Thomas Bodey

Elena Angulo

Alok Bang

Céline Bellard

Jean Fantle-Lepczyk

Bernd Lenzner

Anna Turbelin

Yuya Watari

Franck Courchamp

The economic cost of protecting islands from invasive alien species

Keywords: biodiversity, economic impact, government, InvaCost, overseas territory, political geography, socio-economic

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Alien species, species that have been distributed beyond their natural range through human agency, are one of the driving forces of the restructuring of global and regional species communities [START_REF] Capinha | The dispersal of alien species redefines biogeography in the Anthropocene[END_REF][START_REF] Russell | Invasive alien species on islands: Impacts, distribution, interactions and management[END_REF], IPBES 2019). A subset of these alien species, so termed invasive alien species (IAS), can have severe, multi-faceted impacts in their novel environments [START_REF] Reaser | Ecological and socioeconomic impacts of invasive alien species in island ecosystems[END_REF][START_REF] Blackburn | A proposed unified framework for biological invasions[END_REF], Diagne et al. 2021a). IAS regularly cause ecological devastation within island ecosystems, and are a leading cause of native species extinctions [START_REF] Blackburn | Avian extinction and mammalian introductions on oceanic islands[END_REF][START_REF] Bellard | Global patterns in threats to vertebrates by biological invasions[END_REF][START_REF] Doherty | Invasive predators and global biodiversity loss[END_REF]), a particular problem in these hotspots of global biodiversity with high endemism and evolutionary distinctiveness [START_REF] Whittaker | Island Biogeography: Ecology, Evolution, and Conservation[END_REF][START_REF] Kier | A global assessment of endemism and species richness across island and mainland regions[END_REF][START_REF] Fernandez-Palacios | Scientists' warning -The outstanding biodiversity of islands is in peril[END_REF]. IAS also severely degrade ecosystem integrity (e.g. loss of ecosystem services such as inshore productivity [START_REF] Graham | Seabirds enhance coral reef productivity and functioning in the absence of invasive rats[END_REF]); cause declines in local economies (e.g. through crop damage or reduced availability of wild food stocks [START_REF] Naylor | Invasions in agriculture: Assessing the cost of the golden apple snail in Asia[END_REF][START_REF] Ballew | Invasive lionfish reduce native fish abundance on a regional scale[END_REF]); and can have significant impacts on human health by increasing welfare costs and hospitalisations [START_REF] Mwebaze | Economic valuation of the influence of invasive alien species on the economy of the Seychelles islands[END_REF]. These severe impacts are particularly problematic as, simultaneously, islands are global hotspots of invasion [START_REF] Bellard | Global patterns in threats to vertebrates by biological invasions[END_REF], 2017, Dawson et al. 2017[START_REF] Turbelin | Mapping the global state of invasive alien species: patterns of invasion and policy responses[END_REF], and have experienced dramatic increases in alien species richness [START_REF] Seebens | No saturation in the accumulation of alien species worldwide[END_REF][START_REF] Moser | Remoteness promotes biological invasions on islands worldwide[END_REF]. Increased recognition of the importance of islands to global biodiversity has resulted in substantial IAS management efforts and expenditure on islands worldwide [START_REF] Veitch | Island invasives: eradication and management[END_REF][START_REF] Russell | Invasive alien species on islands: Impacts, distribution, interactions and management[END_REF][START_REF] Bellard | A global picture of biological invasion threat on islands[END_REF][START_REF] Veitch | Island invasives: scaling up to meet the challenge[END_REF]; with the smaller spatial scale of many islands increasing the feasibility and implementation of many management options [START_REF] Jones | Recovery and restoration of seabird islands[END_REF][START_REF] Jones | Invasive mammal eradication on islands results in substantial conservation gains[END_REF]. Indeed, the removal of IAS from islands has proven a successful way to support many endangered species globally [START_REF] Veitch | Island invasives: eradication and management[END_REF][START_REF] Russell | Invasive alien species on islands: Impacts, distribution, interactions and management[END_REF][START_REF] Veitch | Island invasives: scaling up to meet the challenge[END_REF]). In addition, islands have proven to be the ideal testing ground for developing management strategies and techniques across invasion stages, from early detection (e.g. using environmental DNA [START_REF] Takahara | Using Environmental DNA to Estimate the Distribution of an Invasive Fish Species in Ponds[END_REF]), through population management of target IAS (e.g. using stable isotope techniques [Bodey et al. 2011]), to complete eradication [START_REF] Veitch | Island invasives: eradication and management[END_REF][START_REF] Carter | The clock is ticking: temporally prioritizing eradications on islands[END_REF]). While islands have proven fertile locations for such work, many of these management and mitigation strategies for IAS remain costly to implement, requiring significant resources, strategic planning and workforce capability to enact effectively. Such efforts come with unavoidable economic costs (Diagne et al. 2021a), which means that the political and administrative situation of an island can substantially influence the implementation of IAS management efforts. For example, the vast majority of independent small island states are considered to be developing economies. However, in contrast, island overseas territories (OTs) that may be geographically nearby and topographically similar, are exclusively administered by economically more developed nations with greater financial resources to potentially deploy [START_REF] Dawson | Prioritizing islands for the eradication of invasive vertebrates in the United Kingdom overseas territories[END_REF][START_REF] Churchyard | The biodiversity of the United Kingdom's Overseas Territories: A stock take of species occurrence and assessment of key knowledge gaps[END_REF][START_REF] Vaas | Who's in charge here anyway? Polycentric governance configurations and the development of policy on invasive alien species in the semisovereign Caribbean[END_REF][START_REF] Sieber | Hotspots of biodiversity and ecosystem services: the Outermost Regions and Overseas Countries and Territories of the European Union[END_REF]. Examples within the Caribbean include a range of islands comprising OTs (e.g. US Virgin Islands, Montserrat, Martinique), independent nation states (e.g. Jamaica, Haiti, Dominica) and offshore islands of otherwise continental countries (e.g. the Swan Islands, Honduras, and Islas Caracas, Venezuela). This discrepancy in the availability of internal and external financial resources may create differences in the ability or incentives to control IAS. Political and administrative differences may thus significantly affect investment in IAS management when other contrasting, and often urgent, societal needs exist. An economic rationale is often considered necessary for justifying action (or inaction) concerning IAS. Cost-benefit analyses are often conducted as prioritisation exercises [START_REF] Dawson | Prioritizing islands for the eradication of invasive vertebrates in the United Kingdom overseas territories[END_REF][START_REF] Holmes | Bond AL (2019) Globally important islands where eradicating invasive mammals will benefit highly threatened vertebrates[END_REF][START_REF] Carter | The clock is ticking: temporally prioritizing eradications on islands[END_REF]), but typically these tend to be on a case-by-case basis, either operationally or within a specific location.

However, more generalised modelling suggests that prevention of IAS impacts is less cost-intensive than post-establishment responses [START_REF] Leung | An ounce of prevention or a pound of cure: bioeconomic risk analysis of invasive species[END_REF], Ahmed et al. 2022, Cuthbert et al. 2022). Beyond island-, or even country-specific contexts, we are currently lacking a global synthesis of the economic costs of IAS on islands [START_REF] Reaser | Ecological and socioeconomic impacts of invasive alien species in island ecosystems[END_REF]. This is concerning, as such a synthesis would help to identify knowledge and/or management gaps locally and internationally, and would highlight actions that can produce synergies across sectors or locations. Such win-win scenarios are particularly important for addressing multi-faceted threats such as those posed by invasive species. To address the need for a more thorough understanding of the costs of invasive species on islands, we used the recently developed 'InvaCost' database [START_REF] Diagne | InvaCost, a public database of the economic costs of biological invasions worldwide[END_REF] to synthesise total reported costs of IAS on islands worldwide. Total costs are split between management costs (e.g. control, biosecurity, eradication) and damages incurred (e.g. agricultural losses, health costs). Specifically, we compared management costs and damage losses associated with IAS among islands differing in political geography (independent nation states, overseas territories or islands of continental countries). We hypothesised that i) the cost ratio between management and damage will be equivalent across the three island categories. That is, the economic impacts of IAS are felt, and responded to, similarly on all islands. However, ii) there should be a positive relationship between GDP and total expenditure, and iii) as a result of their ties to larger and often financially wealthier states, overseas territories and islands of continental countries should have proportionately greater expenditures compared to independent island nation states.

Finally, in light of the ongoing increase in, and recognition of, IAS impacts, we hypothesised that iv) all islands would experience increases in total costs over time.

Methods

InvaCost dataset

We used the InvaCost database v.4.0 (published June 2021), a publicly available living repository that compiles the monetary impacts of invasive species globally (https://doi.org/10.6084/m9.figshare.12668570). [START_REF] Diagne | InvaCost, a public database of the economic costs of biological invasions worldwide[END_REF] developed InvaCost via standardised literature searches (using the Web of Science platform, Google Scholar and Google search engines), coupled with opportunistic, targeted searches where data gaps were identified. Analogous searches were conducted in >10 non-English languages [START_REF] Angulo | Non-English languages enrich scientific knowledge: the example of economic costs of biological invasions[END_REF]. Additional targeted searches, specifically for economic costs on islands were made within the grey literature and global funding databases (e.g. www.thegef.org). These costs (n = 100) were added and all costs were standardised to a common currency (2017 US$, hereafter $) using the annual average market exchange rate incorporating inflation factors (https://data.worldbank.org/indicator/FP.CPI.TOTL?end=2017&start=1960).

Data selection

We considered only highly reliable (entries assigned as high in the 'Method reliability' column of the database) and observed (entries assigned as observed in the 'Implementation' column) costs. This excludes entries that were not from the peerreviewed literature or official reports, or were otherwise not reproducible (low reliability within the database); and those that were predicted on future organism invasions (labelled as potential within the database). However, we retained studies where costs observed at a small scale within an island were extrapolated to larger areas of the same location. We also omitted the handful of database entries with no value in the 'Cost_estimate_per_year_2017_USD_exchange_rate' column. We consider this approach to provide minimum but robust estimates of the economic costs of IAS.

To derive island costs specifically (Fig 1 ), we screened the full database by the metadata columns 'Official country' and 'Location'. The 'Official_country' column was used to locate all costs associated with island nation states i.e. all countries surrounded by oceanic waters smaller than continental Australia. The column 'Location' was then used to capture all costs associated with islands of continental nations e.g. the Galapagos (offshore islands of continental Ecuador) or Reunion (OT of France). Costs without geographic precision ('islands') or that spanned islands of differing political geographies were excluded (n = 3 in total).

Cost and Island descriptors

All costs were categorised by cost type, impacted sector and species. For cost type, we used the three categories from the 'Type_of_cost_merged' column for categorisation: Management (e.g. control, biosecurity, eradication effort), Damage (e.g. infrastructure repairs/losses, health impacts) and Mixed (costs not accurately separable between the two aforementioned categories, plus a small quantity of unspecified costs [~0.05% of the total]). The 'Impacted_sector' column was combined into broad socio-economic categories, namely: Authorities-Stakeholders (principally governmental and NGO actors), Primary Industries (agriculture, forestry etc.), Health and Social Welfare, Environment and Mixed (costs without definable detail across multiple sectors, including a small unspecified amount [~$270 000]). We used the 'Species' column to determine costs for individual species, with this analysis necessarily restricted to costs with precise values attributable to specific species i.e. a multi-species eradication without detailed breakdowns would be excluded as costs could not be accurately assigned among multiple targeted species.

We then added additional classifiers to identify island types and metrics suggested to predict IAS impacts. 'Island Type' distinguishes islands among three distinct political geographic categories: island Nation States (NSs, all sovereign island nations present); Islands of Continental Countries (ICCs, any island, from small rock stacks to large archipelagos, that is an insular component of an otherwise continental political entity, regardless of distance from continental to insular location e.g. Corsica is an ICC of France); and Overseas Territories (OTs, islands that are politically an integral part of, or in some way dependent on, another state for ultimate governance while being geographically separated from that country's territorial waters e.g. New Caledonia is an OT of France). We added multiple metrics hypothesised to influence the likelihood of IAS impacts or arrival rates (IUCN 2018), namely: i) the World Bank's gross national income (GNI) assessment (four groups: lower, lower middle, upper middle, upper [World Bank Development Indicators 2019]); ii) average GDP of the relevant country from 1970-2018 (World Bank 2021) for the NS itself, or the continental country or ultimate administrative country of the ICC/OT in question.

Such estimates are typically not available for individual OTs due to their complex administrative relationships, and other measures, such as the UN World Economic Situation and Prospects reports, do not provide country-level resolutions; iii) Population density (people per sq. km of land area); iv) merchandise imports (current USD) v) number of international tourist arrivals (all from World Bank 2021, https://data.worldbank.org/) Finally, to allow for comparisons across locations, costs were standardised to a common metric of $ km -2 using the information provided by the 'Spatial_scale' column, where necessary returning to the primary source for clarification. Scaling to the areas over which costs are recorded facilitates comparisons in the same way as scaling to a standard currency. For example, spending $1 million on IAS management over an entire country would potentially produce very different outcomes than the same value directed to a smaller within-country location. When no more precise scale was available, we adopted a conservative approach by assuming that the cost applied either to the entire named island(s) or, if even this level of precision was lacking, the entire country. This resulted in the exclusion of a few references with indeterminable areas (e.g., 'three ponds' or 'fishing boat'). As $ km -2 can be skewed by large costs over very small areas, e.g. eradication of fire ants Solenopsis invicta from <1 ha of Auckland airport, New Zealand at a scaled cost km - 2 of >US$40 million [START_REF] Kean | Global eradication and response database[END_REF], we examined how costs varied when considering i) all cost estimates ii) cost estimates only for areas > 1 ha iii) cost estimates only for areas > 5 ha.

Estimating Total Costs

To assess the total economic impact of average annual and cumulative costs, we annualised the cost values to standardise the temporal frame of occurrence. We expanded entries that reported costs spanning multiple years to determine annual expenditure, providing an accurate assessment of the economic costs of IAS through time. Multi-year costs were expanded using the the expandYearlyCosts function from the 'invacost' package in R v4.0.3 (R Core Team 2020, [START_REF] Leroy | Analysing global economic costs of invasive alien species with the invacost R package[END_REF], based on the 'Probable_starting_year_adjusted' and 'Probable_ending_year_adjusted' columns. When this information was unclear within the primary source (n = 4 entries totalling ~$120 000), we conservatively considered that these costs occurred in a single year, using the ending year determined from the source as the relevant time period. This resulted in an expanded dataset of 4881 annual costs covering 1965-

(Fig 1).

We used this expanded dataset (Table S1) to assess area-corrected costs across islands of differing political geographies in relation to cost types, socio-economic sectors, geographic location, species and economic indicators. For ICCs and OTs, we also compared the costs in these locales with the total documented costs (using the same filtering steps in Data Selection) for the relevant continental country.

Relationships between the total costs recorded for IAS and the four metrics of GDP, population density, imported goods and international arrivals were examined using linear models with a Gaussian error structure, with IAS costs as the independent variable and each metric in turn as the dependent variable. While other potential island-level classifiers are available (e.g. total number of IAS present, arrival dates or extent of spread), such data are temporally and spatially incomplete relative to each other and to the economic cost timeframes, as well as being occasionally contradictory. For example, cost data may relate to a specific IAS but be reported many years before/after any assessment of its distribution. Given these widespread temporal and spatial discrepancies and absences across sources, interpretation of results would be highly problematic, with the potential to conflate genuine absences of relationships with absences of relevant data.

Analysing temporal Trends

For the analysis of temporal trends, we excluded costs that occurred after 2020 (n = 13), although publication lags will result in incomplete data for other recent years as well (median publication lag, the difference between publication and impact years of the study, was three years). Temporal trends were assessed using the summarizeCosts function of the 'invacost' package [START_REF] Leroy | Analysing global economic costs of invasive alien species with the invacost R package[END_REF], and were examined independently for total reported costs across all islands, across island types separately, and for exclusive management and damage costs.

Results

The complete islands dataset consisted of 2183 unique cost entries on a minimum of 246 different islands, resulting in 4881 annualised costs spanning the years 1965-2021. These costs summed to $36.6 billion from a cumulative 396 million km 2 . Total area-corrected costs summed to US$km -2 316 million. Over half of the cost entries and annualised costs were reported from NSs (Fig 1, 2, Table S2). This translated into 52% of the total reported economic costs occurring in NSs, with ICCs reporting 43% of the total costs, and only 5% recorded on OTs (Fig 1, 2, Table S2). S3). The ratio of management spending to damage incurred was of a similar magnitude in NSs and OTs (at over 100:1), although this halved in ICCs, likely as a result of greater quantities of mixed costs (Table S3). While the area over which costs were incurred significantly affected the total amounts recorded, with rapid responses to early IAS detections leading to effective but expensive eradications at small scales of 1-5 ha, it did not qualitatively change comparisons (Fig S1).

Influence of GDP on recorded costs

As a proportion of GDP, there were large differences between island types. NSs lost the greatest fraction of their income to total IAS costs, an order of magnitude higher than the proportion of GDP lost in ICCs, and two orders of magnitude higher than that of any OT's administrative country (Fig 3b, Table S3). In addition, reported costs in OTs were an order of magnitude lower for both management and damage than for other island types (Fig 3a, Table S3). A comparison of the raw 2017 US$ costs on ICCs and OTs to costs of the relevant continental/ultimate governing nations showed that sums incurred on islands are consistently lower (with the exception of Ecuador).

Although the degree of disparity varies considerably, it remains much greater between countries and OTs than ICCs (Table S4). Costs from upper and uppermiddle income countries dominated the dataset, with lower and lower-middle income countries representing only 0.3% of all annualised costs (Table S5).

Temporal trends

When all islands are considered together, there is an increasing trend in total reported costs through each decade from 1960 to 2010, with step changes in orders of magnitude in each decade except for the 1990s. The trend for the 2010s remains incomplete due to publication lags, but may not see as substantial an increase (Fig

Discussion

We found that total reported costs in insular systems to address the economic impacts of IAS comprised over US$ 300 million in area-corrected costs over the past 60 years. However, there was significant variation in the types of costs across island types in relation to their political geography. Our hypothesis that the cost ratio between management and damages would be equivalent across island types was not supported by our results. Reported costs for management were always far greater than those for damages, but ratios were lowest for ICCs (e.g., the Galapagos, Hawai'i and Tierra del Fuego), with management spending approximately twice as high as that in NSs (e.g., Japan, the Seychelles and Palau) and almost three times as high as that in OTs (e.g. Christmas Island, French Polynesia and Guam). In addition, reported costs in OTs substantially lagged behind both ICCs and NSs under any comparisons despite the formers' connections to wealthier governing countries. Similarly, although globally costs have increased through time, trends across island types are very different; with broadly exponential costs in NSs but only incremental changes in ICCs and OTs in recent decades. This likely reflects a complex combination of factors, including the continuing emergence of alien species, variation in capacity or political willingness to address these threats, and changes in available technologies.

Cost Patterns

Management spending dominated the total costs incurred, comprising almost 90% of all reported costs on islands. This proportion may well be even higher given the extent of mixed management-damage costs reported within ICCs (Fig 3). This substantial management-related burden contrasts with most individual countries where costs due to IAS damages consistently outstrip management expenditure (e.g. Haubrock et al. 2021a, Diagne et al. 2021a but see Ballesteros-Mejia et al. 2021). Geographically, management costs were greatest in the Pacific region, with a notable lack of reported management costs from island-rich, biodiverse hotspots such as the Caribbean, SE Asia and the western Indian Ocean [START_REF] Kier | A global assessment of endemism and species richness across island and mainland regions[END_REF].

This suggests that such regions would likely benefit substantially from direct knowledge sharing, facilitating the coordination of approaches in such areas as biosecurity, management techniques and legal instruments. While such island-rich regions would benefit, the same is also true of continental countries given that management costs are already proportionately greater on islands than continental areas. NSs incurred the greatest costs in total and for management, with damage costs marginally highest on ICCs, and with much lower to negligible figures recorded for OTs. NSs also incurred costs to a far greater (although still extremely small) extent as a proportion of their GDP. This may reflect the greater impact of IAS on NS economies where, for example, damage to crops directly impacts the country's capacity to feed itself and increases its import requirements [START_REF] Reaser | Ecological and socioeconomic impacts of invasive alien species in island ecosystems[END_REF][START_REF] Mwebaze | Economic valuation of the influence of invasive alien species on the economy of the Seychelles islands[END_REF]. NSs may also have an increased reliance on their unique biodiversity for generating income from natural capital or ecotourism than either ICCs or OTs [START_REF] Fotiou | Sustainable development of ecotourism in small islands developing states (SIDS) and other small islands[END_REF][START_REF] Dolins | Conservation education in Madagascar: three case studies in the biologically diverse island-continent[END_REF], Ballesteros-Mejia et al. 2021).

However, the low value of damage costs appears to run counter to these suggestions. There are three non-exclusive explanations for this trend. Firstly, lower damage costs may reflect stricter biosecurity protocols that successfully reduce IAS arrivals to some NSs (e.g. New Zealand [START_REF] Cuthbert | Biological Invasion Costs Reveal Insufficient Proactive Management Worldwide[END_REF]) and ICCs managed as strict nature reserves (e.g. the Galapagos [Ballesteros-Mejia et al. 2021]). While such biosecurity expenditure has costs, these chiefly comprise salaries and infrastructure costs that are unlikely to be readily captured by literature searches targeted at costs specifically attributable to invasive species. Such values are not regularly included in reporting of management expenditure (for example because of concerns around confidentiality) in the way that equipment purchases typically are.

Secondly, damage costs may be less frequently recorded to precise areas -for example damage may be reported simply as occurring to forestry within a country, whereas management costs may be more regularly reported in detail e.g. removal of a pest species from a discrete area. As we standardised costs by area, this could mean that imprecisely reported costs in the primary source became comparatively smaller if they were applied over larger spatial scales. Lastly, the limited damage costs reported in ICCs and OTs may reflect combinations of under-reporting and low investment in mitigating such losses due to their relatively small contribution to the governing country's overall economy. This, coupled with small proportions of national populations resident in ICCs and OTs, their spatial isolation and limited electoral influence, may affect the extent of reporting for these island types in comparison to NSs. The mitigation of the costs of IAS in NSs has a clear economic rationale, but we found that costs were broadly comparable between NSs and ICCs for both management and damage. However, ICCs comprise many sparsely inhabited/uninhabited locations (e.g., Dirk Hartog Island, Australia; the Aleutian Island chain, United States; many tropical motu) where significant recordable economic damages or primary industry-driven management requirements are far less likely. Rather, costs in these locations probably reflect losses to biodiversity and ecosystem function. While placing monetary values on such losses is complex, particularly where human livelihoods are negligibly affected [START_REF] Nunes | Economic valuation of biodiversity: sense or nonsense?[END_REF][START_REF] Kallis | To value or not to value? That is not the question[END_REF][START_REF] Roberts | Prioritising invasive species control actions: Evaluating effectiveness, costs, willingness to pay and social acceptance[END_REF], ICCs provided virtually all costs incurred by the environmental sector. This reflects significant management spending on IAS control or eradication for ecological restoration following conservation models established in New Zealand [START_REF] Jones | Invasive mammal eradication on islands results in substantial conservation gains[END_REF][START_REF] Veitch | Island invasives: eradication and management[END_REF], 2019). Under this model, islands can be used to either sustain relict populations or to receive translocations of endangered endemics that may receive exceptional financial support due to their charismatic status and high extinction risk [START_REF] Russell | Predator-Free New Zealand: Conservation Country[END_REF][START_REF] Russell | Invasive alien species on islands: Impacts, distribution, interactions and management[END_REF]. This is demonstrated by the dominance of reported costs from ICCs with successful IAS eradication campaigns including: Macquarie Island, Australia (Helmstedt et al. 2016), the California Channel Islands, USA [START_REF] Parkes | Rapid eradication of feral pigs (Sus scrofa) from Santa Cruz Island, California[END_REF], the Galapagos, Ecuador [START_REF] Carrion | Archipelago-Wide Island Restoration in the Galápagos Islands: Reducing Costs of Invasive Mammal Eradication Programs and Reinvasion Risk[END_REF]) and islands and islets around Mexico (Samaniego-Herrera et al. 2018). In addition, many islands are also protected areas of varying legislative status, making IAS management either a legal requirement or a high priority. The significance of such programmes is seen in the extent to which governmental organisations and other stakeholders, such as conservation NGOs, incurred most of the reported costs across all islands. However, it was also apparent that there was almost no consistency in the species generating management or damage costs across island types. Although a number of known impactful species surprisingly do not feature, for example invasive Rattus species [START_REF] Diagne | Economic costs of invasive rodents worldwide: the tip of the iceberg[END_REF], this analysis was restricted to costs specifically attributed to individual species, so multi-species eradication campaigns and nonspecific damages were excluded. Potentially this variation also reflects a lack of uniformity in the reporting of costs incurred by specific species, and no doubt it also reflects some regional, latitudinal or societal differences. It does, though, strongly suggest there is a need for greater information sharing to synergise experience across locales, particularly because many islands support a relatively homogenous suite of IAS [START_REF] Capinha | The dispersal of alien species redefines biogeography in the Anthropocene[END_REF]. Otherwise, such gaps (whether actual absences or simply a lack of data resolution) have the potential to produce erroneous conclusions on how to effectively allocate budgets and prioritise efforts in what is, necessarily, a triage process [START_REF] Holmes | Bond AL (2019) Globally important islands where eradicating invasive mammals will benefit highly threatened vertebrates[END_REF][START_REF] Carter | The clock is ticking: temporally prioritizing eradications on islands[END_REF]. Consideration of approaches from similar contexts, such as neighbouring islands, should be used to further guide management approaches, including informing horizon scanning exercises to identify potential threats and actions at local and regional scales [START_REF] Ricciardi | Invasion science: a horizon scan of emerging challenges and opportunities[END_REF]. While the number of foreign visitors or extent of imports have been considered highrisk factors for the spread of IAS [START_REF] Pretto | Determinants of non-native plant species richness and composition across small Mediterranean islands[END_REF], IUCN 2018), they did not act as significant predictors of IAS costs incurred on islands. Significant relationships for GDP and imports were only present when the USA (an outlier for both) was included, providing little support for these as predictive metrics. There were also quantitative differences in the reported area-corrected expenditure depending on how small an area was included. Eradications within very small areas (e.g. the detection and removal of red imported fire ants from Auckland airport [START_REF] Kean | Global eradication and response database[END_REF]) produced substantial area-corrected costs, but then such rapid reactive responses likely also substantially reduced ongoing chronic costs, highlighting the importance of preinvasion management [START_REF] Ahmed | Managing biological invasions: the cost of inaction Biological Invasions[END_REF], Cuthbert et al. 2022). Importantly, qualitative patterns, for example the dominance of management over damage costs, remained unaltered if these highly localised costs are omitted.

Underfunded or under-reported?

Regardless of the type of cost, it is clear that costs from OTs are substantially lower than on other island types. This is seen not just in absolute terms, but exceedingly so in relation to the ultimate administrative countries' GDP (Table 2). As these costs are corrected for area, this is not simply a reflection of the smaller landmass OTs occupy. Instead, it appears that countries maintaining OTs either do not record, or do not invest in, IAS mitigation at any significant scale in these locales. There is no evidence to suggest that IAS presence is lower in OTs and, indeed, this would be contrary to the global trend [START_REF] Seebens | No saturation in the accumulation of alien species worldwide[END_REF]. While the InvaCost database collected data only on economic values, and so may fail to capture costs of biodiversity or cultural losses such as the ecosystem functions of extinct island endemics [START_REF] Wood | Island extinctions: processes, patterns, and potential for ecosystem restoration Environmental Conservation[END_REF], this absence should occur across all island types. However, adding additional fields to the database and expanding the search terms used to attempt to capture such costs and losses, would aid with assessing these absences across locations. Indeed, future developments could broaden the scope of the InvaCost database to link monetary values with ecological and socio-economic impacts through integration of information from other standardized global assessments (e.g. EICAT and SEICAT [START_REF] Hawkins | Framework and guidelines for implementing the proposed IUCN Environmental Impact Classification for Alien Taxa (EICAT)[END_REF][START_REF] Bacher | Socio-economic impact classification of alien taxa (SEICAT)[END_REF]), improving our understanding of IAS impacts in the round. Nevertheless, the effort expended on recording IAS costs in OTs appears to vary substantially by location.

For example, New Caledonia represents the highest number of cost entries, though not the highest spending, within the French départements, with Réunion also recording substantial numbers of IAS costs. However, there is simultaneously little data on costs for other French OTs including Mayotte and Martinique (Renault et al. 2021). This pattern of limited reporting reflects a common theme of undervaluing of the ecological richness of these locations, as also seen in the UK [START_REF] Churchyard | The biodiversity of the United Kingdom's Overseas Territories: A stock take of species occurrence and assessment of key knowledge gaps[END_REF]. Most OTs are relatively small islands with limited internal resources, and without substantial external support, they will struggle to mitigate IAS impacts or conserve the biodiversity they support [START_REF] Churchyard | The biodiversity of the United Kingdom's Overseas Territories: A stock take of species occurrence and assessment of key knowledge gaps[END_REF][START_REF] Vaas | Who's in charge here anyway? Polycentric governance configurations and the development of policy on invasive alien species in the semisovereign Caribbean[END_REF]. While targeted funding to tackle the impacts of IAS exists (e.g. Darwin Initiative www.darwininitiative.org.uk), there is clear potential for international collaboration and cooperation to enhance efficacies and synergies, particularly when adjacent countries are likely experiencing similar threats. This can include best practice resources and training courses such as those provided under the Pacific Invasives Initiative (www.pacificinvasivesinitiative.org).

Our work also highlights the almost complete lack of reported costs from lower and lower-middle income economies, with <1% of all reported island costs occurring in such countries. This trend is seen in many aspects of ecology and conservation (Christie et al. 2021[START_REF] Lynch | Culturally diverse expert teams have yet to bring comprehensive linguistic diversity to intergovernmental ecosystem assessments[END_REF][START_REF] Nunez | Making ecology really global[END_REF], and remains here despite significant efforts to incorporate non-English resources [START_REF] Angulo | Non-English languages enrich scientific knowledge: the example of economic costs of biological invasions[END_REF]). It should be noted that very few lower income countries are either islands or even countries with a coastline, and the only lower income island NS -Madagascar -does have some costs recorded. However, there is a clear absence of costs from lower-middle income island nations in SE Asia and the Pacific (Haubrock et al. 2021b). This absence is likely a product of true knowledge gaps resulting from limited studies due to reduced capacity or competing priorities, and an urgent need to increase the ability to disseminate publications and information [START_REF] Wallace | Enabling decisions that make a difference: guidance for improving access to and analysis of invasive species information[END_REF]. Given that several of the largely/completely absent countries represent populous and mega-diverse biological hotspots (e.g., Indonesia and the Philippines), there is an urgent need to improve our understanding of both the damage and management costs of IAS in these locations [START_REF] Kier | A global assessment of endemism and species richness across island and mainland regions[END_REF][START_REF] Fernandez-Palacios | Scientists' warning -The outstanding biodiversity of islands is in peril[END_REF]. Likely impacts include severe negative effects on a range of indicators including health, productivity, happiness, child poverty, ecosystem services and biodiversity (United Nations 2017).

Temporal trends

Given the importance of islands to global diversity, temporal trends in management costs exhibit a potentially encouraging trend. Average decadal management spending showed an increase of over three orders of magnitude between 1960 and 2010, and this value is consistently at least one order of magnitude greater than damage costs (Fig 3). While total management expenditure for the 2010s shows a small decline, this is likely to reflect publication lag times rather than a genuine reduction. However, it appears that the reasons for spending may differ across islands of differing political geography, as trends differ between the three island types. Costs for NSs broadly reflect the overall pattern of increasing magnitudes of expenditure, reflecting a combination of ongoing increases in rates of introductions [START_REF] Seebens | No saturation in the accumulation of alien species worldwide[END_REF], increased recognition of the importance of island biodiversity [START_REF] Fernandez-Palacios | Scientists' warning -The outstanding biodiversity of islands is in peril[END_REF]) and attempts to tackle IAS at larger, more socially complex scales [START_REF] Oppel | Eradication of invasive mammals on islands inhabited by humans and domestic animals[END_REF][START_REF] Russell | Predator-Free New Zealand: Conservation Country[END_REF][START_REF] Veitch | Island invasives: scaling up to meet the challenge[END_REF]). However, costs on ICCs have largely flatlined since the 1980s. This likely reflects early eradication efforts on small uninhabited islands [START_REF] Jones | Recovery and restoration of seabird islands[END_REF], 2016, Samaniego-Herrera et al. 2018) that have led to substantial conservation gains [START_REF] Jones | Invasive mammal eradication on islands results in substantial conservation gains[END_REF][START_REF] Holmes | Bond AL (2019) Globally important islands where eradicating invasive mammals will benefit highly threatened vertebrates[END_REF]. However, translating approaches from uninhabited to inhabited islands is challenging. Logistical difficulties, social challenges and current higher risk of failure can constrain the capacity or willingness of governments to commit to such endeavours [START_REF] Oppel | Eradication of invasive mammals on islands inhabited by humans and domestic animals[END_REF][START_REF] Russell | Predator-Free New Zealand: Conservation Country[END_REF][START_REF] Carter | The clock is ticking: temporally prioritizing eradications on islands[END_REF]. A largely stable, although substantially lower, level of recorded costs is also seen on OTs, again suggesting comparative underinvestment or under-recording, although the complications associated with inhabited ICCs are also relevant here [START_REF] Dawson | Prioritizing islands for the eradication of invasive vertebrates in the United Kingdom overseas territories[END_REF], Renault et al. 2021).

Conclusion

Using the most comprehensive available data on the economic impacts of IAS on islands, we demonstrate that political geography is central to the type and quantity of economic costs and/or the likelihood of reporting costs. Across all locations, losses due to damage were far smaller than costs accrued managing IAS, likely revealing knowledge gaps as to the extent of many socio-economic impacts of IAS and difficulties in estimating damages across all locations (cf Crystal-Ornelas and Lockwood 2020). There is also a clear disparity in the effective dissemination of cost reporting within the literature, whether through differences in terminology, classifications, or access to publishing [START_REF] Wallace | Enabling decisions that make a difference: guidance for improving access to and analysis of invasive species information[END_REF][START_REF] Nunez | Making ecology really global[END_REF].

Importantly, despite islands from all political geographies supporting unique biodiversity, costs were especially low in OTs. Given the importance of OTs to country-specific biodiversity, a greater focus of attention on IAS impacts would improve progress towards the Convention on Biological Diversity's Post-2020 Global Biodiversity Framework (www.cbd.int/conferences/post2020/wg2020-03/documents) in these countries. Such an approach would benefit all islands given the strong overlap between this framework and sustainable development goals [START_REF] Schultz | The 2030 Agenda and Ecosystems -A discussion paper on the links between the Aichi Biodiversity Targets and the Sustainable Development Goals[END_REF]. Nevertheless, the predominance of spending on management approaches (incorporating all stages from biosecurity through to long-term control/eradication), and the continuing temporal increase in costs across all islands, suggests that there continues to be the potential to make substantial conservation and development gains in insular ecosystems. This includes the generation of efficiencies of scale over wider management units, regional cooperation and knowledge sharing, and innovations to enhance efficacy [START_REF] Perrings | International cooperation in the solution to trade-related invasive species risk[END_REF][START_REF] Russell | Predator-Free New Zealand: Conservation Country[END_REF]. In particular, the high economic burden imposed by biological invasions on islands adds to the weight of evidence that prevention rather than management will better protect insular biodiversity [START_REF] Ahmed | Managing biological invasions: the cost of inaction Biological Invasions[END_REF]. International cooperation and the strengthening of legal instruments would also improve approaches, for example by addressing key invasion pathways [START_REF] Turbelin | Introduction pathways of economically costly invasive alien species[END_REF], in turn reducing the financial costs biological invasions continue to accrue worldwide.

  Fig 1. Workflow showing the extraction and filtering processes to determine the

  Fig 2. Global locations and magnitudes of total reported economic costs of invasive alien species on islands. All costs are in 2017 US$ km -2

  Lower-middle income nations are particularly poorly represented, as relatively few lower income countries are island states or coastal nations. The extent of foreign visitors and population density, hypothesised predictors of IAS emergence risk, were not significant predictors of the reported economic costs of IAS across NSs and ICCs (Fig S2). While GDP and import quantities were significantly correlated with IAS costs, these relationships were driven by values for the USA, and were not present if this datapoint was removed (Fig S2). Considering all costs by socio-economic sectors revealed dominant spending incurred by the Authorities-Stakeholders category (89% of annualised costs) i.e. governmental departments and/or non-governmental organisations including conservation agencies or charities concerned with the management of IAS. Most remaining costs were attributed to Primary Industries (6%) and Environment (4%) categories (Fig 3c). However, sector-related impacts were not spread evenly across island types. Primary industry costs were mainly incurred by OTs, with some spending in NSs; environmental costs were dominated by ICCs; and the limited health and social welfare costs were documented overwhelmingly in NSs (Fig 3c). In terms of the costliest species (considering only costs clearly assignable to single species), there was almost no overlap among island types, or between the principal species incurring management or damage costs (Fig 4). Taxonomic groups incurring the highest costs across all island types chiefly comprised insects (137 US$ millions km -2 ) -particularly ants (98% of insect costs) -flowering plants (50 US$ millions km - 2 ), and mammals (16 US$ millions km -2 ) (Fig 4). For example, some of the highest total costs on ICCs and OTs are spent on tropical kudzu (Pueraria phaseoloides), swamp stonecrop (Crassula helmsii) and the little fire ant (Wasmannia auropunctata); whereas in NSs major costs were incurred in relation to the stoat (Mustela erminea), the small Indian mongoose (Urva auropunctata), the African tulip tree (Spathodea campanulata) and, most significantly, the red fire ant (Solenopsis invicta).

Fig 3 .

 3 Fig 3. Distribution of costs in 2017 US$ millions km -2 a) proportion across main cost

  5). As management spending comprises the overwhelming majority of reported costs, trends for management exhibit the same pattern as total costs. Damage costs have similarly made exponential leaps through time while remaining at lower overall values (Fig 5). Trends by island type are far less consistent. NSs show an overall increasing trend but with marked drops in some decades, meaning that it is only since the 2000s that they have consistently surpassed costs on ICCs. In contrast, ICC costs initially rose rapidly but have only increased comparatively incrementally since the 1980s. Costs on OTs are less consistently reported through time, but have been similarly static, even declining in recent decades (Fig 5).

Fig 5 .

 5 Fig 5. Temporal trends in reported total spending on the economic impacts of IAS