Bazzichetto

F Carboni

Massol François
email: francois.massol@univ-lille.fr

Marta Carboni
email: marta.carboni@uniroma3.it

Jonathan Lenoir
email: jonathan.lenoir@u-picardie.fr

Jonas J Lembrechts
email: jonas.lembrechts@uantwerpen.be

Johan

Joly Rémi
email: remi.joly88@hotmail.fr

Renault David
email: david.renault@univ-rennes1.fr

Manuele Bazzichetto
email: manuele.bazzichetto@univ-rennes1.fr

François Massol

Rémi Joly

David Renault

Once upon a time in the far south: Influence of local drivers and functional traits on plant invasion in the harsh sub-Antarctic islands

Keywords: Poaceae, Caryophyllaceae, Juncaceae) alien plants, anthropogenic propagule pressure, invasiveness, plant invasion, species distribution models, sub-Antarctic islands, topoclimate

Aim

Here, we aim to: (i) investigate the local effect of environmental and human-related factors on alien plant invasion in sub-Antarctic islands; (ii) explore the relationship between alien species features and their dependence on anthropogenic propagule pressure; and (iii) unravel key traits conferring invasiveness in the sub-Antarctic.

Location

Possession Island, Crozet archipelago (French sub-Antarctic islands).

Taxon

Non-native vascular plants (

Introduction

Sub-Antarctic islands, archipelagos scattered within the 54-48°S latitudinal ring, are extremely remote territories which harbour a unique biodiversity with a high degree of endemism [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF].

As a consequence of their relatively recent discovery and environmental harshness, these islands have long remained pristine and largely free of human disturbances. Yet, due to the gradual relaxation of these natural barriers, sub-Antarctic islands are now listed among the most threatened environments on Earth. In particular, invasion by alien plants, boosted by ongoing climate changes and increasing human disturbances [START_REF] Duffy | Ice-free area expansion compounds the non-native species threat to Antarctic terrestrial biodiversity[END_REF][START_REF] Hughes | Human-mediated dispersal of terrestrial species between Antarctic biogeographic regions: A preliminary risk assessment[END_REF], has become one of the main threats to the endemic biodiversity of these territories, and is bound to rise in the next decades [START_REF] Lebouvier | The significance of the sub-Antarctic Kerguelen Islands for the assessment of the vulnerability of native communities to climate change, alien insect invasions and plant viruses[END_REF][START_REF] Hughes | Biological invasions in terrestrial Antarctica: what is the current status and can we respond?[END_REF]. Over the past century, alien plants have been increasingly introduced in the sub-Antarctic region [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Huiskes | Aliens in Antarctica: assessing transfer of plant propagules by human visitors to reduce invasion risk[END_REF]. European whalers and scientific activities, respectively in the 19 th and 20 th century, determined the first main introduction events [START_REF] Convey | Environmental change and human impacts on terrestrial ecosystems of the sub-Antarctic islands between their discovery and the mid-twentieth century[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF]. Later on and since the mid-twentieth century, climate warming, strong changes in precipitation regimes and the widespread impacts of non-native vertebrates have progressively favoured the establishment of coldtolerant alien plants on sub-Antarctic islands [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF][START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF][START_REF] Duffy | Ice-free area expansion compounds the non-native species threat to Antarctic terrestrial biodiversity[END_REF].

Nevertheless, despite their demonstrated impacts on native biodiversity, little attention has been given to plant invasions compared to animal invasions on these islands [START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF], leaving a knowledge gap in the mechanisms underpinning plant invasion processes in these unique environments [START_REF] Greve | Terrestrial invasions on sub-Antarctic Marion and Prince Edward Islands[END_REF].

The outcome of any biological invasion is jointly determined by propagule pressure (i.e. frequency of propagules introduction), abiotic conditions (i.e. physico-chemical features of the invaded environment) and biotic features (i.e. alien species characteristics and interactions with the recipient community), with anthropogenic disturbances affecting all three [START_REF] Richardson | Plant invasions: merging the concepts of species invasiveness and community invasibility[END_REF][START_REF] Catford | Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework[END_REF][START_REF] Lembrechts | Disturbance is the key to plant invasions in cold environments[END_REF]. The relative importance of these factors is, however, context-dependent and species-specific [START_REF] Catford | Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework[END_REF]. In sub-Antarctic islands, due to the high specialization but low diversity of the native flora, biotic interactions are thought to play a minor role [START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF][START_REF] Duffy | Barriers to globally invasive species are weakening across the Antarctic[END_REF][START_REF] Moser | Remoteness promotes biological invasions on islands worldwide[END_REF], so it is mainly the first two factors that determine the distribution and spread of alien plants. First, invasions depend on humaninduced propagule pressure: the frequency of propagule introduction correlates with the number of ship landings and is highest in the vicinity of human facilities [START_REF] Huiskes | Aliens in Antarctica: assessing transfer of plant propagules by human visitors to reduce invasion risk[END_REF]. Second, local abiotic conditions are strongly limiting, and particularly the climatic mismatch between the conditions prevailing within the alien species' native range and the conditions prevailing in the sub-Antarctic can strongly constrain invasions [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF]. Some alien plants are more limited during the introduction phase, while others quickly become relatively independent of human-related propagulepressure and seem only climatically limited. Once established, the species which are the least dependent on continuous introductions are the most likely to spread widely and become invasive [START_REF] Richardson | Plant invasions: merging the concepts of species invasiveness and community invasibility[END_REF][START_REF] Catford | Reducing redundancy in invasion ecology by integrating hypotheses into a single theoretical framework[END_REF]. Therefore, quantifying the degree of alien species dependence on propagule pressure might aid at identifying potentially invasive species.

A lower dependence on human-related propagule pressure is potentially related to certain species features which are more generally known to affect invasiveness. First of all, alien species with longer residence times are more likely to become independent of anthropogenic propagule pressure [START_REF] Wilson | Residence time and potential range: crucial considerations in modelling plant invasions[END_REF][START_REF] Pyšek | Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time[END_REF]. Second, certain plant traits are considered key for profiling successful invaders [START_REF] Pyšek | Traits associated with invasiveness in alien plants: where do we stand?[END_REF]: invasive alien plants across most environments are growing faster and taller than non-invasive alien species, and typically produce resource-acquisitive leaves and many small seeds [START_REF] Van Kleunen | A meta-analysis of trait differences between invasive and non-invasive plant species[END_REF][START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF]. More specifically, [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF] performed a first functional comparison between invasive and non-invasive alien species colonizing the sub-Antarctic Marion Island, reporting that species generally considered invasive had lower plant height, smaller leaf area, lower frost tolerance and higher specific leaf area than other alien species. Nevertheless, it is still unclear which traits can actually make some alien plants less dependent on human-related propagule pressure, and thus more likely to become invasive, especially in the sub-Antarctic islands. This knowledge could facilitate the early screening of highly invasive alien plant species in these environments [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF].

Correlative species distribution models (SDMs) are statistical tools that model the speciesenvironment relationship relying on geo-referenced occurrence data and spatial environmental layers [START_REF] Guisan | Habitat suitability and distribution models: with applications in R[END_REF]. Such models already proved to be valuable tools for analysing alien plant invasion in Antarctica and the sub-Antarctic regions. For instance, [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF] modelled the distribution of Poa annua and Poa pratensis in the Antarctic peninsula as a function of bioclimatic variables, while [START_REF] Duffy | Barriers to globally invasive species are weakening across the Antarctic[END_REF] generated future scenarios of invasion across Antarctica and the sub-Antarctic regions using climate-based SDMs. Whilst these previous SDM applications have revealed large-scale determinants of alien plant invasion in the Antarctic biogeographic region, they have up till now failed to account for how environmental and anthropogenic factors regulate alien plant distributions at a spatial resolution that is meaningful for local management. This is chiefly due to the lack of high-resolution environmental (e.g. climatic, topographic) and human-related data layers, which limits the implementation of SDMs at fine spatial resolutions in remote areas [START_REF] Gutt | Correlative and dynamic species distribution modelling for ecological predictions in the Antarctic: a cross-disciplinary concept[END_REF]. A more general limitation inherent to the use of SDMs for modelling biological invasion is that SDMs allow mapping into the geographical space only a snapshot of the current alien species-environment relationship net of dispersal and biotic constraints (i.e. realized distribution), while necessarily underestimating the actual area potentially suitable to a species for establishing and maintaining a viable population (i.e. potential distribution; see [START_REF] Jiménez-Valverde | Use of niche models in invasive species risk assessments[END_REF][START_REF] Srivastava | Species distribution models (SDM): applications, benefits and challenges in invasive species management[END_REF][START_REF] Srivastava | Species distribution models (SDM): applications, benefits and challenges in invasive species management[END_REF].

The sub-Antarctic Possession Island constitutes an ideal arena to analyse alien plant invasions in the sub-Antarctic region. The availability of historical vegetation observations allows retracing the invasion history of most alien plant species on the island. Moreover, this island witnessed past human colonization and climate changes comparable to the other sub-Antarctic islands, allowing inference on the mechanisms underpinning alien plant invasion in these unique areas. Previous work showed that there is considerable variation in the spread of alien plants established on Possession Island, with some species clustering close to their introduction locations and others spreading widely and far from the initial introduction sites [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF], which allows testing for differences in the dependence on human introductions. In the present study, we model the distribution of the most relevant alien plant species colonizing Possession Island using a combination of environmental and human-related spatial data derived at an unprecedented high spatial resolution (i.e. 30-m) for these latitudes that we related to long-term monitoring observations of plant occurrences. Our aim is to test the local effect of environmental and anthropogenic factors on alien plant invasion in sub-Antarctic ecosystems. We hypothesise that both abiotic and human-related factors jointly define the local occurrence of alien plant species, but that these two factors will not be equally important among species. Furthermore, to identify plant characteristics conferring high invasiveness in sub-Antarctic ecosystems, we investigate how plant functional traits affect species dependence on anthropogenic propagule pressures. In this regard, our working hypothesis is that the most invasive species share specific functional characteristics allowing them to become independent of human-assisted introductions and spread widely once established. The study was carried out on Possession Island in the Crozet archipelago, which is included in the Réserve naturelle nationale des terres australes françaises (RNN TAF) and listed as UNESCO World Heritage site since 2019. Possession Island (Figure 1) is characterized by a complex topography, with an altitudinal gradient ranging from 0 to 934 m above the sea level (Pic du Mascarin) over a relatively short spatial extent (147 km 2 ). The island is characterized by a typical sub-Antarctic climate, with mean annual temperature of 5.6 °C and annual precipitation of 2,300 mm (Météo France, data: 1960-2019). Frequent and strong western winds occur throughout most of the year.

Materials and methods

Study area

The first human settlements date back to the 19 th century, when whalers and sealers established on the north-east side of the island during the hunting season, facilitating a first series of alien species introductions. In 1963, a permanent research station (Alfred Faure, hereafter the 'research station') was built on the easternmost area of Possession Island, fostering a new invasion front. Beyond the research station, other shelters (inhabited for short periods) are currently present on each side of the island: north (Pointe Basse); south-west (Baie du La Pérouse); and north-east (Baie Américaine).

Among these, the research station is by far the biggest human settlement and main hub of propagule introduction. The vegetation at Possession Island has experienced relatively low grazing pressure from sheep in the past [START_REF] Convey | Environmental change and human impacts on terrestrial ecosystems of the sub-Antarctic islands between their discovery and the mid-twentieth century[END_REF], in comparison to other sub-Antarctic islands where introduced large herbivores still strongly affect the distribution of alien plants [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF].

Study species

Despite the 68 alien species recorded on Possession Island (see page 99 of the RNN TAF management plan 2018-2027: https://taaf.fr//content/uploads/sites/2/2019/09/180607-Volet-A_pour-CNPN.pdf), preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.

only few have established persistent populations [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF]. In this study, we restricted our analysis to those alien plants that are either known to be generally widespread on sub-Antarctic islands or are particularly widespread on Possession Island, and for which sufficient occurrence data were available (total number of presences > 100). Specifically, we selected the following six species from three different families: Poa annua and Poa pratensis (Poaceae); Cerastium fontanum, Sagina procumbens and Stellaria alsine (Caryophyllaceae); and Juncus bufonius (Juncaceae). The two grasses, P. annua and P. pratensis, have colonised most of the sub-Antarctic islands [START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF] and are the longest-established alien plants in the Antarctic Peninsula [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF]. Cerastium fontanum and S. procumbens are currently widely distributed in this environment [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF] with, in particular, S. procumbens exhibiting the highest rate of spread among the alien plants of Marion and Prince Edward Islands (le Roux et al., 2013). Finally, both J. bufonius and S. alsine currently occur at significant distances from the research station on Possession Island (Frenot el al., 2001). While the former has been recently observed up to the Maritime Antarctica latitudes (Cuba-Díaz, Fuentes, & Rondanelli-Reyes, 2015), the latter has been singled out by some authors as the potentially most problematic future invasive plant species on Possession Island [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF][START_REF] Convey | The establishment of a new ecological guild of pollinating insects on sub-Antarctic South Georgia[END_REF].

Species distribution data

We analysed the invasion patterns of the six selected alien plant species relying on georeferenced occurrence (presence/absence) data collected within the context of a yearly vegetation monitoring survey carried out by the RNN TAF since 2010. The vegetation sampling is implemented within a system of 675 squared cells of 100 × 100 m each, where floristic data (presence and abundance of vascular plant species) are collected along with habitat characteristics through phytosociological relevés [START_REF] Dengler | Phytosociology. International Encyclopedia of Geography: People, the Earth, Environment and Technology: People, the Earth[END_REF]. In this study, we used data collected from 2010 to 2017 (3,354 occurrences for the selected species across 1,572 sampled plots).

Species features and functional trait data

To inform species features (e.g. traits) potentially related to invasiveness, we collected data on plants residence time and functional traits. Residence time positively interacts with propagule pressure in determining plant invasion success [START_REF] Richardson | Plant invasions: merging the concepts of species invasiveness and community invasibility[END_REF][START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF][START_REF] Pyšek | Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time[END_REF], and this relationship was also observed on sub-Antarctic islands [START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF][START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF]. To test how residence time influences alien species' dependence on propagule pressure, we considered the introduction date of the selected plants on Possession Island [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF] and used this information to assign them to two groups: old vs new resident species (Appendix S1, Table S1.1). In particular, we considered as old resident species those which were firstly observed on Possession Island before the research station was built (1963), while referring to the others as new resident species.

We then collated data on seven plant traits commonly used to synthesize species strategies known to be related to invasiveness [START_REF] Van Kleunen | A meta-analysis of trait differences between invasive and non-invasive plant species[END_REF][START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF]: (1) life history (annual vs perennial); (2) plant height; (3) leaf area; (4) specific leaf area (SLA); (5) vegetative reproduction (present vs absent, i.e. sexual and vegetative vs only sexual reproduction); (6) seed dry mass; and (7) number of seeds per plant. We excluded traits related to flowering since pollinating insects are absent from almost all sub-Antarctic islands [START_REF] Convey | The establishment of a new ecological guild of pollinating insects on sub-Antarctic South Georgia[END_REF]. Life history, plant height and leaf area relate to plant persistence and tolerance to environmental stress [START_REF] Cornelissen | A handbook of protocols for standardised and easy measurement of plant functional traits worldwide[END_REF][START_REF] Pérez-Harguindeguy | New handbook for stand-ardised measurement of plant functional traits worldwide[END_REF]. In addition, life history is used to assess maximum lifespan and plant height is associated with competitiveness for light and whole plant fecundity [START_REF] Pérez-Harguindeguy | New handbook for stand-ardised measurement of plant functional traits worldwide[END_REF]. Specific leaf area is the one-sided leaf area per leaf mass and is associated with resource acquisition and photosynthetic rate [START_REF] Pérez-Harguindeguy | New handbook for stand-ardised measurement of plant functional traits worldwide[END_REF]. Reproduction strategy, seed dry mass and number of seeds per plant do not only relate with species persistence, but also with dispersal capacity [START_REF] Ottaviani | Linking Plant Functional Ecology to Island Biogeography[END_REF]. In particular, alien species reproducing predominantly sexually may benefit from lower dispersal limitation and greater genetic diversity [START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF]. At the same time, while small and light seeds are better dispersed at longer distances, largeseeded plants may benefit from more stored resources [START_REF] Van Kleunen | Characteristics of successful alien plants[END_REF].

Functional trait data collected in areas environmentally analogous to Possession Island were compiled from the literature (other sub-Antarctic islands, [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF]Marion Island, Mathakutha et al., 2019). Whenever we could not find information collected in comparable environments, we relied on functional trait data included in the TRY database [START_REF] Kattge | TRY plant trait database-enhanced coverage and open access[END_REF]. For each alien species, the dominant reproduction strategy in the study area was assessed relying on expert-based knowledge (personal communication, Lebouvier, M., & Bittebiere, A.K.). Species-specific values of the functional traits are reported in table S1.1 (Appendix S1) along with literature sources.

Topoclimatic layers

To model the species-environment relationship at fine spatial resolution, we first downloaded coarsegrained temperature (BIO1, BIO5 and BIO6annual mean temperature, max temperature of the warmest month and min temperature of the coldest month) and annual precipitation (BIO12) grid layers at 1-km resolution (at the equator) from the CHELSA database [START_REF] Karger | Climatologies at high resolution for the earth's land surface areas[END_REF] and then disaggregated their spatial resolution using physiographically informed models fitted through geographically weighted regression (GWR;Fotheringham & Rogerson, 2008). This downscaling technique allows statistically predicting the local value of the coarse-grain CHELSA climatic variables as a function of environmental grid layers available at finer spatial resolution (in this study 30-m at the equator, hereafter 30-m) and known to drive microclimate heterogeneity [START_REF] Lenoir | Climatic microrefugia under anthropogenic climate change: implications for species redistribution[END_REF][START_REF] Lembrechts | Comparing temperature data sources for use in species distribution models: From in-situ logging to remote sensing[END_REF]. GWR-derived topoclimatic layers, beyond allowing to model the species-environment relationship at a more meaningful spatial resolution, have already proved to better account for the complex interactions between macroclimate and topography [START_REF] Lenoir | Climatic microrefugia under anthropogenic climate change: implications for species redistribution[END_REF][START_REF] Lembrechts | Comparing temperature data sources for use in species distribution models: From in-situ logging to remote sensing[END_REF].

As using BIO5 (max temperature of the warmest month) and BIO6 (min temperature of the coldest month) in place of BIO1 did not improve species distribution models, we ultimately used BIO1 (hereafter mean temperature) and BIO12 as topoclimatic predictors. A full description of the downscaling procedure is reported in Appendix S2 along with the results of the GWR models.

Human-related layers

As human disturbances are known to favour the establishment of alien plants through propagule introduction and alteration of habitat conditions, we generated a 30-m resolution layer reporting the distance between each human settlement (the research station, Baie du La Pérouse, Pointe Basse and Baie Américaine) and any location on the island. Specifically, assuming that human disturbance is stronger in more accessible areas, we derived a least cost distance grid layer providing a measure of accessibility. Terrain slope changes between both orthogonally and diagonally neighbouring raster cells were used to compute the cost of reaching any location on Possession Island starting from any human settlement and following all potential paths of raster cells (function "accCost", "gdistance" R package; [START_REF] Etten | gdistance: Distances and Routes on Geographical Grids[END_REF]. High costs were thus associated with locations not easily reachable from human settlements due to high topographic roughness (Appendix S3, Figure S3.2).

A network of hiking paths has been designed by RNN TAF to restrict human movements for wildlife conservation purposes, and walking these paths currently constitutes the only authorized way to move across the island. As humans are a critical vector of propagule introduction and dispersal on sub-Antarctic islands, we derived a 30-m resolution raster layer reporting the distance between any location on Possession Island and the closest hiking path using the function "distance" from the "raster" R package [START_REF] Hijmans | raster: Geographic Data Analysis and Modeling. R package version 3[END_REF] (Appendix S3, Figure S3.2).

Alien species distribution modelling

The occurrence probability of the six studied alien plant species was separately modelled as a function of the topoclimatic (mean temperature and annual precipitation) and human-related variables (path distance and least cost) by means of logit binary generalized linear models (GLM). The single-species distribution models (single-SDMs) were trained and tested on datasets obtained through a re-sampling procedure of the presence/absence data performed in the environmental space to reflect all available environmental conditions on Possession Island [START_REF] Lenoir | Cross-scale analysis of the region effect on vascular plant species diversity in southern and northern European mountain ranges[END_REF][START_REF] Hattab | A unified framework to model the potential and realized distributions of invasive species within the invaded range[END_REF]; see Appendix S4). All four topoclimatic and human-related predictors were retained to fit the single-SDMs as the relative variance inflation factor (function "vif", R package "car"; Fox & Weisberg, 2019) was always below a threshold of 3. Second-order polynomial terms were included in the model to allow for intermediate niche optima of the species or in case lack-of-fit tests detected consistent departure from linearity in the profile of Pearson residuals (function "residualPlots", R package "car"; [START_REF] Fox | An R Companion to Applied Regression (Third)[END_REF]. The statistical significance of each predictor was tested using type II analysis of deviance (function "Anova", R package "car"; [START_REF] Fox | An R Companion to Applied Regression (Third)[END_REF]. We then computed the likelihood profile-based 95% confidence intervals of the regression parameters.

Single-SDMs predictive performance was measured using the true skill statistic (TSS, equal to sensitivity + specificity -1; function "ecospat.max.tss", R package "ecospat"; Broennimann, Di Cola, & Guisan, 2018) computed on the testing datasets obtained through the environmental matching described in Appendix S4. We used the TSS as it has desirable properties of other accuracy measures (e.g. Kappa and AUC), while being unaffected by prevalence [START_REF] Allouche | Assessing the accuracy of species distribution models: prevalence, kappa and the true skill statistic (TSS)[END_REF]. Also, we computed the deviance-based R² value as a measure of goodness-of-fit of each single-SDM.

The occurrence probability estimated by the full single-SDMs (including both topoclimatic and human-related predictors) for each alien plant species was mapped on a 30-m raster grid layer to visualise their predicted distribution across Possession Island.

Relationship between plant traits and alien species dependence on propagule pressure

As preliminary analyses, we measured the relative importance of human-related variables in determining alien species occurrence in the single-SDMs. To this aim, we used the sum of Akaike weights (w), which provides an easily interpretable measure of variable importance (it ranges from 0 to 1, with a high value for a given variable indicating its high importance relative to the others; [START_REF] Burnham | Model Selection and Multimodel Inference: a Practical Information-Theoretic Approach[END_REF]. Then we graphically related the species-specific values of the functional traits to the sum of weights to look for relationships between plant traits and the importance of human-related variables (see Appendix S7).

Secondly, we investigated how the interaction between human-related variables and plant traits affected alien species occurrence in a multi-species distribution model (multi-SDM), focusing on those functional traits that showed some relationship with the dependence on human-related variables in the single-SDMs. To this aim, we modelled the occurrence of all alien species together as a function of topoclimatic and human-related variables by means of a logit binary GLM, including the interaction between species identity and topoclimatic variables on the one hand and the interaction between species functional traits and human-related variables on the other hand. This allowed exploring how the effect of human-related variables on alien species occurrence varied according to plant traits, while controlling for species-specific responses to topoclimate. To select the most parsimonious model, we fitted all possible sub-models including different combinations of the functional traits-anthropogenic variables interaction terms (function "dredge", R package "MuMIn"; [START_REF] Barton | MuMIn: Multi-Model Inference[END_REF], always retaining the species-topoclimate interaction terms and the main effect of path distance and least cost in each candidate sub-model. Then, we computed the sum of Akaike weights for each model term and used the evidence ratio as a measure of the relative importance of variables [START_REF] Massol | The influence of trophic status and largescale climatic change on the structure of fish communities in Perialpine lakes[END_REF][START_REF] Burnham | Model Selection and Multimodel Inference: a Practical Information-Theoretic Approach[END_REF]. Specifically, we computed the evidence ratio of the i-th variable (ERi) as the odds of its sum of Akaike weights:

𝐸𝑅𝑖 = 𝑤𝑖

-𝑤𝑖

The evidence ratio was then compared with its expected value (ERnull) under the "null hypothesis" that the variable explained as much deviance as a randomly generated explanatory variable, and would thus be as likely as not to be incorporated in the best models. As all the variables were tested in a balanced design, ERnull = 1 in all tested cases. Following [START_REF] Massol | The influence of trophic status and largescale climatic change on the structure of fish communities in Perialpine lakes[END_REF], the effect of a variable i was deemed unlikely if ERi < 0.37 × ERnull, implausible when 0.37 × ERnull < ERi < ERnull, plausible when ERnull < ERi < 2.72 × ERnull, and likely when ERi > 2.72 × ERnull. These thresholds correspond to differences in Akaike information criterion equal to +2 or -2, which are commonly admitted as a good rule-of-thumb gap to compare model performance.

Results

Effect of topoclimatic and human-related variables on single species distribution

Predictive performances of the single-SDMs varied greatly across species (Table 1): high values of TSS were observed for P. pratensis, S. alsine and J. bufonius (0.80-0.82), while low values were obtained for the remaining species (from 0.09 to 0.29). The R 2 values showed a similar trend, with the highest value obtained for J. bufonius (0.48) and the lowest for C. fontanum (0.02).

Overall, the occurrence of P. pratensis, S. alsine and J. bufonius appeared to be strongly conditioned by both topoclimatic and human-related variables, while C. fontanum, P. annua and S. procumbens were less affected by human-related variables (Table 1, Figures 2 and3).

All alien species, except S. alsine, exhibited a significant positive or humped-shaped relationship with mean temperature (Table 1), meaning that their occurrence probability increased with increasing temperature (Figure 2, Appendix S5). More specifically, the occurrence probability of J. bufonius, S.

procumbens and C. fontanum peaked at mean temperature values around 4.5 °C, while the presence of P. pratensis and P. annua increased more or less linearly with temperature.

Annual precipitation significantly affected the presence of P. pratensis, S. alsine and J. bufonius, while it had a minor influence on the occurrence of the other species (Table 1). In particular, the odds of finding P. pratensis and J. bufonius decreased approximately by 90% for each 500 mm increment in annual precipitation, while the occurrence probability of S. alsine sharply decreased for annual precipitation values above 1,500 mm (Figure 2).

All species except C. fontanum exhibited a significant negative relationship with path distance (i.e.

the occurrence probability of the species decreased at increasing distances from the hiking paths), though its influence varied among species (Table 1, Figure 2, Appendix S5). In this regard, the odds of finding P. pratensis, S. alsine and J. bufonius decreased respectively by 20%, 16% and 19% moving 100 m away from the paths, while the odds of finding P. annua and S. procumbens decreased by 4% and 6%, respectively.

preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.

The copyright holder for this this version posted November 24, 2020. ; https://doi.org/10.1101/2020.07.19.210880 doi: bioRxiv preprint Least cost distance to settlements appeared to influence the occurrence of all analysed species except S. procumbens (Table 1). In particular, the odds of finding P. pratensis, S. alsine and J. bufonius decreased, respectively, by 17%, 13% and 44% for each increment of 5,000 units of cost of travelling a given path from a human settlement (Figure 2, Appendix S5). On the contrary, C. fontanum and P. annua showed a positive relationship with least cost, with their odds of occurring increasing respectively by 9% and 5% for each increment of 5,000 units of cost of travelling a given path from a human settlement (Figure 2 and Appendix S5).

Plant traits and species dependence on propagule pressure

In the preliminary analyses, residence time, life history, vegetative reproduction and plant height showed some relationship with the sum of weights of the human-related variables in the single-SDMs (Appendix S7, Figure S7.6 and S7.8), while seed-and leaf-related traits clearly showed no relationship (Appendix S7, Figure S7.7 and S7.9).

Then, the multi-SDM confirmed significant interactions of residence time, life history and plant height with the human-related variables (Appendix S7, Figure S7.10). Residence time and plant height appeared to interact with both human-related variables, while life history seemed to interact only with least cost in determining alien species occurrence. In particular, the effect of human-related variables on alien species occurrence varied with plant height. For instance, the occurrence probability of taller plants sharply decreased when moving away from human facilities, while a weaker and sometimes opposite trend was observed for plant species of shorter statures (Figure 4a,b and Appendix S7, Figure S7.11). In addition, old residents were on average less affected by the human-related variables than new residents (Figure 4a,b and Appendix S7, Figure S7.11a). Finally, perennials appeared to be on average less negatively affected by least cost distance to human settlements than annuals (Figure 4c and Appendix S7, Figure S7.11b,c).

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Discussion

As hypothesised, both environmental and human-related variables locally affected alien plant species occurrence on Possession Island, though with differences among the studied species. Overall, results confirmed the key role of human-related propagule pressure in favouring alien plant species establishment and spread on sub-Antarctic islands [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Le Roux | Human activities, propagule pressure and alien plants in the sub-Antarctic: Tests of generalities and evidence in support of management[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF], though we also observed a significant effect of abiotic conditions. Indeed, climate barriers seemed to prevent alien plant species occurrence in the most environmentally stressful areas of the island, as found in sub(ant)arctic mountain regions by [START_REF] Lembrechts | Disturbance is the key to plant invasions in cold environments[END_REF]. In particular, our results suggested the existence of two main invasion patterns arising from the species-specific dependence on human-related propagule pressure [START_REF] Frenot | Biological invasions in the Antarctic: extent, impacts and implications[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF]: low-spread species (P. pratensis, S. alsine and J. bufonius) strongly relying on human-assisted dispersal along hiking trails and in the vicinity of human settlements; and high-spread species (C. fontanum, P. annua and S. procumbens) mostly limited by harsh climatic conditions at high altitudes. Differences in plant invasiveness appeared to be influenced by residence time, life history and plant height, with old residents and perennial short species being more invasive.

Due to their dependence on human-related variables, low-spread species were predicted to occur mainly close to hiking paths and human settlements, pointing to the importance of anthropogenic activities as key drivers of continuous propagule pressure favouring species establishment [START_REF] Whinam | Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms[END_REF][START_REF] Pickering | Do tourists disperse weed seed? A global review of unintentional human-mediated terrestrial seed dispersal on clothing, vehicles and horses[END_REF]. Once introduced through ship-to-shore transport, propagules are then likely to be dispersed on hiking paths through trampling [START_REF] Whinam | Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms[END_REF]. However, the harsher environmental conditions characterizing the west side of Possession Island also limit the occurrence of low-spread species to the east. In particular, the west-east gradient of annual precipitation (Appendix S2, Figure S2.1) clearly overlaps with the low-spread species distribution (Figure 3 and Appendix S6), suggesting that their establishment might be prevented in areas with abundant precipitation. Nevertheless, the precipitation gradient is also connected to human presence, so that in the western side of the island (less inhabited and more preserved) anthropogenic propagule pressure is weaker. In any case, our results evidenced that low-spread species may lack important adaptations to successfully colonise less disturbed areas with limiting abiotic conditions, while remaining relegated to areas of high human presence.

On the contrary, high-spread species appeared weakly (yet positively) influenced by human-related variables, suggesting that, in spite of the undisputed importance of anthropogenic activities in promoting alien plants establishment [START_REF] Whinam | Subantarctic hitchhikers: expeditioners as vectors for the introduction of alien organisms[END_REF][START_REF] Huiskes | Aliens in Antarctica: assessing transfer of plant propagules by human visitors to reduce invasion risk[END_REF], high-spread species may possess key traits releasing them from direct dependence on anthropogenic propagule pressure.

Consequently, these species appeared to be mostly limited by the extreme climatic conditions of the high and cold inner sectors of Possession Island. This result is in line with the findings from [START_REF] Chwedorzewska | Poa annua L. in the maritime Antarctic: an overview[END_REF], who documented the rapid expansion of Poa annua from the Arctowski research base (King George Island) towards wilder areas of the maritime Antarctic Peninsula.

Nevertheless, the low predictive performance of high-spread SDMs indicates that the occurrence of these species is poorly explained by the influence of topoclimatic and human-related variables, so that other factors not considered here (e.g. soil properties, plant-soil microbiota interactions, snow cover) may play an important role in driving their distribution at even finer spatial resolutions. In this regard, better performances of the SDMs for high-spread species could have probably been achieved using alien species abundances, which are more informative of the relative habitat suitability than presence-absence data [START_REF] Howard | Improving species distribution models: the value of data on abundance[END_REF]. Critically, although we managed to obtain relatively high-resolution topoclimatic data, it is important to realize that the CHELSA climate for the island (1) might lack the accuracy it has at temperate latitudes, being based on extrapolations from a single weather station, and (2) represents air temperature only, while short plants as those analysed here relate more strongly to soil and nearsurface temperatures [START_REF] Convey | The importance of understanding annual and shorter-term temperature patterns and variation in the surface levels of polar soils for terrestrial biota[END_REF][START_REF] Lembrechts | Comparing temperature data sources for use in species distribution models: From in-situ logging to remote sensing[END_REF]. This highlights the need for in-situ soil-and near-surface temperature measurements in remote locations to get more ecologically meaningful climate data [START_REF] Lembrechts | SoilTemp: a global database of near-surface temperature[END_REF].

Although the small set of analysed alien plant species calls for caution in interpretation, we confirmed here that certain plant traits confer greater invasiveness in sub-Antarctic environments. By relating plant traits to species responses to human-related variables and analysing the effect of their interaction on alien species occurrence, we found evidence that low-stature was a key feature that discriminated invasive from non-invasive alien plant species under the harsh abiotic conditions of Possession Island.

Noteworthy, residence time and life history also appeared to affect species invasiveness.

As similarly reported by [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF], we found that high-spread species were of shorter statures than low-spread species. Consistently, we observed a sharper decrease in the occurrence probability of taller plants moving away from both hiking paths and human settlements. As plant height is generally associated with species adaptations to harsh environments [START_REF] Cornelissen | A handbook of protocols for standardised and easy measurement of plant functional traits worldwide[END_REF] and, specifically, low-stature has been attributed to frost avoidance mechanisms in high mountains [START_REF] Márquez | Freezing tolerance in grasses along an altitudinal gradient in the Venezuelan Andes[END_REF][START_REF] Ladinig | How endangered is sexual reproduction of highmountain plants by summer frosts? Frost resistance, frequency of frost events and risk assessment[END_REF], species of shorter statures may be reasonably favoured in windy and cold sub-Antarctic environments [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF] and therefore be more easily released of human dependence. Indeed, the importance of functional traits providing tolerance to abiotic stress increases with environmental harshness, even under strong anthropogenic disturbance [START_REF] Zefferman | Plant communities in harsh sites are less invaded: a summary of observations and proposed explanations[END_REF]. Further, our results supported the hypothesis that residence time positively affects invasiveness [START_REF] Lockwood | The role of propagule pressure in explaining species invasions[END_REF][START_REF] Pyšek | Naturalization of central European plants in North America: species traits, habitats, propagule pressure, residence time[END_REF], though with some exceptions. Generally, old residents (e.g. C. fontanum and P. annua) were less dependent on human-related propagule pressure and more widely spread than new resident species. Nevertheless, among the old residents, P. pratensis was strongly dependent on human-related variables and was still mostly restricted to the original introduction sites. On the other hand, S. procumbens, a new resident, has been able to spread extensively and quicker than the other (old residents) high-spread species. However, this might be due to the difference in plant height of the two species: while P. pratensis is among the tallest analysed species, S. procumbens is the shortest. The multi-SDM showed that perennials were slightly less dependent on human presence than annuals (Figure 4c). Although annuals might benefit from high dispersal abilities (e.g. abundant light seeds) and usually spread quicker and wider [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF], perennials can sustain short growing seasons [START_REF] Frenot | Human activities, ecosystem disturbance and plant invasions in subantarctic Crozet, Kerguelen and Amsterdam Islands[END_REF][START_REF] Shaw | Southern Ocean Islands invaded: conserving biodiversity in the world's last wilderness[END_REF] and potentially colonise harsher environments [START_REF] Dietz | Recognition that causal processes change during plant invasion helps explain conflicts in evidence[END_REF]. In our case, short perennials (e.g. C. fontanum) might be favoured over tall annuals (e.g. J. bufonius) due to the interaction between stress-tolerant traits, such as plant height, and high abiotic tolerance.

Albeit the interaction between vegetative reproduction and human-related variables was not included in the most parsimonious multi-SDM, alien species may still benefit from sexual reproduction, as suggested from the lower importance of human-related variables for the occurrence of alien plants reproducing sexually in the single-SDMs (Appendix S7). As discussed above, the high dispersal potential of sexually reproducing alien species, together with the ability to form rich seed banks [START_REF] Wódkiewicz | Spatial structure of the soil seed bank of Poa annua L.-alien species in the Antarctica[END_REF], may foster their extensive spread as, for instance, observed for P. annua in the Antarctic Peninsula [START_REF] Pertierra | Global thermal niche models of two European grasses show high invasion risks in Antarctica[END_REF]. Nonetheless, by reproducing vegetatively, perennials might outcompete annuals in maintaining viable and persistent populations during unfavourable seasons. Finally, in spite of their acknowledged importance in conferring invasiveness in sub-Antarctic islands [START_REF] Mathakutha | Invasive species differ in key functional traits from native and non-invasive alien plant species[END_REF], we found no evidence of the role of seed and leaf traits in affecting species dependence on human-related variables. This is possibly due to the small set of analysed alien species or to the lack of abundance data, which might have prevented the emergence of further functional traits-anthropogenic variables relationships.

Despite some limitations inherent to our dataset (e.g. limited number of species, lack of alien species abundance data) and other typical limitations specific to invasive species distribution modelling (e.g. underestimation of invasion potential, see [START_REF] Jiménez-Valverde | Use of niche models in invasive species risk assessments[END_REF], our approach allowed identifying fine-scale drivers of alien plant species distribution, along with the most likely features that favour their spread beyond sources of continuous human-assisted introductions. Combining information on both plant invasiveness and sub-Antarctic islands invasibility, our study provides relevant insights for anticipating future problematic invasions in these remote and unique environments.

Figure 1 -

 1 Figure 1 -Map of Possession Island showing: gross topography using three altitudinal belts (0-300 m; 300-600 m; 600-934 m); human settlements (black dots); and hiking paths (dashed lines). The inset map reports the geographical location of French sub-Antarctic islands, which include Crozet and Kerguelen archipelagos.

Figure 2 -

 2 Figure 2 -Response curves of the analysed alien species in the single-SDMs.

Figure 3 -

 3 Figure 3 -Predicted occurrence of Sagina procumbens and Juncus bufonius. Pocc: occurrence probability. Dashed lines represent hiking paths, while black dots represent human settlements. Occurrence maps of the other alien species are reported in Appendix S6 (Figure S6.4 and S6.5).

Figure 4 -

 4 Figure 4 -Effect of the interaction between human-related variables and plant features on alien species occurrence probability (logit scale). Panel (a): effect of the path distance-plant height interaction on old and new resident occurrence probability. Panel (b): effect of the least cost-plant height interaction on old and new (annual) resident occurrence probability. Panel (c): effect of least cost distance to human settlements on annual and perennial (100 mm height) alien species occurrence (for both old and new residents). All plots are reported on a logit scale.

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Table 1 -

 1 Single-SDM type II analysis of deviance tables and performance measures (R 2 and TSS). LR: Likelihood Ratio statistic; Df: degrees of freedom; p-val: p-value (*** p < 0.001; ** p < 0.01; * p < 0.05).

		Poa pratensis	Juncus bufonius	Stellaria alsine	Poa annua	Sagina procumbens Cerastium fontanum
	Predictors	LR Df	p-val	LR Df	p-val	LR Df	p-val	LR Df	p-val	LR Df	p-val	LR Df	p-val
	Mean temperature	5.083	1	* 32.516	2	***	0.312	1 = 0.576 55.538	1	*** 62.554	2	*** 18.325	2	***
	Annual precipitation 106.443	1	*** 73.647	1	*** 68.406	2	***	4.759	1	*	3.681	1 = 0.055	0.041	1 = 0.840
	Least cost	10.837	1	*** 40.250	1	***	4.420	1	*	5.778	1	*	1.118	1 = 0.290 12.483	1	***
	Path distance	15.623	1	***	8.740	1	**	7.524	1	**	3.927	1	*	6.782	1	**	2.877	1 = 0.089
	R 2			0.46			0.48			0.36			0.06			0.10			0.02
	TSS			0.80			0.81			0.82			0.19			0.29			0.09

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Acknowledgements

We would like to thank Marc Lebouvier (Ing. CNRS) for his suggestions which strongly improved the manuscript. We acknowledge the contribution of Lise Chambrin (Ing. RNN TAF) and all civil volunteers of the Institut Polaire Français Paul-Emile Victor (IPEV) who collected data used in this research. Also, we thank Dr. Matos, Prof. Convey and Prof. Hortal for their helpful suggestions and comments, which allowed strongly improving the first version of the manuscript. Funding: Project Granted by the French Polar Institute Paul-Emile Victor 'IPEV 136 Subanteco', Institut Universitaire de France (IUF) 'ENVIE', Stratégie d'attractivité durable (SAD) Région Bretagne 'INFLICT'. JJL is funded by the Research Foundation Flanders (FWO, project OZ7828), MC by a Rita Levi-Montalcini grant.

Data availability statement

Data and R code available on Zenodo: https://doi.org/10.5281/zenodo.4287498

Authors' contribution

DR, MB, FM and JL conceived the idea; MB analysed the data with FM, MC and JL; MB led the writing of the manuscript. All authors contributed critically to the drafts and gave final approval for publication.

Conflict of interest disclosure

The authors of this article declare that they have no financial conflict of interest with the content of this article.


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