the subject in this part of the world [START_REF] Speziale | Dealing with nonnative species: what makes the difference in South America?[END_REF]. Nevertheless, invasive species were estimated to cost Brazil alone USD$ 49.3 billion per year, to the turn of the century, through losses to the crops, pastures, forests and the environment at large [START_REF] Pimentel | Economic and environmental threats of alien plant, animal, and microbe invasions[END_REF]. In fact, 40 of the top 100 world's worst invasive species have been introduced to South America (IUCN-GISD, 2017) and most of the introduced species that went on to become invasive in S. America came from Eurasia [START_REF] Speziale | Patterns of alien plant invasions in northwestern Patagonia, Argentina[END_REF][START_REF] Speziale | Dealing with nonnative species: what makes the difference in South America?[END_REF]. Various Asian carp species, closely associated with initial introductions of P. parva to Europe (Gozlan et al., 2010), are invasive in S. America (IUCN -GISD, 2017). Despite this, P. parva are, as yet, not known to be presentperhaps due to historically lower propagule pressure, relative to introductions into Europealthough it is likely that introduced species here are under-recorded [START_REF] Quiroz | Análisis cuantitativo de la investigación en invasiones biológicas en Chile: tendencias y desafíos[END_REF]. Nonetheless, we can expect to see an increase in the number of non-native introductions, analogous to the trends of increasing GDP in a number of S. American countries (Pyšek et al., 2008;Hulme et al., 2009), so the risks should be taken seriously, particularly because the region hosts a number of biodiversity hotspots (Myers et al., 2000) and is lagging behind in terms of environmental policies and risk assessments.

In South America, the outlook for preventative action is less positive than that of N. America, in relation to P. parva. There has been a recent raft of proposed and implemented legislative change in Brazil, designed to nurture economic growth, particularly in the aquaculture sector, including for food production and the ornamental trade [START_REF] Vitule | Ecology: Preserve Brazil's aquatic biodiversity[END_REF][START_REF] Pelicice | A Serious New Threat to Brazilian Freshwater Ecosystems: The Naturalization of Nonnative Fish by Decree[END_REF][START_REF] Vitule | Aquarium trade: Monitor Brazil's fish sampling closely[END_REF][START_REF] Junior | Dams, politics and drought threat: the march of folly in Brazilian freshwaters ecosystems[END_REF][START_REF] Azevedo-Santos | Removing the abyss between conservation science and policy decisions in Brazil[END_REF]. However little thought has been given to the likely negative consequences of such activity, which will undoubtedly increase the number of non-native introductions [START_REF] Vitule | Aquarium trade: Monitor Brazil's fish sampling closely[END_REF] and also provide a system where spread is enabled. For instance, a number of non-native fish, notorious for their high invasive potential, are being exempted from import restrictions [START_REF] Pelicice | A Serious New Threat to Brazilian Freshwater Ecosystems: The Naturalization of Nonnative Fish by Decree[END_REF][START_REF] Junior | Dams, politics and drought threat: the march of folly in Brazilian freshwaters ecosystems[END_REF], so that they can be reared and traded. Further, rules on transporting live fish are being relaxed and inter-catchment connectivity is being increased by means of large-scale water diversion, man-made channels and the construction of hydroelectric dams [START_REF] Pelicice | A Serious New Threat to Brazilian Freshwater Ecosystems: The Naturalization of Nonnative Fish by Decree[END_REF][START_REF] Vitule | Brazil's drought: Protect biodiversity[END_REF]. Despite opposition from the scientific community, the Brazilian government have pushed ahead with such policies, seemingly unperturbed [START_REF] Azevedo-Santos | Removing the abyss between conservation science and policy decisions in Brazil[END_REF]. If the current trend of increased non-native aquatic species introductions continues, and it seems likely to, then parts of South America could be facing similar magnitudes of invasion and consequent impacts to those seen in the North American Great Lakes, one of the most heavily invaded ecosystems on the planet [START_REF] Holeck | Bridging troubled waters: biological invasions, transoceanic shipping, and the Laurentian Great Lakes[END_REF][START_REF] Ricciardi | Patterns of invasion in the Laurentian Great Lakes in relation to changes in vector activity[END_REF]. Makhrov et al. (2013) found evidence of a reproducing P. parva population in Tibet, at an approximate altitude of 3600 m above sea level. My model predictions generally

show that mountainous regions are not suitable for P. parva, however this could be due to lack of propagule pressure, as these are unlikely locations for introductions, given that most introductions were associated with aquaculture. As Makhrov et al.

(2013) point out, it is traditional to release live fish into water bodies, in Asian Buddhist culture, so this is a likely source of its introduction there. The presence of P.

parva in this Tibetan location implies that perhaps there is still more scope for this species to spread than my model in Chapter II predicts.

My final risk map integrated the Vörösmarty et al. (2010) "Aquaculture Pressure" dataset, a surrogate for introduction likelihood, in order to provide an estimate of successful invasion likelihood. I used an equal weighting with bioclimatic suitability, implying that both factors are equally important to invasion success, however one could argue that suitability is more important, with suitable locations only needing as few as one propagule introduced to lead to an invasion. Whilst integration of introduction likelihood may well provide a refined risk assessment, able to differentiate between suitable locations with high and low likelihood of introduction, care should be taken to maintain a precautionary approach, whereby any suitable location is at potential risk. Furthermore, and directly related to the output of the risk map, these "Aquaculture Pressure" data are already out of date, with Brazil in particular likely to harbour much larger values now, due to the recent legislativebacked growth in the aquaculture sector.

CLIMATIC ADAPTATIONS

In climatic comparisons of the two key genetic lineages, in both native and non-native ranges, I found no evidence to support the idea that the lineages represent local climatic adaptations in the native range, or that either lineage was differentially associated with any particular climate-type in the non-native range. Similarly, the thermal response experiments identified no significant differentiation between populations from two differing climate-types. The most parsimonious explanation for these results is that, instead of local genetic adaptations, P. parva has very broad tolerances and maintains fitness in a wide variety of conditions. Despite their seeming ubiquity in the literature, there are few empirically proven examples of contemporary genetic adaptations (Gienapp et al., 2008). Biological invasions are often typified by introduction of P. parva provides a good basis to investigate what these other factors may be. For instance, we know that P. parva is a healthy carrier for an infectious disease that causes mortality and morbidity in other fish species (Andreou et al., 2012). The presence of this disease could, in theory, depress competitiveness of naïve communities where P. parva is introduced, thus facilitating establishment. It could be informative, then, to investigate if susceptible fishes are more abundant at P. parva invaded locations.

Although total reproductive output was not affected by temperature, in the thermal response experiments (Chapter IV), P. parva did exhibit a strong response of temporal reproductive strategy, with intensity positively related to temperature.

Variations in growth and reproductive strategies typically reflect trade-offs between juvenile mortality, adult fecundity and age at maturity, in order to maximise annual recruitment to the population (Garvey et al., 2002). In the case of P. parva, this strong thermal response may enhance their ability to rapidly form dense populations in favourable warm conditions, maximising growth potential in progeny by concentrating reproductive effort at the beginning of the productive warm season, which will in turn enhance overwinter survival of juveniles and their subsequent fecundity, when sexually mature. At more poleward latitudes, where conditions are colder and the productive season is shorter, the observed shift in strategy may enhance medium-term persistence of adults by minimising wasteful reproductive effort (i.e. on producing offspring in climates where early growth would be minimal and overwinter mortality severe), thus maintaining body condition and increasing adult overwinter survival. In an invasion context, this would still mean that this population had invasion potential, particularly if it was associated with the aquaculture trade, wherein it could contaminate outgoing consignments and flourish when introduced to locations with warmer conditions.

Whilst my study showed no significant difference between populations from Poland and England, there are populations in locations that differ from one another in climate more strongly than these two locations. Selection of populations for future tests could, in theory, be selected on an a priori basis of being climatically dissimilar and include populations from the native range too, in order to corroborate the implied lack of adaptation from native to introduced ranges observed in Chapter III. A large number of replicates would allow for other important factors to be investigated, which could shed light on important determinants of overall reproductive performance of invasive fish, such as overwinter conditions and food availability (as discussed in Chapter IV).

These factors were not varied in my experiments, as there were insufficient numbers of fish and also capacity for replication, without which statistical power in analyses would be unacceptably low.

DISPERSAL AND MORPHOLOGY

In my dispersal and morphology study (Chapter V), I found a strong gradient of morphological change, corresponding to distance from the invasion front. Spatial sorting theory predicts that where there is differential dispersal between individuals in range-expanding populations, individuals exhibiting greater dispersal ability will tend to aggregate along the expansion fronts (Shine et al., 2011). Dispersal ability is often linked to morphological characteristics or traits and so dispersal-types, idiosyncratic by their morphology are often observed to be at higher prevalence on expanding range fronts (e.g. [START_REF] Phillips | Invasion and the evolution of speed in toads[END_REF][START_REF] Hughes | Modelling and analysing evolution of dispersal in populations at expanding range boundaries[END_REF][START_REF] Forsman | Phenotypic evolution of dispersal-enhancing traits in insular voles[END_REF]. I found that the morphological gradient was not significantly related to dispersal, with only size a significant factor for dispersal distance in experimental systems. That size was the most significant predictor of dispersal in my experiments, was not especially remarkableit is a common phenomenon across a number of different taxa, including fishes (Peters, 1986;Minns, 1995;Radinger & Wolter, 2014) -however, the lack of significant relationship between the strong morphological change gradient and dispersal was unexpected, as was the similar lack of significant relationship between dispersal propensity and distance from invasion front. Spatial sorting theory states that dispersal phenotypes will tend to have higher incidence in colonising populations (Shine et al., 2011), so it seems atypical to not find a connection with either morphology (commonly associated with differential dispersal ability) or distance from invasion front.

Higher propensity to disperse in larger individuals could facilitate the invasion process, purely by the mechanism of more fecund individuals (see Chapter IV) being more likely to disperse. It is, however, also possible, as mentioned in Chapter V, that the experimental design used did not allow enough scope for the differentiation in dispersal amongst individuals to be expressed. For instance, whereas my experiments were conducted under still water conditions, other assessments of dispersal (e.g. Cote et al., 2010;Ashenden et al., 2017) used systems with a channel of flowing water.

The additional effort and swimming ability necessary from individuals in order to classify as dispersers may refine the response and provide a clearer picture. Future studies could incorporate such a design and should also investigate the potentially important factor of population density (i.e. starting density), as this could be an important link to social behaviour, which could express itself as density-dependant dispersal propensity, such as that seen in Neogobius malanostomus, where smaller relate to feeding behaviour, which may directly influence the impact on native taxa through trophic overlap. Moreover, it could mean that morphological measures could be used to differentiate vanguard populations from older populations, which would undoubtedly be useful when formulating plans for invasion management and eradication.

PERSPECTIVES

My global risk map shows scope for further spread of P. parva, which, if realized, will likely result in negative impacts on invaded ecosystems. There are also possible economic implications if these P. parva populations increase commercial fisheries' exposure to disease (e.g. S. destruens). Whilst I found no evidence to suggest that there have been climatic adaptations in P. parva, at either the level of genetic lineages, or individual populations within the introduced range, my dispersal and morphology study did identify a strong morphological cline in the context of landscape scale metapopulations (i.e. populations along the same river channel).

Intraspecific morphological variability, particularly in head-related measurements, in fishes often relates to prey type, or diet (e.g. in Lepomis gibbosus, [START_REF] Mittelbach | Variation in feeding morphology between pumpkinseed populations: phenotypic plasticity or evolution?[END_REF] in Archosargus probatocephalus, [START_REF] Cutwa | Intralocality variation in feeding biomechanics and prey use in Archosargus probatocephalus (Teleostei, Sparidae), with implications for the ecomorphology of fishes[END_REF] in Perca fluviatilis and Rutilus rutilus, [START_REF] Svanbäck | Intraspecific competition drives multiple species resource polymorphism in fish communities[END_REF]. Trophic plasticity is a trait exhibited by a number of other invasive freshwater cyprinids [START_REF] Almeida | Feeding tactics and body condition of two introduced populations of pumpkinseed Lepomis gibbosus: taking advantages of human disturbances?[END_REF][START_REF] Godard | Plasticity of diel patterns in the diet and habitat use of feral, non-native fathead minnow Pimephales promelas (Actinopterygii, Cyprinidae) in a pond-dwelling population in England[END_REF] and flexibility in diet may facilitate the invasion of novel systems (Almeida et al., 2016). Mittlebach et al. (1999) observed that another invasive fish species, Lepomis gibbosus, exhibit plasticity in jaw morphology, directly related to diet. Their study tested for phenotypic plasticity versus localized genetic adaptation, by using common garden experiments to compare two morphologically divergent populations, exposing the lab-reared offspring from each of these populations to two differing diets. Such experimental methods could also be used to test for the same in P. parva, perhaps in addition to, or in conjunction with, gut content analysis on the fish from the locations I sampled along the River Garonne (Chapter V).

Although the analysis in Chapter III found no evidence for climatic adaptations of the two native lineages to the conditions of their respective native distributions (i.e.

northern and southern), the results did highlight the difference in occupied environmental conditions between the native and European ranges. Chapter IV tested for differences in life history trait thermal response between fish from non-native populations, but I was unable to include fish from native populations, due to a combination of time constraints and difficulties in obtaining and shipping samples to the UK. The majority of the studies in the literature conducted on P. parva are based on observations in the introduced range (e.g. Rosecchi et al., 1993;Pinder et al., 2005;[START_REF] Britton | Contrasting life history traits of invasive topmouth gudgeon (Pseudorasbora parva) in adjacent ponds in England[END_REF]Britton et al., , 2010)) The plasticity exhibited by P. parva in the non-native range (e.g. in life history traits, [START_REF] Britton | Contrasting life history traits of invasive topmouth gudgeon (Pseudorasbora parva) in adjacent ponds in England[END_REF] in morphology, Chapter V) may also be present in the native range, but it could equally be an evolutionary response to release from natural enemies in the introduced range. Non-native populations often evolve to be more plastic than their native conspecifics (e.g. [START_REF] Lavergne | Increased genetic variation and evolutionary potential drive the success of an invasive grass[END_REF][START_REF] Caño | Increased fitness and plasticity of an invasive species in its introduced range: a study using Senecio pterophorus[END_REF] and this evolved plasticity can be a product of release from enemies, or stress in the native range [START_REF] Huang | Stress relief may promote the evolution of greater phenotypic plasticity in exotic invasive species: a hypothesis[END_REF]. Release from native range enemies may significantly shift the balance of costs and benefits of developing plasticity, which may be too costly in a strongly competitive or stressful native range, but feasible in naive ecosystems. P. parva could be a successful invader because it evolved in a highly competitive environment and consequently is able to outcompete many other species in new ecosystems. However, it is possible that evolved plasticity gives P.

parva a degree of flexibility in behavior, morphology and life history traits that allows it to rapidly adapt to varying conditions. Further, it is possible that any adaptation to conditions associated with human activities (i.e. degraded, or disturbed conditions) could facilitate spread and augment their invasiveness, through increased exposure to possible accidental transportation to new locations.

  CLIMATIC ADAPTATIONS .............................................................................................................. DISPERSAL AND MORPHOLOGY ................................................................................................ CONCLUSIONS ...................................................................................................................................... PERSPECTIVES ...................................................................................................................................... REFERENCES .........................................................................................................................................

Figure 4 . 1 Figure 4 . 2 Figure 4 . 3 Figure 4 . 4 Figure 4 . 5 Figure 4 . 6 Figure 4 . 7 Figure 4 . 8 Figure 4 . 9 FittedFigure 4 .Figure 4 .Figure 4 .Figure 5 . 4

 41424344454647484944454 Figure 1.1 Heuristic diagram, adapted from Soberón & Peterson (2005), depicting visual representations of the three categories of constrains that determine a species distribution; Abiotic constraints (A); Biotic constraints (B); Physical barriers to dispersal (P) .....................................................................................................36

  risques d'invasion à l'échelle globale. Ceci constitue un des manques de connaissances les plus criants pour cette espèce et cela pourrait fortement limiter les efforts pouvant être fait pour limiter davantage une dispersion future et les impacts en résultants. Au cours de cette thèse, mon objectif général était de quantifier les risques d'invasion de Pseudorasbora parva à l'échelle globale en utilisant des approches corrélatives de modélisation de la l'enveloppe climatique (Chapitre II). Ces approches corrélatives dépendent d'hypothèses de bases liées à la présence ou l'absence d'adaptation aux échelles locales et régionales. J'ai donc testé par la suite les évidences pour des adaptations de ce type au sein des principales lignées génétiques de P. parva et de certaines de ses populations en comparant d'une part les enveloppesles enveloppes climatiques modélisées entre les aires natives et non-natives (Chapitre III) et d'autre part en menant des expériences en laboratoires sur la réponse thermique de traits d'histoire de vie majeurs (Chapitre IV). L'analyse du risque d'invasion proposée dans le premier chapitre ne prend pas en compte un facteur clef des invasions ; à savoir la capacité de dispersion naturelle. La capacité de dispersion a souvent été montré comme étant sujet à la sélection naturelle lors des phénomènes d'invasions et l'adaptation de certains traits liés à la dispersion pourrait permettre à certaines espèces (ou populations) d'améliorer leur pouvoir de propagation. Par exemple, lors de l'invasion du crapaud buffle en Australie, il a été observé que le taux de dispersion dans le paysage a été multiplié par cinq au cours des 70 dernières années du fait de l'adaptation génétique de certains traits liés à la dispersion (la longueur des jambes) le

Figure

  Figure 1.1 Heuristic diagram, adapted from Soberón & Peterson (2005), depicting visual representations of the three categories of constrains that determine a species distribution; Abiotic constraints (A); Biotic constraints (B); Physical barriers to dispersal (P).

Figure 1 . 2

 12 Figure 1.2 Photograph of female (top) and male (bottom) Pseudorasbora parva.

Figure 1 . 3

 13 Figure 1.3 Global map displaying river basins where Pseudorasbora parva occurs, in the native (blue) and non-native (red) ranges, according to Tedesco et al. (2017).

Figure 2 . 1

 21 Figure 2.1 Box-plots displaying the Area Under Curve (AUC), Receiver Operating Characteristic (ROC) evaluation scores for all models, grouped by modelling method. Components of box-plots represent minimum, lower quartile, mean upper quartile and maximum values for each modelling method. For each group n = 100, except for EMmw, where n = 1.

Figure 2 . 2

 22 Figure 2.2 Density plots of top three most important variables in final mean-weighted Ensemble Model (EMmw), in order of importance; (a) Bio14 -Precipitation of driest month (mm); (b) Bio3 -Isothermality (mean diurnal temperature range divided by annual temperature range; (c) TWIS -Topographic Wetness Index.

Figure 2 . 3

 23 Figure 2.3 Suitability (a) and Risk (b) maps. Both maps use the same colour scale, from low value of 0 to high value of 1, indicating climatic suitability and risk of successful invasion, respectively. Final layout of map panels was generated using Esri ArcMap (Version 10.0, Build 2414. url: https://www.arcgis.com/).

  phenotypic plasticity, including the interplay between the two mechanisms, is key to the development of effective conservation strategies in the face of sustained climate change. Biological invasions can closely match the fundamental process of climate change; in that introduction of an organism to a new location with novel climatic conditions can constitute a rapid change in experienced climate -from native range to introduced range (Losos et al., 2001). As such, biological invasions can be studied to elucidate the mechanisms by which organisms can adapt to climate change. A central theme to the research on biological invasions is the prediction of future invasions (Vander Zanden & Olden, 2008; Moran & Alexander, 2014).

Figure 3 . 1

 31 Figure 3.1 Map displaying the locations of populations sampled and the identified lineages, including relative proportions depicted in pie charts, in both the native and introduced ranges identified by Simon et al., 2011, 2015 and Hardouin et al., 2018).

Figure 3 . 2

 32 Figure 3.2 Kernel-smoothed output of PCA trained on data from native range, depicting bioclimatic niche of Native Northern lineage (a), Native Southern Lineage (b) and correlation circle of the PCA (c). Solid and dashed contours represent 100% and 50% of background environmental data.

Figure 3 . 4

 34 Figure 3.4 Kernel-smoothed output of PCA trained on data from introduced range, depicting bioclimatic niche of Introduced Northern lineage (a), Introduced Southern Lineage (b) and correlation circle of the PCA (c). Solid and dashed contours represent 100% and 50% of background environmental data.

Figure 4 . 2

 42 Figure 4.2 Photograph showing experimental unit, with the male guarding the nest, made from an upturned length of plastic guttering.

  size, (iv) Total eggs laid. Generalised Linear Models (GLMs) were used to compare the responses between temperatures and populations, with Period and SL as covariates and Temperature and Population as factors, as well as assessing the interactions between Period, Temperature and Population. Post hoc tests were carried out, where slopes of individual regression lines were compared in pairwise fashion, within-population groups and within-temperature groups, by testing the significance of the interaction terms in an ANOVA framework (i.e. F-tests of model with interaction term vs. model without interaction term). These tests were performed on corresponding pairwise subsets of the data (e.g. PL1 15 °C and PL1 20 °C for between-temperature test, or PL1 15 °C and UK 15°C for between-population test)

Figure 4 . 4

 44 Figure 4.4 Fitted relationship of total fecundity (full duration of study) of Pseudorasbora parva with Standard Length (SL), with standard errors.

Figure 4 . 5

 45 Figure 4.5 Fitted number of clutches by Pseudorasbora parva population, with standard errors.

(

  GLM: F 2, 281 = 7.16, p < 0.001). The proportion of eggs produced per 25-day period diminished much more rapidly in the PL2 fish than either of the other two populations (GLM: F 1, 141 = 17.67, p < 0.001 and F 1, 181 = 7.30, p = 0.008, for PL1 vs PL2 and UK vs PL2 respectively). The rate of egg production diminished more rapidly with increasing temperature, with post hoc tests confirming a decreasing tendency from 15 °C to 20 °C (GLM: F 1, 181 = 3.59, p = 0.060) and highly significant decreasing tendencies from 15 °C to 25 °C and from 20 °C to 25 °C (GLM: F 1, 201 = 60.56, p < 0.001 and F 1, 186 = 39.18, p < 0.001, respectively).

Figure 4 . 6

 46 Figure 4.6 Slope coefficients, for proportion of Pseudorasbora parva eggs laid over time, by population and temperature treatments, with standard errors.

(Fig

  Fig. 4.7).

Figure 4 . 8

 48 Figure 4.8 Fitted clutch size over time, by Pseudorasbora parva population, with standard errors.

Figure 4 . 9 Fitted

 49 Figure 4.9 Fitted Pseudorasbora parva clutch size over time, by temperature treatment, with standard errors.

Figure 4 Figure 4 .

 44 Figure 4.10 Fitted relationship between Pseudorasbora parva fecundity (25-day aggregated data) and Standard Length (SL), with standard errors.

Figure 4 .

 4 Figure 4.12 Fitted growth rates of fish progeny for all Pseudorasbora parva populations, at all temperature treatments, with standard errors.

  water and 40 L of oxygen (n ≤ 25 in each sack), then transported to Station d'Ecologie Théorique et Expérimentale (SETE), where they remained in the sacks to acclimate to holding-tank conditions for a minimum of 10 hours, before being released into the holding tanks.

  photographed on a gridded background (see Fig.5.2) then individually tagged, using

Figure 5 . 2

 52 Figure 5.2 Photograph of Pseudorasbora parva sample, with digitised landmarks (LMs) indicated and numbered. Morphometrics calculated are: Preorbital distance (1-2); Head length (1-7); Eye diameter (2-4); Postorbital distance (4-7); Head depth (5-6); Body length (7-14); Pectoral fin to pelvic fin (8-10); Body depth (9-10); Pelvic fin to anal fin (10-11); Caudal peduncle length (11-13); Caudal peduncle depth (12-13).

Figure 5 . 4

 54 Figure 5.4 First two linear discriminants from linear discriminant analysis of 11 measured morphometric traits, with colours and ellipses identifying the population groups: Portet-sur-Garonne (POR); Grenade (GRE); Bourret (BOU); Auvillar (AUV); Tonneins (TON); Couthuressur-Garonne (COU).

Figure 5 . 6

 56 Figure 5.6 Activity (distance travelled in 15 minutes) plotted against distance from Pseudorasbora parva invasion front, with standard errors and linear regression line.

Figure 5 . 7

 57 Figure 5.7 Relative importance of parameters in full-averaged model, composed of all fitted models where ∆ AICc < 2. Models terms are: log(SL) (1); LD1 (2); PropFront (3); LD1:PropFront (4); Act_km (5); LD2 (6); Act_km:PropFront (7); LD2:PropFront (8); Act_km:LD1 (9); log(SL):PropFront (10); LD1:LD2 (11).

Figure 5 . 8

 58 Figure 5.8 Full model averaged parameter coefficient estimates (unfilled circles), with adjusted standard errors (heavier lines) and 95% confidence intervals (lighter lines). Parameters containing the variable 'Act_km' are plotted on a separate axis, as the values are an order of magnitude greater than that of other parameters.

  introduction likelihood was added, N. America appears most at risk. In investigating the climatic niches of the lineages, in the native and non-native ranges (Chapter III), I found no evidence to suggest that the genetic lineages represented local adaptations to their respective native climates. There was little or no differentiation of the lineages' climatic niches in the introduced range. It was also apparent, from the climatic comparisons, that the conditions in European range constituted climatic conditions unoccupied in the native range. I found that thermal responses of important life history traits (Chapter IV) were not significantly different between populations from two different climates in the nonnative range; one strongly seasonal continental and the other mild temperate maritime. The overall reproductive output of females did not vary according to breeding season temperature, however, temporal reproductive strategy showed a strong response, with lower temperatures inducing a protracted breeding season and higher temperatures inducing rapid and intense reproductive output. In the dispersal and morphology-related study (Chapter V), I found a strong gradient of morphological change, corresponding with distance from invasion front, demonstrating a high degree of plasticity in morphology. However, this morphology

  the Eastern regions of the USA, my model's suitability predictions are largely concordant with those of the USFWS climate matching assessment (Section 6; USFSW, 2016), however their assessment predicts high levels of climatic match across much of Central and Western Mainland US, where my model predicts low suitability. The USFWS climate matching analysis, using CLIMATCH (Australian Bureau of Rural Sciences 2018), appears to be missing P. parva source locations in the introduced range. Two particular areas where P. parva is present but underrepresented in the USFWS training dataset are northern Central Europe (i.e. Germany and Poland) and parts of the Middle East. The omission of these locations, where P. parva have successfully established, could mean that certain climatic conditions are missing from the characterised climatic niche, used to compare to conditions at weather stations in the target area (i.e. the US). Nonetheless, the CLIMATCH method uses Euclidean distances, based on 16 climate variables, as a means of characterising the climate at included weather stations, which essentially

  , with relatively few accessible examples of native population studies. Comparisons of native and non-native populations could prove essential to identifying what makes P. parva such a successful invader and how it is able to establish populations in novel conditions. For instance, Nolte et al. (2005) compared native populations and non-native populations of Cottus gobio and identified a morphological shift and a new ecological potential, which appeared to facilitate invasion, as a result of hybridization between two genetic lineages of Cottus gobio. They found that the new hybrid invaded locations with environmental conditions beyond the range of occupied conditions previously observed in either genetic lineage.

Table 2 .

 2 1 Description of abiotic descriptor datasets used in analysis and source URLs. Variables in the column entitled code marked with an asterisk were retained for modelling procedures...................................................................................................62Table 2.2 Mean variable importance by modelling method, as a percentage. Variable importance scores, as measured by randomization technique, calculated for individual models as 1-Pearson's correlation between predictions, before and after randomization. Scores were then converted into a % of the sum of all variable importance scores for each modelling method. The scores of the three most important variables in the final EMmw model are highlighted in bold...........................................................................................................................................68

Table 3

 3

.1 GIS layer datasets, used in analysis. Those with a short 'code' were retained for final analysis.................................................................................................................100

Table 3.2 List of entities

  axis, as the values are an order of magnitude greater than that of other parameters............................................................................................................................188

	RÉSUMÉ EXÉCUTIF EN FRANÇAIS
	INTRODUCTION
	Les invasions biologiques posent de sérieux problèmes pour la biodiversité et le Berthou (2005) ont par ailleurs démontrés en se basant sur la liste rouge IUCN que
	fonctionnement des écosystèmes. La dispersion des espèces au delà de leur aire native 54% des espèces listées comme étant en cours d'extinction (et pour lesquelles les
	est une des causes majeures du changement global. La globalisation du commerce et causes étaient connues) étaient concernées par les invasions biologiques. Les
	des transports a largement contribuée à la translocation et la dispersion des espèces invasions biologiques sont reconnues comme une cause majeure des problèmes
	non-natives, que ce soit de façon intentionnelle ou non (Perrings et al., 2005; environnementaux contemporains, comme en atteste le nombre florissant d'études sur
	le sujet

Meyerson & Mooney 2007; Crowl et al., 2008)

. Les voies d'introduction d'espèces non-natives sont nombreuses et au delà des introductions intentionnelles, elles peuvent être la conséquence de la contamination des biens, des gens ou des animaux d'élevages par des pathogènes, de la dispersion naturelle des organismes ou des changements environnementaux majeurs (par ex. construction de ponts, de tunnels, de canaux…).

Les impacts écologiques générés par l'introduction d'espèces non-natives peuvent se manifester de différentes façons. Une espèce introduite peut rentrer en compétition avec les espèces natives pour une ressource limitée (territoires, nourritures…) et conduire à un déplacement des espèces natives, voir à une extinction locale ou régionale. Par ailleurs, les espèces non-natives peuvent agir comme nouveaux vecteurs de maladies émergentes ou amplificateurs de maladies locales. Les impacts des espèces non-natives peuvent se révéler à l'échelle des individus, mais aussi à l'échelle de l'écosystème dans sa globalité. Par exemple, certaines plantes invasives peuvent affecter les concentrations en carbone ou en azote du sol

[START_REF] Liao | Altered ecosystem carbon and nitrogen cycles by plant invasion: A meta-analysis[END_REF]

et d'autres organismes non-natifs peuvent modifier complètement l'habitat physique en ayant un rôle « d'ingénieur »

[START_REF] Davis | Invasion Biology[END_REF]

.

L'introduction d'espèces non-natives est responsable de pertes économiques majeures (équivalente à 5% du Produit Interieur Brut global) et conduit à des pertes de l'ordre de 20% de la production alimentaire globale

[START_REF] Nentwig | Biological Invasions[END_REF]

.

Clavero & Garcia-

  depuis sont introduction en Europe dans les années 1960. En plus des impacts négatifs directs que cette espèce peut avoir sur les populations natives de poissons, P. parva peut transmettre une maladie infectieuse émergente qui peut provoquer la mort de nombreuses autres espèces. Afin de quantifier les risques d'invasion de cette espèce dans les régions ou l'espèce pourrait être introduite, j'ai développé des modèles dit « de niches » qui m'ont permis de construire un modèle d'ensemble prédisant l'adéquation de l'enveloppe climatique de chaque région étudiée et un proxy pour la probabilité d'introduction. Ces modèles ont montrés que

Pouvoir prédire les régions à fort risque d'introductions d'espèces non-natives est un enjeu fondamental pour mettre en place des actions de prévention efficaces et des mesures de gestion des risques appropriées. P. parva est rapidement devenu une espèce très invasive P. parva pourrait envahir de nombreuses régions actuellement non concernées par son introduction. Cela inclue des régions dans plusieurs continents, avec en particulier la mise en évidence de points chauds d'adéquation climatique et de risque d'introduction. Ces modèles sont facilement adaptables pour de nombreuses autres espèces envahissantes et les cartes de risques en résultant peuvent être utilisées par les gestionnaires et les décideurs pour accroitre les surveillances dans les régions concernées et mettre en place des systèmes de « early-warming » permettant d'éviter des introductions et invasions futures.

  Le tôt de croissance précoce des rejetons était plus élevé à des températures élevées et en particulier significativement différent en les températures de 25 °C et de 15 °C. Ces résultats ont d'importantes implications pour la gestion des espèces de poissons introduites, particulièrement d'un point de vue de la saisonnalité des mesures d'interventions. Ceci suggère que les études futures combinent des facteurs liés à la température et à la disponibilité en nourriture pour développer nos connaissances relatives quant à la façon dont ces facteurs intéragissentpour influencer le succès d'invasion. Au cours de ces expériences, j'ai montré que le seul prédicteur significatif de la propension à disperser était la taille corporelle des poisson qui cependant ne variait pas entre les populations. Bien que nous ayons put mettre en évidence une cline morphologie le long du gradient d'invasion, nous n'avons trouvé aucun lien significatif entre ces variations morphologiques et la propension à disperser. Les différences morphologiques étaient principalement liées à la taille relative de la tête et

	variations morphologiques liées à la distance au front d'invasion, ce qui suggère un
	fort degré de plasticité phénotypique par P. parva dans la zone envahie. Bien que ces
	variations n'étaient pas liées significativement à la dispersion des individus, cette
	plasticité pourrait avoir une influence sur le potentiel invasif de cette espèce.
	Davantage d'études devraient être menées car ces variations morphologiques
	pourraient être liées aux comportements de nutrition et ainsi avoir des effets sur la
	faune locale via un chevauchement trophique. De plus, cela indiquerait que ces
	mesures morphologiques pourraient être utilisées pour différencier les populations en
	cours d'expansion des populations plus âgées, ce qui pourrait être utile afin de
	proposer des plans de gestions et d'éradication pertinents.

La gestion des espèces non-native basée sur l'analyse des risques dépend de notre compréhension de leur potentiel d'invasion, qui est lui même largement déterminée par l'expression des traits d'histoire de vie (THV) dans leur milieux d'accueil.

Comprendre comment les THV varient avec les conditions environnementales permet de mieux prédire le potentiel invasif des espèces. Après un phénomène d'introduction, un prérequis pour que les individus fondateurs survivent (puis s'établissent) est que les conditions environnementales locales permettent l'expression de THV facilitant l'établissement

[START_REF] Moyle | Fish invasions in California: do abiotic factors determine success[END_REF]

. Pour les poissons d'eau douce, ces conditions dépendent beaucoup du climat et -dans les habitats lentiques-en particulier de la température

[START_REF] Crozier | Plastic and evolutionary responses to climate change in fish[END_REF]

. Ainsi, là ou de nombreuses populations introduites d'espèces non-natives ce sont adaptées aux conditions climatiques du milieu d'accueil (e.g. régime de température,

Kavanagh et al., 2010)

, cela indiquerait que la provenance des populations pourrait fortement influencer le potentiel invasif dans n'importe quelle autre localité secondaire d'introduction ou de translocation (e.g.

[START_REF] Rey | Where do adaptive shifts occur during invasion? A multidisciplinary approach to unravelling cold adaptation in a tropical ant species invading the Mediterranean area[END_REF]

. Dans ce chapitre, j'ai réalisé une expérience en laboratoire afin de quantifier les réponses liées à la capacité reproductrice d'adultes et à la croissance de leurs rejetons à trois températures (15 °C, 20 °C and 25 °C) chez P. parva. Afin de tester l'adaptation locale, j'ai comparé la réponse thermique des adultes et de leur rejetons provenant de trois populations non-natives de deux pays et représentatives de deux climats très contrastés ; climat continental froid de Pologne et climat tempéré maritime du Royaume-Uni. Ces expériences n'ont pas permis de mettre en évidence de différentiation significative pour ce qui est de la réponse thermique entre les différentes populations, suggérant une absence d'adaptation au climat local. Nos résultats suggèrent que la taille du corps, pas la température, détermine le rendement reproducteur brut des femelles sur l'ensemble de la saison de reproduction, avec les individus les plus grands qui étaient les plus féconds. Cependant, la température affectait la stratégie reproductive temporelle, avec des rendements reproductifs prolongés et moins intenses à des températures basses (et au contraire plus rapides et avec des pics plus élevés à des fortes températures). SYNTHESE DU CHAPITRE V Le processus d'invasion biologique est caractérisé par un certain nombre d'étapes, chacune d'elle est liée à une barrière pouvant limier le succès d'invasion. Les caractéristiques (i.e. les traits) liées à la dispersion sont un des facteurs pouvant déterminer la capacité des espèces à franchir ces barrières à l'invasion. Il y a en effet de plus en plus d'évidence que les populations situées sur les fronts d'invasion sont phénotypiquement différentes par rapport à des populations établies depuis longtemps et une partie de ces différences impliquent des traits liés à la dispersion. La dispersion est souvent liée à des traits morphologiques, mais peut aussi être dépendants de traits comportementaux. Ces traits peuvent former des ensembles cohérents comme par exemple des « phénotypes dispersants » dont la prévalence devrait diminuer avec l'éloignement au front d'invasion. La présence de ces phénotypes sur les fronts d'invasions pourrait être un déterminant majeur du succès d'expansions et donc de l'invasibilité d'une espèce. Dans ce chapitre j'ai échantilloné six populations de P. parva le long d'un gradient d'invasion (de 0 à 210 km depuis le front d'invation ) le long de la Garonne en France. J'ai utilisé une approche expérimentale en mésocosme pour quantifier la propension à disperser, le niveau d'activité ainsi que la morphologie générale de chaque population. J'ai testé si des covaritions existaient entre ces traits, si elles variaient entre les populations et par rapport à la distance au front d'invasion. au diamètre des yeux, ce qui pourrait indiquer un shift dans la diète ou le mode de vie le long du gradient d'invasion. Nos résultats ont montré que les individus les plus gros étaient ceux qui étaient le plus apte à disperser. Les individus les plus gros sont aussi les plus féconds, ce qui pourrait permettre à P. parva d'étendre rapidement son aire de répartition et à coloniser de nouveaux habitats plus rapidement qu'au hasard. Cependant, d'autres facteurs pourraient influencer la dispersion, comme par exemple la sociabilité et les effets densité-dépendances. CONCLUSIONS Cette thèse a permis de générer une carte mondiale des risques d'invasion de P. parva, ce qui a démontré que l'aire potentielle d'invasion de cette espèce est extrêmement grande, en particulier sur les continents Sud et Nord Américains. Bien que les résultats de cette thèse ne suggèrent pas que ces prédictions soient particulièrement affectées par les différentiations observées aux échelles des lignées ou des populations, j'ai pus démontré qu'un certain degré d'incertitudes dans ces prédictions persistait, principalement du fait que la distribution native ne prédisait pas correctement la distribution actuelle. Néanmoins, en combinant les données de distributions connues dans les aires natives en non-natives, ces incertitudes étaient plus faibles qu'en utilisant uniquement les données disponibles dans l'aire native. L'absence d'évidence pour de l'adaptation locale (Chapitres III et IV) devraient être complémentés par des expériences impliquant des populations de l'aire native ainsi que des populations non-natives sélectionnées a priori. et qui présenteraient des différences quant à l'enveloppe climatique dans lesquelles elles vivent. Un examen plus approfondi des zones inoccupées de l'aire natives, correspondant à des conditions climatiques appropriées, pourrait nous aider à mieux comprendre les facteurs limitants importants pour P. parva (par ex. facteurs biotiques, facteurs abiotiques à l'échelle locale) et à identifier de nouveaux moyens de gérer les populations non-natives. Cette thèse a également permis de montrer de très fortes

Table 2

 2

	.1 for individual

Table 2 .2 Mean variable importance by modelling method, as a percentage. Variable importance scores, as measured by randomization technique, calculated for individual models as 1-Pearson's correlation between predictions, before and after randomization. Scores were then converted into a % of the sum of all variable importance scores for each modelling method. The scores of the three most important variables in the final EMmw model are highlighted in bold.

 2

	Model	Bio1	Bio2	Bio3	Bio8	Bio14 Bio18 Bio19 TpH	INMSR TWIS
	ANN	12.50	4.33 8.96	6.79 30.11	12.43	16.46	0.42	2.54 5.44
	GLM	3.05	2.89 22.36	6.63 41.50	1.17	5.00	0.79	4.37 12.24
	GBM	4.36	2.42 25.89	1.65 46.73	1.63	1.08	0.03	2.05 14.16
	SRE	10.08 10.12 10.11	10.45 7.28	9.60	9.37 10.52	11.25 11.22
	CTA	9.87	5.64 20.10	4.67 34.33	5.24	3.60	0.60	3.52 12.43
	RF	12.62	5.62 24.48	3.98 18.69	6.21	6.15	2.51	7.86 11.88
	MARS	7.50	5.48 17.18	5.25 41.93	1.26	6.96	0.10	3.10 11.24
	FDA	7.48	3.50 22.83	5.42 35.04	1.42	4.70	0.38	6.70 12.54
	GAM	7.85	4.01 30.64	6.74 15.49	3.75	11.60	1.27	5.61 13.05
	EMmw	8.15	4.23 21.60	5.13 32.94	4.13	6.93	0.77	4.48 11.64

Table 3 .2 List of entities compared and respective data used to calibrate each of the PCA-Env runs.

 3

	Entity a	Entity b	Calibration data
	NNL	NSL	Native
	INL	ISL	Introduced
	NNL	INL	Native + Introduced
	NSL	ISL	Native + Introduced
	Nat	Int	Native + Introduced
	Broennimann et al., 2012). The regions of interest (native and introducedranges) were
	delineated as all cells within a 1000 km buffer around occurrence data. The first two
	principal components (PCs) were then divided into a 100 by 100 grid, each cell
	representing a unique set of environmental conditions. 'Smoothed' occurrence
	densities (entities), in reduced-dimensionality environment-space (i.e. PC1 and PC2,

I calculated three commonly used metrics of niche comparisonoverlap

(Schoener's D, Schoener, 1968; Warren et al., 2008)

, equivalency and similarity -following the Kernel Smoother ('KS') methodology of

Petitpierre et al. (2012)

and

Broennimann et al. (2012)

. This methodology relies initially on calibrating a PCA on the entire environmental data for the region, or regions, of interest (i.e. 'PCA-Env' in

Table 4 .1 Names, locations and climatic descriptors for the three sampled populations. Climatic descriptors were extracted from Hijmans et al. (2005) 'bioclim' dataset, based on mean monthly temperatures, but converted to °C where necessary. Seasonality is calculated as the standard deviation multiplied by one hundred.

 4

	Population Location	Mean Seasonality Max Min	Range
	PL1	N: 51 42`54.45", E: 19 30`46.99"	7.92	783.74 21.81 -8.70	30.51
	PL2	N: 52 16`3.9", E: 21 17`42.11"	8.13	793.52	22.5 -8.51	31.01
	UK	N: 51 00`00.1", W: 1 27`01.1"	10.48	449.03 20.34	0.68	19.67

  PropFront' and log(SL) were used in place of distance from invasion front (km) and SL in order to maintain similar range magnitudes for all the predictor variables, as this aids convergence of the models. Prior to the modelling phase of this analysis, tests for collinearity between independant variables were carried out, using Variance Inflation Factor (VIF), Corrected VIF (CVIF), Tolerance, Klein's and Leamer's methods. All tests indicated no significant degree of collinearity that would contraindicate inclusion of any of the five variables. I used the 'dredge' function, in MuMIn, to construct, fit and rank by AICc, models containing all combinations of the five predictor variables, including all two-way and three-way interactions. All models also contained a random intercept term, structured by experimental system used and experimental run (i.e.

	LD2 distance from invasion front); 'LD1' and 'LD2' (first two linear discriminants). 30.3%
	TON
	AUV
	BOU
	POR	LD1
	COU	39.6%
	GRE
		Prop. of trace
	LDA

'

1|Sys/Run), to account for pseudoreplication. I then discarded all those models that had a ∆ AICc of > 2, or failed to converge. I calculated estimates, standard errors, 95% confidence intervals and relative importance of the model parameters by full model averaging, based on this subset of 29 best models (i.e. ∆ AICc of ≤ 2).

Table 5 .1 Proportion of between-group variance explained by each of the five linear discriminants.

 5

	LD1	LD2	LD3	LD4	LD5
	39.6% 30.3% 16.4% 12.0% 1.7%

Table 5 .2 Coefficients of linear discriminants for LDA of 11 morphometric characteristics of Pseudorasbora parva, using population location as grouping factor.

 5

	Description	LD1	LD2
	SL minus head length	0.00 0.02
	Preorbital distance	5.57 -4.12
	Eye diameter	11.24 -3.34
	Head depth	-0.58 0.17
	Postorbital distance	6.32 -7.42
	Pectoral fin to pelvic fin	0.14 0.15
	Pelvic fin to anal fin	0.19 0.07
	Depth of caudal peduncle	0.78 1.01
	Body depth	0.18 -1.27
	Head length	-7.99 5.90
	Anal fin to lower caudal fin	0.08 -0.04

OBJECTIVES OF THE THESIS

The main objective of this thesis is to address the prediction of invasion risk for a major invasive species, by using a multi-tiered approach, combining different tools to consider the implications of adaptations at varying spatial and temporal scales. The process was broken down into four stages, each stage addressing one of the following specific questions: (i) What is the global scope for invasion risk under the null model that there is not intraspecific adaptation? (ii) Is there evidence for intraspecific adaptation at the level of lineages? (iii) Is there evidence for intraspecific contemporary adaptation, at the population level? (iv) Is there evidence of intraspecific adaptation at the invasion front?

CHAPTER II

Predicting global invasion risks: a management tool to prevent future introductions.

ABSTRACT

Predicting regions at risk from introductions of non-native species and the subsequent invasions is a fundamental aspect of horizon scanning activities that enable the development of more effective preventative actions and planning of management measures. The Asian cyprinid fish topmouth gudgeon Pseudorasbora parva has proved highly invasive across Europe since its introduction in the 1960s. In addition to direct negative impacts on native fish populations, P. parva has potential for further damage through transmission of an emergent infectious disease, known to cause mortality in other species. To quantify its invasion risk, in regions where it has yet to be introduced, I trained 900 ecological niche models and constructed an Ensemble Model predicting suitability, then integrated a proxy for introduction likelihood. This revealed high potential for P. parva to invade regions well beyond its current introduced range. These included areas in all modelled continents, with several hotspots of climatic suitability and risk of introduction. I believe that these methods are easily adapted for a variety of other invasive species and that such risk maps could be used by policy-makers and managers in hotspots to formulate increased surveillance and early-warning systems that aim to prevent introductions and subsequent invasions. parva (Scott et al., 2007) and inclusion of this aspect could help refine model predictions for this species. Soil salinity could be a good indicator (due to surface water runoff,) of local water salinity properties [START_REF] Venkatesan | Systems dynamic model to forecast salinity load to the Colorado River due to urbanization within the Las Vegas Valley[END_REF]; however, a search revealed that most available soil salinity datasets at a global extent consist of either qualitative (e.g. the Harmonized World Soil Database 'Excess salts' dataset of [START_REF] Fischer | Global Agro-ecological Zones Assessment for Agriculture (GAEZ 2008)[END_REF] or semi-quantitative categorical variables (e.g. the 'Global saltaffected soils' map of [START_REF] Wicke | The global technical and economic potential of bioenergy from salt-affected soils[END_REF], which limit the choice of feasible modelling techniques [START_REF] Pearson | Species' distribution modeling for conservation educators and practitioners[END_REF]. Instead, I chose to include Topsoil pH, which was available at a global extent in a suitable continuous floating-point value format.

Although it is not a universally applicable surrogate for salinity, it does correlate strongly in regions of calcareous soils [START_REF] Al-Busaidi | Salinity-pH relationships in calcareous soils[END_REF]. assumptions are: i) assumed niche conservatism, ii) genetic and phenotypic uniformity over space and time (see Jeschke & Strayer, 2008 for a more exhaustive list of assumptions). One direct consequence of these assumptions is that such models often underestimate the size of the non-native range (Fitzpatrick et al., 2007;Loo et al., 2007;Broennimann & Guisan, 2008;Medley, 2010;Hill et al., 2013), as the projected realised niche (only a sub-set of the fundamental niche) is not always conserved in the introduced range (Rödder & Lötters, 2009;Guisan et al., 2014). The realised niche in the introduced range can represent an addition to and/or shift in the environment-space defining the realised niche in the native range. However, these modelling methods necessarily use the realised niche, based on presence location data, as there is insufficient data to model the fundamental niche.

Species with a large native distribution across a range of habitats and climates often include a high degree of genetic and/or phenotypic variation that corresponds to regional or historical adaptations [START_REF] Oberdorff | Global and regional patterns in riverine fish species richness: a review[END_REF]. Where this is the case, the source location of the introduced individuals may have a direct effect on their ability to survive and reproduce in the conditions present at introduction sites. For example, many historical attempts to introduce Pacific pink salmon (Oncorhynchus gorbuscha)

to the European North of Russia (the White and Barents Sea basins) failed, assumed to be a result of the selection of southern native populations (mainly from around Sakhalin Island) as the source of the introductions [START_REF] Gordeeva | Experimental microevolution: transplantation of pink salmon into the European North[END_REF].

When individuals from a more northerly (several degrees higher latitude) source population were introduced in 1985 the fish established a self-reproducing population [START_REF] Gordeeva | Genetic differentiation of Pacific pink salmon during colonization of a new area[END_REF], with adult returns reaching 60,000 in just four years [START_REF] Gordeeva | Experimental microevolution: transplantation of pink salmon into the European North[END_REF]. It follows then that such local or regional adaptations to climatic conditions can have a profound effect on the potential collinearity. The final refined dataset comprised 11 variables, which were given short codes (X1 -X11), for ease of labelling (see Table 3.1 for codes). All variables were used at a spatial resolution of 10 x 10 km.

Aquarium facility and initial holding conditions

The aquarium facility at Bournemouth University, Southern England, comprised of a discrete room where light: dark cycle and water temperatures could be controlled. The fish were initially held in 90 L tanks, arranged in a column of three tanks on a recirculating flow through filtration system (2.5 L/min) for the maintenance of water quality. In-line refrigeration units and heaters were used to control and maintain water temperatures. Aquaria and pipes were insulated to minimise temperature drop across systems, which was maintained at a differential, from flow entry to exit, of 0.5 °C (see Fig. 4.1). In order to be able to measure the subsequent reproductive output of the adult P.

parva, they had to experience an initial 'winter' period for gonadal development.

Data analyses

Reproductive effort was assessed using two-way analysis of covariance (ANCOVA) that tested differences in the three response variables (i.e. total clutches laid, mean clutch size and total eggs laid) across the treatments (i.e. temperatures, populations).

Data on 'total eggs laid' were log-transformed, as they violated assumptions of normality in their raw state. The effect of fish size was controlled using the mean standard length (SL) of the female fish as a covariate (after Almeida et al., 2016).

These were followed by post hoc Tukey-Kramer honest significant difference (HSD)

tests. Assumptions of normality and homogeneity of variance were verified using Shapiro-Wilk and Levene's tests, respectively.

In order to assess temporal differences in reproductive effort across temperatures and populations, data were aggregated into 25-day laying periods (i.e. a total of five 25day periods for the 125 day observed laying season). Aggregation by 25-day periods was chosen in order to smooth out the variation present at higher temporal resolutions, and because the total observation period, 125 days, divided into a whole and manageable number of periods, i.e. five, for robust statistical analyses.

Furthermore, studies of temporal gonadosomatic index (GSI) variation in P. parva (e.g. [START_REF] Yan | Variations in reproductive strategies between one invasive population and two native populations of Pseudorasbora parva[END_REF]Zahorska & Kovac, 2013;Zahorska et al., 2014) indicate that general shifts in GSI in the wild are captured at this level of temporal resolution.

Before analysis, 'number of eggs laid' and 'number of clutches' were transformed into proportion of total laid (i.e. quantity laid in each 25-day period, expressed as a fraction of quantity laid over entire duration of trial) in order to allow the shape of the relationship with time (i.e. period) to be assessed. 'Clutch size' data were also aggregated into the 25-day periods, to give a total of four response variables; (i) Proportion of total eggs laid, (ii) Proportion of total clutches laid, (iii) Mean clutch

RESULTS

Reproductive effort

Across the three populations, there were significant differences in the size of clutches laid (ANCOVA: F 2, 49 = 5.56, P < 0.01), but Temperature and SL were not significant predictors of clutch size (F 2, 49 = 0.17, p = 0.84 and F 1, 49 = 1.15, p = 0.29, respectively), including the interaction term between Population and Temperature (F 3, 49 = 0.30, p = 0.82). Pairwise post hoc tests showed that mean clutch size (see Fig. Tests on log-transformed data showed that total fecundity, over the entire duration of the trials, was not significantly different between temperature treatments, or populations (ANCOVA: F 2, 49 = 1.43, p = 0.25 and F 2, 49 = 0.72, p = 0.49, respectively). Nor was there a significant interaction between temperature and population (ANCOVA: F 3, 49 = 0.60, p = 0.62). Fecundity was, however, significantly There was a borderline significant interaction between Period and Population (GLM:

F 2, 281 = 2.99, p = 0.052) and a significant interaction between Period and Temperature (GLM: F 2, 279 = 13.24, p < 0.001). Post hoc tests showed that clutch size declined with increasing temperature, significantly so between 15 °C and 25 °C (GLM: F 1, 202 = 18.18, p < 0.001) and between 20 °C and 25 °C (GLM: F 1, 187 = 11.20, p < 0.001). Clutch sizes in UK fish were significantly higher than PL1 (GLM:

F 1, 245 = 5.90, p = 0.016), but not significantly higher than PL2 (GLM: F 1, 180 = 2.25, p = 0.14).

The rate at which clutch size diminished over time (i.e. steepness of slopesee Fig.

Eggs per clutch SL (mm)

There would be considerable merit in further studies to investigate the interactive effects of temperature and food ration of P. parva adults on reproductive output and growth of progeny, particularly if a reliable protocol could be developed for a surrogate of nutritional status, such as those discussed in [START_REF] Congleton | Blood-chemistry indicators of nutritional status in juvenile salmonids[END_REF]. Such a metric of nutritional status, or condition, could then potentially be used in field studies to quantify the parameter for studies on individuals or populations.

Once these factors that are testable in a controlled environment are investigated and the corresponding relationships quantified, it will make in situ studies easier to interpret, as we will then know the combined effects of, for instance, temperature and food rations.

CONCLUSION

Whilst the present study did not identify the temperature at which reproductive output exhibited suppression, it did confirm the broad tolerances of P. parva and highlight its ability to maintain gross reproductive output across a wide range of simulated summer temperatures (i.e. 15-25 °C). The study also identified the strong link between temperature and temporal reproductive output strategy, which could have implications for the management of detected introductions with respect to both eradication and dispersal prevention, as protracted breeding seasons may increase the risks of fluvial larval dispersal to secondary locations. Furthermore, where temperatures are optimal for rapid growth -25 °C in this studyreproductive output is short and intensive; meaning that utilization of limited warmer conditions is maximized, potentially leading to higher likelihood of winter survival and reaching sexual maturity in time for the following breeding season.

CHAPTER V

Biological drivers of fish dispersal during an invasion process.

ABSTRACT

The process of biological invasion has a number of distinct stages, each containing barriers to invasion success. Dispersal characteristics have been suggested as important factors in determining success at overcoming these barriers. There is increasing evidence that populations on expanding range fronts are phenotypically different from longer-established populations, with part of these differences being related to dispersal characteristics. Dispersal ability is often linked to morphological traits, but can also be connected to behavior and together these factors can combine to form a dispersing phenotype (dispersal-type), which will diminish in prevalence with distance from invasion front. The presence of such dispersal-types at range edges can be a strong determinant of range expansion success and hence invasiveness. Here I use six populations of an invasive fish, Pseudorasbora parva, spread along the River Garonne in France, at incremental distances from the invasion front (from 0 -210 km). I experimentally quantified dispersal propensity in mesocosms and I measured activity levels and characterized morphology in all six populations. I tested for covariation patterns in measured traits, relative to one another and to distance from invasion front. The only significant predictor of dispersal propensity in the present study was fish size and this did not vary significantly with distance from invasion front. Although I found a strong morphological cline, concordant with distance from invasion front, I found no significant link between this morphological variation and dispersal propensity or activity levels. The morphological differences were predominantly explained by relative head-size and eye diameter, which could indicate a shift in diet, or mode of life. These results showed that larger individuals have a higher propensity to disperse. As larger individuals are generally more fecund, this could enable P. parva to expand its range and recolonize new areas more quickly than

MATERIALS & METHODS

Sampling methods and locations

Fish were sampled using electrofishing at a total of six sampling locations along the River Garonne (see Fig. Each fish received a unique two-tag combination from a choice of four coloured elastomer tagsone on each side of the base of the dorsal finand each group of eight fish were returned to a separate cage in holding pools, organised by source populations.Captured photographs were processed in tpsDig software [START_REF] Rohlf | TpsDig, ver. 2. 16[END_REF], where 14 landmarks (see Fig. 5.2) were digitised, and in ImageJ software (Schneider et al., 2012), to calibrate lengths against the gridded background. Digitisation was carried out on photographs in a randomised order, so as to avoid bias associated with the sequence of specimens. The tps file was imported into R v.3.3.2 (R development core team 2015), where the 11 morphometric characteristics (see Fig. 5.2) were measured from the landmarks and standardised using standard length (SL), to provide shape characteristics of individual fish.

Dispersal propensity

Dispersal assays were conducted in custom-built experimental systems. Each system comprised three 1100 L circular MDPE water tanks, linked in a linear configuration (Fig. 5.3) by 1m lengths of 110 mm diameter PVC pipe at 35 cm height. (0) (4) (2)

(3) (1)

Activity

Likelihood ratio tests on the GLMs showed that 'Activity' was not significantly associated with distance from invasion front (c 2 1 = 0.49494, P = 0.4817, see Fig.

CHAPTER VI GENERAL DISCUSSION

APPROACH

After the initial summary section, I will approach this discussion by looking at the data chapters in sequence, but combining the Chapters II and III, as they both relate to climatic adaptations. I will first consider the relevance of the main findings and their practical implications, then opportunities for improvement and possible future research directions. I will finish by giving a brief personal perspective.

SUMMARY

In this thesis I set out to quantify the risk of invasion from the invasive freshwater fish, Pseudorasbora parva, at a global extent, using traditional correlative ecological niche modelling approaches. These correlative approaches rely upon key assumptions relating to the presence or absence of local or regional adaptations, and so I subsequently tested for evidence of such adaptations in genetic lineages and individual populations, by analysing climatic differentiation in the native and introduced ranges (Chapter III) and by conducting lab experiments comparing thermal responses of important life history traits (Chapter IV). The initial risk assessment (Chapter II) carried out did not account for a key factor in invasions; namely natural dispersal. Natural dispersal has been observed to be subject to selection in vanguard populations of invasive species, and adaptation of dispersal traits can infer additional invasive vigour, allowing the species to spread across the landscape quicker. For example, in the Australian cane toad invasion, a fivefold increase in rate of spread over 70 years was attributed to genetic dispersal adaptations in individuals at the expansion front [START_REF] Phillips | Evolutionarily accelerated invasions: the rate of dispersal evolves upwards during the range advance of cane toads[END_REF]. I quantified dispersal, activity and morphological differences, often associated with differential dispersal ability, in six populations along a distance-gradient from an invasion front, in order to identify if P.

was not significantly related to propensity to disperse in my experiments and nor were activity levels. Size was the only significant explanatory factor in dispersal in my study.

RISK MAP

The fact that North America is predicted to be at high risk from invasion of P. parva, seems wholly reasonable given the history of this region with regard to aquatic invasions. A number of Ponto-Caspian species have invaded both European and North American inland waters (e.g. Hemimysis anomala - [START_REF] Audzijonyte | Tracing recent invasions of the Ponto-Caspian mysid shrimp Hemimysis anomala across Europe and to North America with mitochondrial DNA[END_REF] and various others -see [START_REF] Leppäkoski | The meltdown of biogeographical peculiarities of the Baltic Sea: the interaction of natural and man-made processes[END_REF], demonstrating both the similarity in climatic conditions between these regions [START_REF] Mills | Exotic species and the integrity of the Great Lakes[END_REF] and the high incidence of introductions from Eurasia to N. America. Studies of biological invasions of N. American inland waters have shown the possible impacts of aquatic invasions are severe, both in terms of biodiversity and in economic terms [START_REF] Vanderploeg | Dispersal and emerging ecological impacts of Ponto-Caspian species in the Laurentian Great Lakes[END_REF]Pimentel et al., 2005). Aside from the direct impacts of invasions, the increased number of introduced species in the N. American Great Lakes region is possibly facilitating further invasions, making it increasingly easy for new introductions to establish [START_REF] Ricciardi | Facilitative interactions among aquatic invaders: is an" invasional meltdown" occurring in the Great Lakes?[END_REF]. Further, a successful invasion by P.

parva would bring with it the associated issue of this fish being a healthy carrier of Sphaerothecum. destruens, a pathogen previously observed to have had severe impacts on North American Salmonid populations (Gozlan et al., 2005;Arkush et al., 2003). Established P. parva populations could act as major reservoirs of the pathogen, increasing likelihood of outbreaks [START_REF] Peeler | Nonnative aquatic animals introductions have driven disease emergence in Europe[END_REF], Ercan et al., 2015).

having small, genetically depauperate founder populations and generally speaking, the literature says that small populations, bottlenecks and generally low genetic diversity should preclude adaptation, or at least provide little opportunity for it to occur [START_REF] Allendorf | Introduction: population biology, evolution, and control of invasive species[END_REF]. Specialist, or local adaptations typically have tradeoffs, so broader tolerances could suffer as a result for attenuated fitness in specific conditions (Hereford, 2009). If the conditions are tolerable without adaptation, then adaptation is unlikely to occur in the short-term, as there is a lack of selective pressure.

The fact that the native and non-native climatic niches did not overlap significantly implies that for some species the current range is not necessarily a good predictor of the potential range (see Chapter I). Although this phenomenon is not in itself a particularly remarkable or rare observation (Fitzpatrick et al., 2007;Loo et al., 2007;Hill et al., 2013), it does highlight that predicting invasive potential using such climatic niche-based approaches (i.e. ENMs) is prone to significant uncertainty.

Further, species with large native ranges, which are not overtly constrained by extensive physical geography are typically thought to realize much of their fundamental niche (Early & Sax, 2014), so predictions based on native distributions should be good. In the case of P. parva, this means that the lack of niche overlap between native and introduced ranges is perhaps uncharacteristic, given the sprawling nature of its native range, both in terms of spatial extent and also in terms of climatic breadth. This raises two important implications: (i) if the potential niche was greater than the realised niche, prior to invasion, then there is a possibility that the potential is yet greater than predicted in my risk maps, meaning that wider areas are potentially at risk; (ii) other factors, not captured in the predictor dataset, must be important in determining the distribution of P. parva. With this in mind, the data available on the individuals were likely being forced to disperse out of areas of higher population density by larger, aggressive males (Brownscombe & Fox 2012).

CONCLUSIONS

This work provides a global scale invasion risk map for P. parva, which shows significant scope for further spread of the established range, particularly in areas of North and South America. Whilst I found no evidence to suggest that these predictions are hampered by differentiation in responses at either lineage or population levels, the findings of Chapter III do highlight the uncertainties surrounding the degree of conservatism in such predictions, mainly owing to the fact that past distribution does not accurately represent the current non-native distribution.

Nonetheless, using the introduced and native distributions, as in Chapter II, does mean that predictions are less conservative than those based purely on the native range. However, further investigation of the native distribution of P. parva and the factors limiting its occupation of seemingly suitable climatic conditions (i.e.

those corresponding with conditions occupied by European populations) could provide information useful for providing more accurate predictions of potential distributions and for the control of already established populations.

The results of Chapter V show a strong morphological shift with distance from a spreading invasion front, which emphasizes the high degree of phenotypic plasticity exhibited by P. parva in an invasion context. Although this was not significantly linked to dispersal in my experimental systems, this plasticity could have an impact on invasiveness. Further investigation of this strong pattern is warranted, as it could