Emilie Ladent

Agathe Leriche
email: agathe.leriche@imbe.fr

Bastien Thomas

Alexandre Millon

Orcid

Weak evidence of spatial segregation between the vulnerable southern water vole ( Arvicola sapidus ) and the two main invasive mammals of European freshwater ecosystems

Keywords: Ondatra zibethicus, Myocastor coypus, invasive species, native species, habitat segregation, nested spatial scales Ondatra zibethicus, Myocastor coypus, espèces invasives, espèce native, ségrégation de

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

Introduction

Niche differentiation represents a process allowing species to cope with competitively superior species [START_REF] Harrington | The impact of native competitors on an alien invasive: temporal niche shifts to avoid interspecific aggression[END_REF], and in particular with introduced competitive species. Indeed, the introduction of an alien species does not systematically lead to competitive exclusion of native species [START_REF] Cayuela | Multi-event models reveal the absence of interaction between an invasive frog and a native endangered amphibian[END_REF]). Niche differentiation involves three non-mutually exclusive dimensions: spatial, temporal and trophic [START_REF] Pianka | Sympatry of desert lizards (Ctenotus) in Western Australia[END_REF][START_REF] Case | Interference competition and niche theory[END_REF][START_REF] Albrecht | Spatial and temporal niche partitioning in grassland ants[END_REF][START_REF] Wereszczuk | Spatial niche segregation of sympatric stone marten and pine marten -avoidance of competition or selection of optimal habitat?[END_REF], with spatial habitat segregation being the most frequent [START_REF] Schoener | Resource partitioning in ecological communities[END_REF]. Spatial habitat segregation is therefore identified as an important mechanism to facilitate the coexistence of species sharing similar resources. Competition for resources decreases when species use different parts of a same area, either spatially or temporally [START_REF] Pita | Spatial segregation of two vole species (Arvicola sapidus and Microtus cabrerae) within habitat patches in a highly fragmented farmland landscape[END_REF]. Species coexistence is a scale-dependent process: coexistence at the large regional scale of the community may be facilitated by spatial segregation at a smaller, local scale [START_REF] Patterson | Regionally nested patterns of species composition in granivorous rodent assemblages[END_REF][START_REF] Brown | Assembly rules: desert rodent communities are structured at scales from local to continental[END_REF][START_REF] Amarasekare | Competitive coexistence in spatially structured environments: a synthesis[END_REF][START_REF] Pita | Hierarchical spatial segregation of two Mediterranean vole species: the role of patch-network structure and matrix composition[END_REF]. Conditions for coexistence between native and invasive species have received relatively little attention so far [START_REF] Peters | Habitat, predation, and coexistence between invasive and native crayfishes: prioritizing lakes for invasion prevention[END_REF]. However, it seems essential to understand the mechanisms leading to species coexistence and to identify the spatial scale at which these processes occur, in order to implement the appropriate management measures in a context of invasion.

When alien species are introduced into a new environment, they can alter the organisation and functioning of ecosystems [START_REF] Vilà | How well do we understand the impacts of alien species on ecosystem services? A pan-European, cross-taxa assessment[END_REF][START_REF] Pyšek | Invasive species, environmental change and management, and health[END_REF] through various processes such as predation [START_REF] Woodroffe | Patterns in the production of latrines by water voles (Arvicola terrestris) and their use as indices of abundance in population surveys[END_REF][START_REF] Bryce | Turning back the tide of American mink invasion at an unprecedented scale through community participation and adaptive management[END_REF], parasite and disease transmission [START_REF] Dunn | Competition and parasitism in the native white clawed crayfish Austropotamobius pallipes and the invasive signal crayfish Pacifastacus leniusculus in the UK[END_REF], hybridization [START_REF] Huxel | Rapid displacement of native species by invasive species: effects of hybridization[END_REF]) and competitive exclusion [START_REF] Byers | Competition between two estuarine snails: implications for invasions of exotic species[END_REF], which may ultimately lead to the extinction of native species [START_REF] Mooney | The evolutionary impact of invasive species[END_REF]. Buffering the potential impact of biological invasions is challenging and requires the identification of the process by which native species are impacted [START_REF] Mooney | The evolutionary impact of invasive species[END_REF][START_REF] Ricciardi | Predicting the impacts of an introduced species from its invasion history: an empirical approach applied to zebra mussel invasions[END_REF][START_REF] Rouget | Invasion debt -quantifying future Biol Invasions[END_REF]. Competition with native species is one of the most common consequences of alien species' introduction [START_REF] Cadi | Impact of the introduction of the red-eared slider (Trachemys scripta elegans) on survival rates of the European pond turtle (Emys orbicularis)[END_REF][START_REF] Violle | Phylogenetic limiting similarity and competitive exclusion[END_REF]. Many studies have indeed shown that successful invasive species are competitively superior to native species [START_REF] Heard | Coexistence between native and exotic species is facilitated by asymmetries in competitive ability and susceptibility to herbivores[END_REF].

The southern water vole Arvicola sapidus [START_REF] Miller | XXXI.-Eighteen new European voles[END_REF]; hereafter SWV) is a native semi-aquatic rodent occurring in freshwater habitats in parts of France, Spain and Portugal. Its distribution and habitat selection are mostly determined by the combined presence of freshwater, food and vegetation cover [START_REF] Bonesi | The combined effect of environmental factors and neighbouring populations on the distribution and abundance of Arvicola terrestris. An approach using rule-based models[END_REF]. SWV populations are declining in a large part of its distribution and therefore this species is classified as vulnerable in the Red List of Threatened Species (IUCN 2017). The decline of SWV populations is mostly due to the destruction and fragmentation of its habitat [START_REF] Pita | Influence of land mosaic composition and structure on patchy populations: the case of the water vole (Arvicola sapidus) in Mediterranean farmland[END_REF][START_REF] Grilo | Species-specific movement traits and specialization determine the spatial responses of small mammals towards roads[END_REF]) and predation by an invasive predator, the American mink Neovison vison [START_REF] Melero | Evaluating the effect of American mink, an alien invasive species, on the abundance of a native community: is coexistence possible?[END_REF][START_REF] García-Díaz | The impact of the American mink (Neovison vison) on native vertebrates in mountainous streams in central Spain[END_REF]. Grey literature also suggests the impact of competition with other invasive species such as the coypu (Myocastor coypus Molina) and the muskrat (Ondatra zibethicus L.) (e.g. Rigaux 2015) but to our knowledge no formal scientific studies have confirmed this. Muskrat and coypu, two semi-aquatic rodents, were introduced in Europe between the late 19 th -early 20 th centuries for fur trade. In France, individuals were released into the wild following farm closures, and the two species rapidly colonised most of the territory, during the 1940s-1950s for muskrat and 1960s-1970s for coypu [START_REF] Maurin | L'Homme et les mammifères de France métropolitaine: évolution historique et introductions d'espèces dans les milieux humides et aquatiques[END_REF]. Their ecological plasticity facilitated their expansion over a wide range of freshwater and brackish ecosystems across Europe, where their excavation activities alter riverside ecosystems [START_REF] Carter | Modeling the effects of nutria (Myocastor coypus) on wetland loss[END_REF][START_REF] Mackenzie | The impacts of non-native species on UK biodiversity and the effectiveness of control[END_REF].

About sixty years after colonisation by coypu and muskrat, very little is known about the current impact of these two invasive aquatic rodents on the SWV. Coexistence of these species has been recorded, suggesting that processes such as spatial habitat segregation (i.e. niche differentiation) may have taken place.

Here we tested for the occurrence of niche differentiation between SWV and the two larger-bodied aquatic rodents at two nested spatial scales in a large river catchment of northwest France (2,300 km 2 ). Specifically, we conducted a site-occupancy survey, accounting for imperfect detection, to assess the SWV distribution in relation with the presence of invasive species and riparian habitat characteristics at the catchment scale. Then, in each cell occupied by SWV, we further explored the pattern of spatial co-occurrence between species at the scale of a rodent home range.

Material and methods

Study species

Weighting up to 280 g, the southern water vole is the largest vole of Europe. It is considered highly dependent on freshwater [START_REF] Garde | The diet of the southern water vole, Arvicola sapidus in Southern Navarra (Spain)[END_REF], although the species may be found in dry streams, ponds, irrigation ditches and wetlands, sometimes far from main water bodies [START_REF] Fedriani | Local and landscape habitat determinants of water vole distribution in a patchy Mediterranean environment[END_REF][START_REF] Pita | Spatial segregation of two vole species (Arvicola sapidus and Microtus cabrerae) within habitat patches in a highly fragmented farmland landscape[END_REF]. Its main habitat consists of tall and dense herbaceous vegetation patches dominated by a hygrophilic plant community [START_REF] Fedriani | Local and landscape habitat determinants of water vole distribution in a patchy Mediterranean environment[END_REF][START_REF] Pita | Spatial segregation of two vole species (Arvicola sapidus and Microtus cabrerae) within habitat patches in a highly fragmented farmland landscape[END_REF][START_REF] Peralta | Factors affecting Southern water vole (Arvicola sapidus) detection and occupancy probabilities in Mediterranean farmland[END_REF]. Generally, SWV colonies comprise between 6 and 31 individuals (mean ± 1 SD: 19 ± 8; Centeno-Cuadros et al. 2011) distributed over a suitable patch of 80-150 m long [START_REF] Rigaux | Élements sur la densité locale et l'utilisation de l'espace du campagnol amphibie (Arvicola sapidus) : exemple de trois sites en Auvergne[END_REF], with a strong site fidelity. The home range size depends on habitat quality (mean size of 946 ± 28.2 m 2 [START_REF] Pita | Spatial segregation of two vole species (Arvicola sapidus and Microtus cabrerae) within habitat patches in a highly fragmented farmland landscape[END_REF]; 2615 ± 801 m 2 for individuals with stable home range [START_REF] Mate | Spatial organization and intraspecific relationships of the southern water vole (Arvicola sapidus) in a Mediterranean mountain river: what is the role of habitat quality?[END_REF]).

Originating from North America, the muskrat is a semi-aquatic rodent that weights from 0.9 to 1.36 kg [START_REF] Donohoe | Muskrat reproduction in areas of controlled and uncontrolled water-level units[END_REF]. This species colonised northwest France in the 1940s [START_REF] Maurin | L'Homme et les mammifères de France métropolitaine: évolution historique et introductions d'espèces dans les milieux humides et aquatiques[END_REF] and is currently present in a wide variety of semi-aquatic habitats such as ponds, rivers, lakes, streams, irrigation canal and road ditches [START_REF] Bourget | Stratégie de gestion de la faune déprédatrice en milieu agricole. Rimouski (Québec): Direction de l'expertise Faune-Forêts-Territoire[END_REF]. The home range of muskrats spans over 582 ± 56 m of riverside with a using space aggregated around established burrows [START_REF] Ahlers | Home ranges and space use of muskrats Ondatra zibethicus in restricted linear habitats[END_REF]. Native from South America, the coypu is a large semi-aquatic rodent (6.5-10 kg) which colonized northwest France in the 1970s [START_REF] Maurin | L'Homme et les mammifères de France métropolitaine: évolution historique et introductions d'espèces dans les milieux humides et aquatiques[END_REF] and lives in similar habitats to those used by muskrats [START_REF] Baroch | Nutria (Myocaster coypus) in Louisiana[END_REF]. Rarely observed further than 100 m away from rivers [START_REF] Bertolino | Effectiveness of coypu control in small Italian wetland areas[END_REF], coypus have home ranges varying from 2 to 5 ha (up to 12 ha) and can travel distances between 40 m and 1.25 km per day, although most do not go further than 400 m a day [START_REF] Micol | Démographie du ragondin[END_REF].

Study area

We conducted this study in the Risle river catchment, located in northwest France (49.095°N 0.566°E EPSG: 4326; Figure 1). The catchment area of this river and its major tributary, the Charentonne, covers an area of 2,300 km 2 . The Risle river runs for 210 km before joining into the Seine River. With a total of 36 secondary tributaries, the catchment area comprises almost 760 km of permanent rivers and streams. This area is characterized by oceanic climate with moderate precipitations well distributed over the year (ca. 170 rainy days). Nevertheless, the average annual rainfall varies from 840 mm per year in the west to 620 mm in the east. The upstream of the river is made of impermeable soils (marls, hard limestone, sandy or siliceous clays), inducing sudden change in flow rate in this area following rainy events (SAGE Risle et Charentonne 2005). The downstream of the river has on the other hand a porous substrate, ensuring a more regular flow. Riversides of the Risle and its tributaries are relatively well preserved and mainly constituted of permanent pastures and wetlands areas. The riparian woodland is mostly discontinuous and narrow with denser patches occurring along small tributaries.

Field procedures and survey protocols

The permanent natural (streams, rivers) and artificial (ponds) water bodies occurring in the study area were represented into a random-origin grid of 606 cells of 1 × 1 km using Quantum GIS v.2.14.3 (QGIS Development Team 2016), among which 150 cells were randomly selected for surveying (Figure 1). A further selection was conducted using aerial photographs to exclude cells with unsuitable areas for SWV, defined as dense riparian woodland and urbanised areas without riparian vegetation. During the first field prospection in March 2016, hygrophilic riparian vegetation, i.e. the suitable habitat for SWV [START_REF] Pita | Assessing habitat differentiation between coexisting species: the role of spatial scale[END_REF][START_REF] Pita | Influence of land mosaic composition and structure on patchy populations: the case of the water vole (Arvicola sapidus) in Mediterranean farmland[END_REF], was searched for in every selected cell. If no suitable habitat was found in a cell, the latter was no longer prospected and was excluded from the dataset. Within each selected cell (N = 98, 16.2% of the study area), a 100 m-long transect (hereafter 'site') was surveyed on the riverside judged as the most favourable for the SWV (same transect length as used by the SWV French National Survey [START_REF] Rigaux | Les campagnols aquatiques en France -histoire, écologie, bilan de l'enquête 2009-2014[END_REF]. A visual cue was positioned (and recorded with GPS) at the beginning of each site to ensure the same site was surveyed at every visit. A unique, previously trained, observer (EL) meticulously waded along the transect, seeking for faeces for 35 minutes within 2 m from the riverside. Faeces of the SWV [START_REF] Fedriani | Local and landscape habitat determinants of water vole distribution in a patchy Mediterranean environment[END_REF][START_REF] Mate | Habitat selection by Southern water vole (Arvicola sapidus) in riparian environments of Mediterranean mountain areas: a conservation tool for the species[END_REF][START_REF] Pita | Influence of land mosaic composition and structure on patchy populations: the case of the water vole (Arvicola sapidus) in Mediterranean farmland[END_REF]) are easily distinguishable from the two invasive species [START_REF] Rezendes | Tracking and the art of seeing, 2nd edition: how to read animal tracks and signs[END_REF] thanks to their colour and size and therefore provide a reliable method for recording SWV presence. Transect survey was stopped as soon as faeces of the three species were found. The same observer visited each site at three occasions in 2016, between 16 March and 14 May, i.e. during the SWV breeding season where patch occupancy can be assumed constant (for all species). Visits were at least 2 weeks apart to minimise observer bias due to memory effect. We assumed that no site colonisation or extinction occurred during the survey (breeding) period.

Detection probability varies spatially due to sitespecific habitat characteristics patterns and temporally in relation to breeding behaviours and changing environmental conditions [START_REF] Bailey | Estimating site occupancy and species detection probability for terrestrial salamanders[END_REF]). Surveys were not conducted during heavy rains as they reduce faeces detection [START_REF] Fusillo | Survey of an otter Lutra lutra population in Southern Italy: site occupancy and influence of sampling season on species detection[END_REF]). We recorded rainy events occurring within the last three days preceding the survey (Rainfall, a binary variable) to account for its effect on detectability. Other survey-specific covariates included date of prospection (Date: we used Julian date with day 1 = March 16), the height of riparian vegetation (Veg_h: <30 cm or ≥30 cm, a binary variable) and the water level in the river (Water_rise: 0/1, a binary variable). For the former parameter, it has been defined that Water_rise = 1 when the water level was above the riparian vegetation (>10 cm) at least in part of the transect. As the vegetation height was sufficiently homogeneous along the whole site for only one of the two categories (< 30 cm or ≥ 30 cm) to be represented, only one measurement of this binary variable was made.

In order to characterize the habitat of SWV, in addition to vegetation hight (Veg_h) we measured sitespecific covariates potentially relevant for occupancy alongside a 100 m-long transect. We collected data on the type of the most representative riparian vegetation located within 2 m from the riverside (Veg_type) at each site: mesophilic or hygrophilic plants, woody vegetation. Vegetation height (Veg_h) was used to model both detection and occupancy of SWV. At each site, percentage of cover by aquatic vegetation (Veg_co) was visually estimated on the whole transect using four vegetation cover classes: 0, 1-25%, 26-50% and 51-100%. Vegetation cover might positively affect the capacity of SWV to move alongside the river. From aerial photos taken in 2012-2015, we estimated river width (River_w) on QGIS, from an average over 10 points located at equal intervals alongside the transect.

As our intent was also to determine if the presence of SWV was affected by the presence of invasive species (muskrat and coypu), while accounting for habitat features, we assessed the use of a particular site by the invasive rodents (D_Onda; D_Myo) on a scale ranging from 0 to 3, based on the number of detections after three occasions. This covariate was also altered into presence-absence data (species-specific presence: P/ A_Onda; P/A_Myo; presence of at least one invasive rodent: P/A_inv).

As a second step, we assessed whether spatial segregation in habitat use between the three species occurs at small spatial scale (SWV home range). To achieve this, we selected, among the 98 sites surveyed for the large-scale protocol those with both SWV and at least one of the two invasive species detected during at least two occasions (N = 27). Each original transect was lengthened to 200 m and subdivided into 10 segments of 20 m long that constituted the adjacent spatial replicates. Each segment was sampled only once (between 15 June and 30 June) for 10 minutes, by the same observer to detect faeces of SWV, muskrat and coypu. Presence of muskrat (P/A _Onda ), presence of coypu (P/A _Myo ), presence of any of these two species: 0 or 1 (P/A _inv ) and species richness of invasive species: 0, 1 or 2 (Rich _inv ), were treated as categorical variables. In each segment, we also characterised the herbaceous plant type (mesophilic or hygrophilic) and the height (≥30 cm) of riparian vegetation located between 0 and 0.5 m (Veg_type1; Veg_h1) and 0.5-2 m (Veg_type2_H; Veg_h2) from the riverside and presence of wood between 0.5-2 m (Veg_type2_W). Local scale surveys were always carried out in periods without precipitation over the last three days. Only 18 sites of the 27 original ones were surveyed because of accessibility issues (reluctant landowners, development of dense thorns). The same observer visited every segment in June 2016. Two of these 18 sites were excluded following the important deterioration of the riverside vegetation due to intensive browsing by cows; the final dataset included 16 sites.

Statistical procedures

For analysing the large-scale survey, we used hierarchical occupancy models to estimate SWV occupancy probability in relation to habitat features and occurrence of invasive species [START_REF] Mackenzie | The impacts of non-native species on UK biodiversity and the effectiveness of control[END_REF]. We modelled the probability of detection p and occupancy rate ψ using single-season occupancy models with Program PRESENCE v.11.0 [START_REF] Hines | Program PRESENCE[END_REF]. Model assumptions were that (1) occupied sites remain occupied for the duration of the survey, (2) the species is not detected when absent (i.e. no false positive), and (3) detection at one site is independent of detection at all other sites [START_REF] Mackenzie | The impacts of non-native species on UK biodiversity and the effectiveness of control[END_REF]. The probability of detecting the invasive species at least once over three surveys was very close to 1 (see results), such that a multispecies occupancy modelling approach would have brought only little added value but too many parameters given the sample size available.

First, we explored the effects of different covariates on detectability, using a full parameterisation for occupancy (P/A_inv + Veg_h + Veg_co + Veg_type + River_w). Models were run with detectability set as a function of Date, Rainfall, Veg_h, Water_rise (all additive combinations tested). We selected the best detectability model based upon the Akaike Information Criterion (AIC; [START_REF] Burnham | Multimodel inference[END_REF]. This parameterisation was then held constant while a similar model selection procedure was performed regarding occupancy. Occupancy was modelled as a function of habitat features and presence of invasive species: Veg_type, Veg_h, Veg_co, River_w, D_Onda, D_Myo, P/A_Onda, P/A_Myo, P/ A_inv. All combinations of additive models were tested, with a maximum of 5 covariates for occupancy, keeping the ratio N/K ≥ 10 (N: sample size, K: number of parameters used in the model). Moreover, the sum of AIC model weights for every covariate was calculated to demonstrate the influence of each covariate [START_REF] Burnham | Multimodel inference[END_REF]. Similar models (intercept only) were run for both the coypu and the muskrat to derive species-specific p and ψ.

The sampling design used at a local spatial scale involved a component of spatial dependency between replicates (segments). To account for nonindependence in the data, we used the recently developed Markovian occupancy models designed for spatially adjacent replicates [START_REF] Hines | Tigers on trails: occupancy modeling for cluster sampling[END_REF]. This model estimated four parameters: p, the probability of detecting the SWV in a segment conditional on the presence of the species on the site; ψ, the probability that a site is occupied or used by a species; 00, the probability that a species is present on a segment given that the site is occupied but the species was absent on the previous adjacent segment; and 01, the probability that a species is present on the segment given that the site is occupied and that the species was present on the previous adjacent segment [START_REF] Hines | Tigers on trails: occupancy modeling for cluster sampling[END_REF][START_REF] Karanth | Monitoring carnivore populations at the landscape scale: occupancy modelling of tigers from sign surveys[END_REF][START_REF] Charbonnel | Spatial replicates as an alternative to temporal replicates for occupancy modelling when surveys are based on linear features of the landscape[END_REF][START_REF] Charbonnel | Integrating hydrological features and genetically validated occurrence data in occupancy modelling of an endemic and endangered semi-aquatic mammal, Galemys pyrenaicus, in a Pyrenean catchment[END_REF]. Surveys based on adjacent spatial replicates along streams and the Markovian occupancy model parameter settings are adequate for a species with a small and linear home range [START_REF] Charbonnel | Spatial replicates as an alternative to temporal replicates for occupancy modelling when surveys are based on linear features of the landscape[END_REF]). In the sampling design, each segment was sampled only once, such that we assumed p = 1 for every segment. We tested the influence of each covariate (Veg_type1, Veg_type2_H, Veg_type2_W, Veg_h1, Veg_h2, P/A_Onda, P/A_Myo, P/ A_inv and Rich_inv) on ψ. Model selection procedure was similar to previously described. Estimates are presented as mean ± SE.

Results

Southern water vole habitat and species co-occurrence at a catchment spatial scale

The southern water vole was detected at least in one of the three occasions in 47 of the 98 cells sampled and the occupancy rate, accounting for imperfect detection, averaged 0.52 ± 0.06. Detectability of the SWV averaged 0.73 ± 0.04 (95% CI = [0.64; 0.80]) per occasion. Over three occasions, the cumulated probability of detection of the species reached 0.98. Detectability decreased with survey date (effect size β = --0.034 ± 0.012 per day on logit scale). For an average survey date (day = 33.6), detectability following the rise of the river level decreases from 0.77 to 0.05. We did not find evidence for an effect of rainfall or vegetation height on the detectability of the SWV (Table 1). The two invasive species showed higher occupancy rates with 0.80 ± 0.04 and 0.58 ± 0.05 for the muskrat and the coypu, respectively (Figure 2). The probability of detecting a coypu or a muskrat at least once over three surveys was very close to one: 0.97 and 0.99, respectively.

Among the habitat covariates tested for occupancy, the best model retained vegetation height and river width (Table 2). Specifically, vegetation height on the river side was also highly influential with occupancy rate rising from ψ = 0.08 when it was shorter than 30 cm, to ψ = 0.74 when it was higher than 30 cm (Figure 3). Secondarily, larger parts of the river tended to host SWV more often (β = 0.69 ± 0.34 on logit scale; Figure 3).

SWV occupancy was not affected by the presence of neither of the two invasive aquatic rodents (coypu or muskrat; Figure 2). This result held true considering the presence of either species ('invasive' considered as present when at least one species was present), the presence of each species separately (two-level factors) or the sum of detection over the three surveys (a proxy of habitat uses for each species separately).

Patterns of habitat use of the three aquatic rodents at a small spatial scale

On the 16 sites selected to evaluate a potential spatial segregation at a local scale between SWV and the two invasive species, muskrat was ubiquitous (all sites), and coypu was detected at 10 sites. Muskrat was detected at 102 out of the 160 segments (63.8%), while SWV was detected at 74 (46.3%) and coypu at 62 (38.8%).

We compared the relative performance of a set of plausible alternative models for SWV habitat use. For the 7 models showing a difference in AIC ≤ 2, a common structure with 3 covariates (Veg_type1, Veg_h1, Veg_h2) was retained (Table 2). The sum of AIC model weights for every covariate also indicated that these covariates were the most relevant for explaining SWV occupancy (Figure 4). These models suggested that habitat use by SWV was positively influenced by the riparian vegetation height (Veg_h2), specifically between 0.5 and 2 m of the riverside (from the best model: β = 2.13 ± 0.86, on logit scale; Figure 4). SWV occupancy increased when the height of the riparian vegetation between 0 and 50 cm (Veg_h1) was greater than 30 cm (β = 1.99 ± 0.92). Habitat use of SWV was affected by type of riparian vegetation between 0 and 50 cm (Veg_type1) of the riverside with a decrease of occupancy when vegetation was mesophilic (β = -1.75 ± 0.61) compared with a hygrophilic vegetation. The most parsimonious model (Table 2) also included the presence of muskrat, but the decrease in AIC was marginal compared to the second-best model with the three habitat variables only (∆AIC = 0.06). At this small spatial scale, the presence of muskrat tended to decrease SWV habitat use, though not significantly (β = -1.12 ± 0.83).

Discussion

The occurrence of SWV was primarily dependent upon the presence of hygrophilic vegetation on the very edge of the river. We found no clear evidence for a competitive exclusion process operating at either small (species home range) or large (river catchment) spatial scales. While we cannot rule out the negative impact of the large-bodied rodents on SWV during the early phase of the invasion phase in the second half of the 20 th century, the three species seem to currently coexist in riparian habitats of northwest France.

Riparian habitats and occurrence of the Southern water vole

At both large (catchment) and small (home range) spatial scales, SWV occurrence/habitat use was primarily influenced by the type and height of the riverside vegetation. These results are consistent with previous studies [START_REF] Pita | Assessing habitat differentiation between coexisting species: the role of spatial scale[END_REF][START_REF] Mate | Habitat selection by Southern water vole (Arvicola sapidus) in riparian environments of Mediterranean mountain areas: a conservation tool for the species[END_REF][START_REF] Peralta | Factors affecting Southern water vole (Arvicola sapidus) detection and occupancy probabilities in Mediterranean farmland[END_REF]. SWV is considered as a specialist of habitats with dense hygrophilic vegetation [START_REF] Pita | Assessing habitat differentiation between coexisting species: the role of spatial scale[END_REF][START_REF] Mate | Habitat selection by Southern water vole (Arvicola sapidus) in riparian environments of Mediterranean mountain areas: a conservation tool for the species[END_REF][START_REF] Mate | Spatial organization and intraspecific relationships of the southern water vole (Arvicola sapidus) in a Mediterranean mountain river: what is the role of habitat quality?[END_REF] with Juncus spp., Carex spp., Phragmites spp. and Typha spp. This permanent vegetation cover provides food resources, shelter from predators as well as nesting sites [START_REF] Barreto | The role of habitat and mink predation in determining the status and distribution of water voles in England[END_REF][START_REF] Fedriani | Local and landscape habitat determinants of water vole distribution in a patchy Mediterranean environment[END_REF][START_REF] Mate | Habitat selection by Southern water vole (Arvicola sapidus) in riparian environments of Mediterranean mountain areas: a conservation tool for the species[END_REF][START_REF] Peralta | Factors affecting Southern water vole (Arvicola sapidus) detection and occupancy probabilities in Mediterranean farmland[END_REF]. As [START_REF] Fedriani | Local and landscape habitat determinants of water vole distribution in a patchy Mediterranean environment[END_REF], we found that patches with such vegetation cover ≥ 30 cm tall provide suitable sections of riverside for SWV. At the spatial scale of the river catchment, we found quite unexpectedly that SWV was slightly more likely to occur alongside large rivers (mean river width in the area: 5.7 ± 6.0, range: 0.6-26.4 m). This finding contradicts previous results from a survey conducted at the national level in France and showing that the species was slightly more likely to be found alongside rivers of 0.5-1 m wide [START_REF] Rigaux | Les campagnols aquatiques en France -histoire, écologie, bilan de l'enquête 2009-2014[END_REF]. Two hypotheses could explain this discrepancy. First, narrow rivers in our Table 1. Results of the model selection regarding the detection probability p and the probability of occurrence (ψ) of the Southern water vole. The number of parameters contributing at the model (k), the Akaike Information Criterion (AIC), the Delta AIC (Δ AIC) (i.e. the difference in AIC between focal and best models). To estimate the probability of detection, the probability of occurrence (ψ) for all models was held constant: ψ(P/A_inv + Veg_h + Veg_co + Veg_type + River_w). To model the occupancy propability, the detection probability (p) was held constant (with the best parameterisation found p(Date+Water_rise). Only a subset of models are shown. catchment mainly occur upstream and typically show extensive riparian woodland areas at the expense of dense herbaceous riverbed, due to competition for light [START_REF] Ricard | Abundance, growth and allometry of red raspberry (Rubus idaeus L.) along a natural light gradient in a northern hardwood forest[END_REF]. Second, upstream rivers occur on more impermeable soils compared to downstream and concentrate small hydraulic structures (ca. 400 hydraulic structures in the catchment area) causing important and rapid fluctuations of water level (SAGE Risle et Charentonne 2005), that can lead to the flooding of SWV burrows [START_REF] Carter | Reedbeds as refuges for water voles (Arvicola terrestris) from predation by introduced mink (Mustela vison)[END_REF]. Many aquatic and semi-aquatic species have evolved with annual flood pulses and are able to adapt to seasonal and cyclic changes in river flows [START_REF] Sparks | Need for ecosystem management of large rivers and their floodplains[END_REF]. Nevertheless, extreme fluctuations in water levels caused by river management can have both direct and indirect negative impacts on those species [START_REF] Alho | Effects of severe floods and droughts on wildlife of the Pantanal wetland (Brazil) -A review[END_REF]. Direct impacts involve burrow flooding that can cause litter drowning and force individuals to move in sub-optimal habitats where they might become more vulnerable to predators [START_REF] Moorhouse | Effects of habitat quality upon reintroduction success in water voles: evidence from a replicated experiment[END_REF].

Coexistence of native and invasive semi-aquatic rodent species

At large spatial scale, we did not detect any negative effect of invasive species on SWV. The three species seem to coexist at the scale of a river catchment in northwest France, with a relatively high level of patch occupancy (0.52, 0.80 and 0.58 for SWV, muskrat and coypu, respectively). We further tested whether this coexistence at the large-scale could be the result of a spatial segregation operating at the home-range level. The spatial segregation in habitat use may be measurable as a reduced occupation probability by one species when the other is present [START_REF] Amarasekare | Competitive coexistence in spatially structured environments: a synthesis[END_REF][START_REF] Fisher | Spatial segregation of sympatric marten and fishers: the influence of landscapes and species-scapes[END_REF]). However, we only found a weak support for this hypothesis, the presence of muskrat retained in the best model showing a negative but not significant effect on SWV habitat use. However, this analysis may suffer from reduced statistical power as the occupancy of adjacent segments was surveyed only once. Imperfect detection tends to underestimate the effect of covariates [START_REF] Tyre | Improving precision and reducing bias in biological surveys: estimating false-negative error rates[END_REF]. SWV territories can overlap both with those from conspecifics and with other rodents species [START_REF] Pita | Spatial segregation of two vole species (Arvicola sapidus and Microtus cabrerae) within habitat patches in a highly fragmented farmland landscape[END_REF]. Our test of spatial segregation occurred over a month in a single year, and we cannot therefore exclude the possibility of stronger segregation during another season [START_REF] South | Extrapolating from individual movement behaviour to population spacing patterns in a ranging mammal[END_REF] or in years with reduced food availability [START_REF] Andrzejewski | Abundance of food supply and size of the bank vole's home range[END_REF]. A telemetry study conducted on both SWV and muskrat could provide additional information on a potential spatial segregation [START_REF] Benson | Inter-specific territoriality in a Canis hybrid zone: spatial segregation between wolves, coyotes, and hybrids[END_REF]. Coexistence of species sharing similar resources can be possible if the overall carrying capacity of the environment is not reached [START_REF] Sayre | The genesis, history, and limits of carrying capacity[END_REF]). Here, SWV, muskrat and coypu consume overall, the same food resource consisting of hygrophilic vegetation. Our sampling design was focused on suitable habitats for SWV (i.e. dense herbaceous vegetation). In such optimal habitats, spatial segregation appeared to be weak, possibly because the carrying capacity is sufficient to meet the needs of the three species. Whether SWV suffers more from the competition with muskrat and coypu in sub-optimal habitats, therefore limiting its potential distribution, remains to be assessed.

Our results show weak, if any, spatial segregation between the two invasive species and the SWV, whatever the spatial scale considered. However, this finding does not mean that coypu and muskrat have not induced in the past a shrinkage in the distribution and abundance of the SWV. Indeed, invasive species impact native communities through different processes alongside the successive phases of colonisation, establishment, spread and equilibrium, the first three ones being the most deleterious [START_REF] Salo | Red brome (Bromus rubens subsp. madritensis) in North America: possible modes for early introductions, subsequent spread[END_REF][START_REF] Ricciardi | Predicting the impacts of an introduced species from its invasion history: an empirical approach applied to zebra mussel invasions[END_REF]). The contemporary situation may reflect a new stable equilibrium, with the native species occurring at a much lower density than before the invasions. For instance, [START_REF] Ruffino | Invasive rats and seabirds after 2,000 years of an unwanted coexistence on Mediterranean islands[END_REF] showed that, at the regional scale, black rat (Rattus rattus) Note: Vegetation type of riparian vegetation located between 0-50 cm from the riverside (Veg_type1), herbaceous type of riparian vegetation located between 50 cm-2 m from the riverside (Veg_type2_H), presence of wood on riparian vegetation located between 50 cm-2 m from the riverside (Veg_type2_W), height of vegetation of riparian vegetation located between 0-50 cm from the riverside (Veg_h1), height of riparian vegetation located between 50 cm-2 m from the riverside (Veg_h2), presence of muskrat (P/A_Onda), presence of coypu (P/A_Myo), presence of any of these two species (P/A_inv) and species richness of invasive species (Rich_inv).

Figure 3. Probability of occurrence of the southern water vole at the catchment scale according to log-transformed river width, derived from the best occupancy model (ψ (Veg_h + River_w), see Table 1), accounting for detectability (p (Date + Water_rise)). Grey and open circles correspond to tall (> 30 cm) and short (< 30 cm) riparian hygrophytic vegetation. A small jitter has been added on the y-axis to improve the visibility of data points.

presence was only a limiting factor in the abundance of the smallest seabird, the storm petrel (Hydrobates pelagicus), while the distribution and abundance of three large-bodied shearwaters were more influenced by island characteristics in the Mediterranean. However, a rat removal experiment conducted on a Tunisian island showed an immediate increase in breeding numbers of Scopoli's shearwater (Calonectris diomedea), demonstrating that in the apparent equilibrium previously observed, the native species was kept at very low density by invasive rats [START_REF] Bourgeois | Dramatic increase in the zembretta yelkouan shearwater breeding population following ship rat eradication spurs interest in managing a 1500-year old invasion[END_REF]. Many removal programs are currently conducted on muskrat and coypu throughout France and looking at their impact on SWV populations, in a properly designed experiment, would be insightful for optimising conservation actions on riparian ecosystems.

Figure 1 .

 1 Figure 1. Study area in the Risle river catchment in northwest France, and its major tributary, the Charentonne river. The stream network is indicated in grey. Squares represent random site selection with black squares indicating patches with favourable habitat used in the survey (N = 98). Grey squares indicate patches of unsuitable habitat that were excluded from the survey (N = 52).

Figure 2 .

 2 Figure 2. Observed (dark bars) vs. expected (light bars) species occurrence and co-occurrence (number of sites) of the three aquatic rodent species: southern water vole (Arvi), coypu (Myoc) and muskrat (Onda). The expected frequencies were derived from the specific average occupancy rates ψ. Total number of sites surveyed was 98.

Table 2 .

 2 Results of the Markovian occupancy model selection regarding the probability of occupancy ψ of the water vole using Akaike Information Criterion (AIC), Delta AIC (ΔAIC) and number of parameters (k). The detection probability p for all models was held constant with p(.) and ψ(.), 00(.), 01(.), p(.) represents the null model. Best model in bold. Only a subset of models are shown.

	Model specification for occupancy ψ	k	AIC	∆ AIC
	Veg_type1+ Veg_h1+ Veg_h2 + P/A_Onda	8	149.81	0.00
	Veg_type1+ Veg_h1+ Veg_h2	7	149.87	0.06
	Veg_type1+ Veg_type2_H+ Veg_h1+ Veg_h2	8	150.38	0.57
	Veg_type1+ Veg_h1+ Veg_h2+ Rich_inv	8	151.30	1.49
	Veg_type1+ Veg_type2_W+ Veg_h1+ Veg_h2	8	151.35	1.54
	Veg_type1+ Veg_h1+ Veg_h2 + P/A_Myo	8	151.61	1.80
	Veg_type1+ Veg_h1+ Veg_h2 + P/A_inv	8	151.81	2.00
	Veg_type1+ Veg_type2_H+ Veg_h2 + P/A_Onda	8	152.92	3.11
	Veg_type1+ Veg_h2 + P/A_Onda	7	153.70	3.89
	Veg_type1+ Veg_h1	6	154.26	4.45
	Veg_type1+ Veg_h1+ Rich_inv	7	154.52	4.71
	null model	4	195.24	45.43

Acknowledgments

The paper was written as part of a master course (SET-EEGB) at Aix Marseille University. Emilie Ladent thanks Aurélien Besnard, Ricardo Pita, James Hines and Darryl MacKenzie for having kindly answered her queries.

Funding

This study led by the Groupe Mammalogique Normand was supported by the Agence de l'Eau Seine-Normandie.

Disclosure statement

No potential conflict of interest was reported by the authors.