; Le Viol

Frédéric ; Jiguet

; Machon

; Scher

; Kerbiriou
email: kerbiriou@mnhn.fr

Road network in an agrarian landscape: potential habitat, corridor or barrier for small mammals ?

Keywords: Dispersal, marginal areas, roadside, road management, Sorex coronatus, Microtus arvalis. Highlights

If the negative effects of road networks on biodiversity are now recognized, their role as barriers, habitats or corridors remain to be clarified in human altered landscapes in which road verges often constitute the few semi-natural habitats where a part of biodiversity important for ecosystem functioning may maintain. In human-dominated landscape, their roles are crucial to precise in comparison to other habitats for small mammal species considered as major natural actors (pests (voles) or biological control agents (shrew)).

We studied these roles through the comparison of small mammal abundance captured (418 individuals belonging to 8 species) using non-attractive pitfall traps (n = 813) in 176 sampled sites distributed in marginal zones of road and crop, in natural areas and in fields. We examined the effect of roadside width and isolation of sites.

We found the higher small mammal abundances in roadside verges and an effect of width margins for shrews. The significant effect of the distance to the next adjacent natural habitat at the same side of the road on the relative abundance of S. coronatus, and the absence of a significant effect of distance to the next natural habitat at the opposite side of road, suggest that highway and road verges could be used as corridor for their dispersal, but have also a barrier effect for shrews. Our results show that in intensive agricultural landscapes roadside and highway verges may often serve as refuge, habitat and corridor for small mammals depending on species and margin characteristics.

Introduction

Road networks have strongly expanded over large areas with human population growth [START_REF] Watts | Roadless Space of the Conterminous United States[END_REF]) so that the majority of the total area for the more developed countries is under their influence (Reijnen et al. 1995;Forman 2000;[START_REF] Selva | Roadless and Low-Traffic Areas as Conservation Targets[END_REF]. Unfortunately, roads are known to have major negative impacts on species and ecosystem dynamics, modifying landscape structure, through habitat destruction, alteration and fragmentation [START_REF] Forman | Roads and their major ecological effects[END_REF][START_REF] Sauvajo | Patterns of human disturbance and response by small mammals and birds in chaparral near urban development[END_REF]Trombulak and Frissel 2000;[START_REF] Liu | Evaluating the influence of road networks on landscape and regional ecological risk -A case study in Lancang River Valley of Southwest China[END_REF]. One major impact identified is the reduction of populations size of a wide variety of species [START_REF] Fahrig | Effects of roads on animal abundance: an empirical review and synthesis[END_REF][START_REF] Benítez-López | The impacts of roads and other infrastructure on mammal and bird populations: a meta-analysis[END_REF][START_REF] Rytwinski | Do species life history traits explain population responses to roads? A meta-analysis[END_REF] through increase in mortality by collision [START_REF] Shuttleworth | Traffic related mortality in a red squirrel (Sciurus vulgaris) population receiving supplemental feeding[END_REF], fragmentation of home ranges, habitat destruction, disturbance of foraging and reproductive behaviors [START_REF] Siemers | Hunting at the highway: traffic noise reduces foraging efficiency in acoustic predators[END_REF] and barriers to movements which decrease landscape connectivity (Rico et al. 2007).

This fragmentation may result in the ultimate division of the population adjacent to roads into smaller isolated subpopulation involving a decrease in the genetic diversity of such isolated population [START_REF] Rico | Can the barrier effect of highways cause genetic subdivision in small mammals?[END_REF]). However, the potential biological value of road verges in anthropogenic altered landscape has also long been recognized [START_REF] Way | Roadside verge and conservation in Britain: A review[END_REF]. Sides of linear transport structures, i.e. linear areas of semi-natural vegetation, may provide refuges and/or corridors [START_REF] Davies | Are hedgerows effective corridors between fragments of woodland habitat? An evidence-based approach[END_REF] to a large number of taxa [START_REF] Hansen | The vegetation on roadsides on Denmark: a qualitative and quantitative comparison[END_REF][START_REF] Bennett | Habitat corridors and the conservation of small mammals in a fragmented landscape forest environment[END_REF][START_REF] Merriam | Corridor use by small mammals: field measurement for three experimental types of Peromyscus leucopus[END_REF][START_REF] Hodkinson | Plant dispersal: The role of man[END_REF][START_REF] Penone | Do railway edges provide functional connectivity for plant communities in an urban context?[END_REF]. In some cases, they are known to significantly contribute to the conservation of indigenous flora [START_REF] Spooner | Spatial analysis of anthropogenic disturbance regimes and roadside shrubs in a fragmented agricultural landscapes[END_REF][START_REF] O'farrell | Road Verge and Rangeland Plant Communities in the Southern Karoo: Exploring What Influences Diversity, Dominance and Cover[END_REF] and fauna [START_REF] Meunier | Use of roadsides by diurnal raptors in agricultural landscapes[END_REF][START_REF] Ries | Conservation value of roadside prairie restoration to butterfly communities[END_REF].

Actually, their role as refuges may depend on their surrounding landscape: in natural habitats, generally supporting a high species diversity, road verges do not provide particular habitat to threatened local species [START_REF] O'farrell | Road Verge and Rangeland Plant Communities in the Southern Karoo: Exploring What Influences Diversity, Dominance and Cover[END_REF]. [START_REF] Ranta | The importance of traffic corridors as urban habitats for plants in Finland[END_REF] showed that the usual extensive management techniques were not sustainable for the survival of endangered species on a roadside site. Roads network even can have negative effects, especially in favoring invasion by exotic species [START_REF] Hansen | The influence of disturbance and habitat on the presence of non-native plant species along transport corridors[END_REF][START_REF] Brown | Toad on the road: Use of roads as dispersal corridors by cane toads (Bufo marinus) at an invasion front in tropical Australia[END_REF]. By contrast, in human dominated areas such as intensive agricultural landscapes, where non-agricultural habitats (e.g. edges) are sparse and critical to the conservation of biological diversity and ecological processes [START_REF] Burel | Hedgerows and Their Role in Agricultural Landscapes[END_REF], road verges should play a crucial role as a refuge and ecological corridors [START_REF] Dawson | Are habitat corridors conduits for animals and plants in a fragmented landscape? A review of the scientific evidence[END_REF][START_REF] Tikka | Road and railway verges serve as dispersal corridors for grassland plants[END_REF][START_REF] Dawson | Roads and habitat corridors for animals and plants[END_REF][START_REF] Viol | Plant and spider communities benefit differently from the presence of planted hedgerows in highway verges[END_REF], 2012;[START_REF] Penone | Do railway edges provide functional connectivity for plant communities in an urban context?[END_REF].

Hence, there is a crucial need to assess the roles of road verges as refuges, corridors or barriers, and more particularly for small mammals in agrarian landscapes that are supposed to take an important place in the ecosystem functioning. Small mammals are indeed known to be major natural actors of agrarian ecosystems: some of them are considered as pests [START_REF] Sullivan | Population dynamics of small mammals in relation to vegetation management in orchard agroecosystems: compensatory responses in abundance and biomass[END_REF]) while others strongly regulate invertebrate populations in fields and grasslands [START_REF] Churchfield | The effects of small mammal predators on grassland invertebrates, investigated by field exclosure experiment[END_REF]. These are crucial to ensure good agricultural production [START_REF] Schoener | Leaf damage in island buttonwood, Conocarpus erectus: Correlations with pubescence, island area, isolation and the distribution of major carnivores[END_REF][START_REF] Spiller | A terrestrial field experiment showing the impact of eliminating top predators on foliage damage[END_REF][START_REF] Dial | Experimental removal of insectivores from rain forest canopy: Direct and indirect effects[END_REF] because they control for low pest species abundances [START_REF] Maisonneuve | Importance of riparian habitats for small mammals and herpetofaunal communities in agricultural landscapes of southern Québec[END_REF]. However, while small mammals are regularly observed or trapped in field margins of agrarian landscapes, the role of road verges as refuge or corridor in comparison to other habitats at the landscape scale has rarely been assessed and results are still scarce and often contradictory. In some studies, small mammals have been observed to spread tens of kilometres along highway roadsides [START_REF] Getz | Interstate roadsides as dispersal routes for Microtus pennsylvanicus[END_REF]. Roadsides are thus considered as effective corridors for them [START_REF] Suckling | Population ecology of the sugar glider, Petaurus breviceps, in a system of fragmented habitats[END_REF][START_REF] Bennett | Habitat corridors and the conservation of small mammals in a fragmented landscape forest environment[END_REF][START_REF] Verkaar | Corridors as a tool for plant species conservation? Species Dispersal in Agricultural Habitats[END_REF]. Other studies showed that roads could be a significant barrier to dispersal of many animals (Rico et al. 2007), particularly in the case of high traffic rates [START_REF] Harris | Forest fragmentation and the conservation of biological diversity[END_REF]). Given the high potential impact of these animals on local agricultural economies, it is important to better understand the role of roadsides among other habitats on the population dynamics of small mammals. In this way, according to the results on the ecological importance of road verges in agricultural landscapes, it will be possible to determine how much roadside management should take into account populations of small mammals.

The aim of the present study is to assess the role of road verges as habitats, corridors or barriers for small mammal species in an intensive agricultural ecosystem. First, their role as habitat was evaluated by comparing the relative abundance of small mammals in road verges, according to their characteristics (width…) and compared to their abundance in other habitats such as fields, field margins and woods. Second, in order to evaluate their potential corridor effect, we investigated the relationship between small mammal abundances in portions of verges and their distance to the closest adjacent (= connected) natural habitats (such as woods or meadows). Finally, we attempted to identify barrier effects by defining the relationships between small mammal abundance in portion of verges and their distance to the nearest natural habitat adjacent to the verge of the other side of the road and compare it to the relationship with distance to the natural habitats of the same side of the road.

Materials and methods

Study area

The study area is an intensive agricultural plain located in the region Ile-de-France in the surroundings of Paris (48° 51′ North -2° 21′ East, Fig. 1). It is thus mainly composed of urban areas covering 18.2% of the global area and of agricultural fields used for intensive cropping, especially wheat, sugar beet and rapeseed covering 49.0%). The main "natural" habitat is woodland representing about 23.7% of land cover (see Appendix A). In the study area, the road network is 38,906 km long, i.e. 3.23 km of highways and roads per km 2 . Road network verges (highway or roadside) and field margins are herbaceous strip mowed once or twice each year, differing mainly by their width (15.7 ± 6.6 m [10.0 -21.5] for highway verges, HV,] for road verges, RV, and 4.0 ± 0.4 m for [2.0 -10.0] for field margin, FM). The average height of grass strips is less than 1 meter in June while five plant species dominate this communities (Dactylis glomerata, Festuca ssp, Heracleum sphondylium, Galium ssp, Plantago lanceolata have an occurrence exceeding 50%). All sampled sites (highway, roads and fields) are close to crops typical of intensive agrarian landscape (wheat and rape totalized 81% of crops in the sampling, with no obvious bias among site types: Fisher's exact test, P=0.31). We also verified that studied highway verges and road verges have similar distance ranges to woods (490 ± 83 m [10 -2100] for highway verges, HV, and 973 ± 118 m for [80 -4450] for road verges, RV). Verges of this road network represent 1.6% of the total area of the region (highway verges: 0.4%, road verges: 1.2%) and field margins (margins comprised between two adjacent agricultural fields), 1.5% of the territory.

Sampling design

To examine the habitat, corridor and barrier roles of road verges, we sampled the two types of road verges: we studied 31 sites along highways (noted HV) and 48 sites along roads (noted RV). A site consisted of 5 traps placed linearly every twenty meters along the way in the middle of the verges (for more details see Le [START_REF] Viol | Plant and spider communities benefit differently from the presence of planted hedgerows in highway verges[END_REF]Redon (de) et al. 2010).

To assess the relative importance of verges for small mammals in the landscape, we also sampled the main habitat present in this area: fields, n = 65 sites and wood, n = 32. Fields margins and inside field were sampled (Fig. 1) as following: margins (65 fields, noted FM) each consisting of three traps placed around the field separated from each other by at least 100 meters); inside fields at 25 m from the hedges (65 fields, noted F25, consisting of two traps with a distance of twenty meters), and inside filed at 50 m from the hedges (56 fields, noted F50, consisting of two traps with a distance of twenty meters). Woods (32 sites, noted WD) were sampled with one trap randomly placed in each studied wood (Fig. 1). A total of 864 traps were thus installed in the 176 sites, but 813 traps installed were recovered (Table 1).

Most traps destroyed during the sampling period were located within or around the fields: in field margins (FM: 4% of traps destroyed), fields-25m (F25: 15%) and fields-50m (F50: 21%).

Small mammals capture and determination

The number of caught individuals was used as a proxy to estimate their relative abundance in the sites. Sampling was carried out simultaneously in 2006 from May the 2 nd to June the 4 th , (i.e. 31 nights of trapping for each site). For more technical details see Appendix B and Le [START_REF] Viol | Plant and spider communities benefit differently from the presence of planted hedgerows in highway verges[END_REF]Redon (de) et al. 2010;[START_REF] Vergnes | Ecological corridors also operate in an urban matrix: A test case with garden shrews[END_REF]. Each animal was dissected and identified mainly using its morphological cranial characteristics, such as teeth, according to [START_REF] Chaline | Les proies des rapaces[END_REF].

Data analysis 2.4.1. Variation of species richness among habitat

In order to estimate potential differences in richness among habitats, we assessed an estimated richness per habitat using species accumulation method [START_REF] Kindt | Patterns of species richness at varying scales in western Kenya: planning for agroecosystem diversification[END_REF]; R package vegan, function: specaccum) which allow to estimate the number of species from a certain number of sampled sites (here a random sample of 30 traps for each habitat: HV, RV, FM, F25, F50 & WD).

Verges and margins as habitats

To identify the habitats preferentially used by small mammals, we evaluated their relative abundance (particularly that of the two most observed species: Microtus arvalis and Sorex coronatus) at the trap scale in the habitats (HV, RV, FM, F25, F50 & WD). According to the hierarchical structure of the sampling design (traps within site), we used a Generalized Linear Mixed Model (GLMMs, package lme4, [START_REF] Zuur | Mixed effects models and extensions in ecology with R[END_REF]) with a Poisson error distribution (count data) and with a random effect of sites. This approach allows accounting for potential spatial pseudoreplication, linked to the hierarchical structure in the sampling design [START_REF] Crawley | The R book[END_REF]. We explore data for potential spatial autocorrelation using a variogram tool (R package spatial, [START_REF] Bivand | Applied spatial data analysis with R[END_REF]) and according to the slight spatial trend detected, we added an autocovariate (i.e., a distance-weighted function of neighboring response values, here weights by the square of inverse distance; [START_REF] Dormann | Methods to account for spatial autocorrelation in the analysis of species distributional data: a review[END_REF][START_REF] Penone | Assessing anthropogenic pressures on Orthoptera communities using large scale acoustic monitoring[END_REF] with the autocov dist function in R (package spdep, Roger Bivand).

Environmental variables influencing the abundance of M. arvalis and S. coronatus on highway and road verges

To go further, we focused on highway and road verges at the site scale. We assessed how abundances of M. arvalis and S. coronatus were influenced by margin types (HV, RV) compared to other environmental variables. These latest were car traffic (CT), margin width (MW), distance to nearest wood (distance as the crow flies NW), distance to the nearest crop (distance as the crow flies NC), proportion of forest habitat in a 250 meters buffer (W250), proportion of forest habitat in a 500 meters buffer (W500) and crop type in the field (CT). We simultaneously took into account all habitat variables in the analysis using Hierarchical Partitioning (R package hier.part, Chris Walsh), a multiple regression analytical method that allows to identify the most likely causal factors while alleviating multicollinearity problems [START_REF] Nally | Regression and model building in conservation biology, biogeography and ecology: the distinction between and reconciliation of 'predictive' and 'explanatory' models[END_REF].

In a second step, we focused on the potential influence of margin width among HV, RV and FM on the relative abundance of mammals using the same GLMM modelling structure than previously. The only difference was that the effect of margin width was adjusted to the distance to the nearest crop for S. coronatus; and to the crop type in the field for M. arvalis, according to Hierarchical Partitioning results (see 3.3). In addition, we used a Generalized Additive Model (GAM, package mgcv, [START_REF] Hastie | Generalized Additive Models[END_REF]) to identify graphically the potential non-linear effects of margin width on the relative abundances of mammals.

Highway and road verges as corridors

If small mammals move along their territories located in preferred natural habitats through the verges, we should expect more captures in verges located in the neighbourhood of natural habitats. Thus, we focused at the site scale (sum of the 5 traps/site), with a restriction to highway and road verges and calculated the distance between each verge site (HV & RV sites) and the nearest adjacent (= connected) meadows or woods (distance noted DC, Fig. 2) and tested its effect, on the relative abundance of M. arvalis and S. coronatus using a GLM. This effect was adjusted to the margin width and to the distance to the nearest crop for S. coronatus; to the margin width and to the crop type for M. arvalis according to Hierarchical Partitioning result (see 3.3). For each species, we performed Generalized Linear Models (GLM), a first one for road sites and a second for highway sites. Since S. coronatus is a territorial species (Neet and Hausser 1990), we used a second approach to evaluate the effect of the distance to the nearest adjacent natural habitat. According to [START_REF] Cantoni | Social and spacial organization of free-ranging shrews, Sorex coronatus and Neomys fodiens (Insectivora, Mammalian)[END_REF], its population densities are about 10.8 individuals per ha i.e. a territory can be approximately represented by a disk of 926m², we then sorted verge sites in two categories: sites near a natural habitat (sites A), i.e. less distant than ten territories diameters (343m), and sites far from a natural habitat (sites B), i.e. more distant than ten territories diameters. We then tested for differences in species abundance between the two site categories with a GLM.

Highway and roadside verges as barrier for S. coronatus

If roads and highways are barriers for S. coronatus and if the previous hypothesis is verified (i.e. significant effects of distance to nearest adjacent habitat, DC, on shrew abundance), we should find no effect of distance to the nearest natural habitat located on the other side of the road on the abundance of S. coronatus. Thus we calculated the distance between each verge of RV and the next natural habitat of the other side of the road (distance noted DX, Fig. 2). As previously, we sorted verges in two categories (A: <343m and B: >343m) and ran GLM to test the difference of abundance of shrews between the sites of the two categories. Since wood or meadows are expected not to be a favourable habitat for voles (agrarian species) we are not able to test effect of road as corridor for these species.

All linear models (GLMs and GLMMs) were performed using R (R Development Core Team, 2010) and with a Poisson error distribution (count data). For GLMs, results were evaluated using a type II ANOVA with a F-test (R package car; Fox and Weisberg, 2011) and P-values were corrected for potential overdispersion following [START_REF] Faraway | Extending the linear model with R, generalized linear, mixed effects and nonparametric regression models[END_REF].

Results

Variation of species richness among habitat

A total of eight species were trapped among the different habitats: three shrew species (Crocidura russula, S. coronatus and S. minutus), three vole species (Clethrionomys glareolus, Microtus agrestis and M. arvalis) and two mice species (Apodemus flavicollis and Apodemus sylvaticus). According to estimated richness (Table 1), woods, highway verges and road verges, exhibited a quite similar richness. However their richness are higher than fields and field margins. Two species (M. arvalis and S. coronatus) represented 78% of collected individuals (Table 1), and we further only focused on these two species. Sample sizes for the other species were too small to be properly analysed.

Verges and margins as habitats

GLMMs indicated that the relative abundance of M. arvalis (Fig. 3a) in roadside verges did not differ significantly from their abundance in highway verges (t-value = 0.648; P = 0.52), respectively HV: 0.24 ± 0.05 SE and RV: 0.30 ± 0.02 SE individuals/trap. In field habitats, densities are significantly lower (See Table 1; F25: z-value = -2.389; P = 0.02; F50: z-value = 2.321; P = 0.02). In field margins, which are very similar habitat to highway and roadside verges (z-value = -1.945; P = 0.04) their density was also slightly lower. M. arvalis was not detected in woods (WD).

S. coronatus (Fig. 3b) varied in abundance among habitats. They were abundant in verge habitats but significantly more in highway than in road verges (HV: 0.63 ± 0.08 SE & RV: 0.26 ± 0.05 SE individuals/trap; t-value = -4.189, P < 0.0001). In fields, their abundances were very low (FM: 0.05 ± 0.02 SE individuals/trap) and significantly lower than in highway verges (FM: t-value = -3.384; P < 0.0001; F25: z-value = -5.212; P < 0.0001; none was captured in field-50m, F50). This species was detected in woods but significantly at lower rate than in highway verges (WD: 0.16 ± 0.08 SE individuals/trap; t-value = -2.439; P = 0.02).

Environmental variables influencing the abundance of M. arvalis and S. coronatus on verges and margins

The main variable influencing the abundance of M. arvalis is the crop type in the field and margin width, whereas for S. coronatus margins types (HV vs RV), distance to the nearest crop and margin width appeared to have a similar importance (Table 2). The influence of the distance to the nearest wood, the proportion of forest habitat in a 250 or 500 meters buffer and the car traffic were weak predictors of abundance for both species (Table 2).

The margin width did not influence the relative abundance of M. arvalis (Chisq=1.262 ; df=1 ; P=0.26)., note that crop type was not significant (Chisq=5.786 ; df=2 ; P=0.06). For S. coronatus, we detected a significant positive effect of margin width on shrew abundance (Chisq=7.973 ; df=1 ; P=0.005), again the variable of adjustment (here distance to the nearest crop) was not significant (Chisq=1.144 ; df=1 ; P=0.28). We further detected a non-linear pattern: the abundance of shrew increased drastically with an increase of margin width until 12 meters (see Appendix C for pattern of with effect on relative abundance).

Verges and margins as corridors

In the HV sites, we did not detect any effect of the distance to the nearest adjacent natural habitat, (DC) neither for S. coronatus (F1,26 = 0.52; P = 0.47) nor for M. arvalis (F1,26 = 0.36; P = 0.55). In the RV sites, S. coronatus abundance was negatively correlated to the distance to the nearest adjacent natural habitat, DC (F1,44 = 2.21; P = 0.03) whereas no effect was detected for M. arvalis (F1,43 = 2.67; P = 0.11). When considering the difference between verge sites separated in two categories of distance (i.e. sites less distant than ten territories diameters (site A), versus sites far from a natural habitat (sites B)), the pattern was even strengthened for S. coronatus: In HV sites and for S. coronatus, we again did not detect an influence of the distance to the nearest connected natural habitat, DC (F1,26 = 0.78; P = 0.39; Fig. 4). But in RV sites, abundances of S. coronatus were negatively correlated to the distance to the nearest connected natural habitat, DC (F1,43 = 25.06; P < 0.001; Fig. 4).

Roads as barrier for shrews

Because we did not found any effect of connected distance (DC) on shrew abundances in highway verges, we focused these analyses on road verges and found no correlation between shrew densities and the distance to the nearest adjacent natural habitat situated on the opposite side of road (DX) (F1,47 = 0.50; P = 0.48).

Discussion

This set of experiment was conducted with traps usually used for soil arthropods, during another experiment (Le [START_REF] Viol | Plant and spider communities benefit differently from the presence of planted hedgerows in highway verges[END_REF] which had unfortunately led to the capture of small mammals. For monitoring most terrestrial small mammals, the Sherman live trap has become the standard foldable; however, pitfalls work also well [START_REF] Hoffmann | Chapter 19 -Field Methods and Techniques for Monitoring Mammals[END_REF]). Their placement is often rewarded by the capture of small shrews less sampled with any other method [START_REF] Kalko | Comparative studies of small mammal populations with transects of snap traps and pitfall arrays in southwestern Virginia[END_REF][START_REF] Colyn | Relative efficiency of three types of small mammal traps in an African rainforest[END_REF]. In addition, pitfall set-up and results can sometimes be shared with entomologists and herpetologists who might be working on the same inventory [START_REF] Hoffmann | Chapter 19 -Field Methods and Techniques for Monitoring Mammals[END_REF]. Despite our sampling covers only one month at a suboptimal period (the best time for effective trapping of small terrestrial mammals being in autumn), given the number of animals collected (n=418) and efficient sampling plots (n=813), statistical analyses were possible, and hypothesis concerning the ecological role of verges could be tested, in assuming that, at least for two of the species, the number of mammals collected was a good proxy of their densities or rate of passage in the study sites (see Redon (de) et al. 2009, a study using such pitfall capture and that allow identify effects of roadside verges on vole outbreaks).

First, confirming previous published observations [START_REF] Adams | Effects of roads on small mammals[END_REF], densities of small mammals were important in verge habitats compare to neighbouring habitats (wood, fields and field margins). Furthermore, when looking at richness highway verges and road verges present a similar diversity than natural areas (woods) and a greater diversity than fields. This suggest the importance of these marginal areas to host small mammals in intensive agrarian landscapes and the role they could play for biodiversity conservation. Nevertheless, whereas both frequented by voles (M. arvalis) and shrews (S. coronatus), the role of these margins seemed to differ between these species. Thus, our results confirmed that the ecological functions of different elements of a landscape are strongly species dependant [START_REF] Noss | Corridors in Real Landscapes: A Reply to Simberloff and Cox[END_REF][START_REF] Beier | Do habitat corridors provide connectivity?[END_REF].

Verges as habitat for voles

Higher relative abundance of voles (M. arvalis) were found in marginal zones compared to those in fields, in accordance with Adams and Geis' study (1983). We suggest that these margins represent the main habitats of the landscape used by these species. Thus the presence of such verges maintains a reservoir of rodents considered as agricultural pests in the neighbourhood of the fields. [START_REF] Delattre | Vole outbreaks in a landscape landcape context: Evidence from a six year study of Microtus arvalis[END_REF] showed for this species that the dynamics of abundance cycles depended on landscape structure. Indeed, vole populations display pluriannual density variations whose amplitude is greater in large open fields. This could be due to difference in predation pressure. Rodent predators (mustelidae, raptors) may be more efficient in small fields using linear marginal areas or fixed hunting posts (such as edges, poles…) than in large uniform fields, because predator generally avoid to cross such open habitat [START_REF] Hansson | The domestic cat as a possible modifier of vole dynamics[END_REF][START_REF] Meunier | Use of roadsides by diurnal raptors in agricultural landscapes[END_REF][START_REF] Rondinini | Habitat use and preference by polecats (Mustela putorius L.) in a Mediterranean agricultural landscape[END_REF]). As a consequence, vole populations could be better regulated in margins. Local extinctions could more often occur in smaller fields and population's outbreaks more chronic only in large fields. This result was confirmed in the same study area where previous work found a negative relationship between variation in local vole abundance and roadside network densities (Redon (de) et al. 2010).

Verges and margins as habitat and corridor for shrews

The results suggest that highway verges appears as a habitat widely used by S. coronatus in the study landscape because among the habitat harbors the highest abundance of individuals captured among the sampled habitats. The suitability of highway verges (e.g. margins of a large width) as habitat for shrews is also supported by our captures of S. minutus, considered as one of the small mammals the most sensitive to habitat degradation and agriculture intensification [START_REF] Peña (de) N | Response of the small mammals community to changes in western French agricultural landscapes[END_REF]) and here only recorded in highway verges.

The abundance of S. coronatus was lower in road verges than in highway verges, and we detected a significant influence of the distance to the nearest adjacent natural habitat. [START_REF] Vergnes | Ecological corridors also operate in an urban matrix: A test case with garden shrews[END_REF]2013 showed that the probability of shrew occurrence in garden was negatively correlated to the distance to woodlot and to corridors (these corridors included railway and road verges). Thus we rather believe that roadside verges could be used as corridor for their dispersal and are of lower habitat quality than highway verges for this species. Those results are in accordance with the biology of S. coronatus which move intensively all along their life because it is a very active territorial animal with important energetic needs [START_REF] Genoud | Activity of Sorex coronatus (Insectivore, Soricidae) in the field[END_REF] and whose males disperse to meet females during the reproductive season, [START_REF] Cantoni | Social and spacial organization of free-ranging shrews, Sorex coronatus and Neomys fodiens (Insectivora, Mammalian)[END_REF]. [START_REF] Oxley | The effects of roads on populations of small mammals[END_REF] already reported that road networks often attract small mammals by facilitating access to food or increasing food supply, in our study we found that invertebrate biomass (potential preys of shrew) was higher in margins (see Appendix C) Tichendorf and Wissel (1997) found that corridor widths have strong effects on small mammal dispersion. In our study area, highway and road verges were very similar in plant composition and management (Le [START_REF] Viol | Plant and spider communities benefit differently from the presence of planted hedgerows in highway verges[END_REF]Redon (de) et al. 2010). They may differ by noise, soil pollution and ground vibration caused by car traffic, but in our case we showed that the main factor influencing the abundance was the margin width, which was also the main difference between highway and road verges. We may hypothesize that highways verges, thanks to their wideness, are more used by shrews as habitat but also as corridor than road verges. In field margins, which are even narrower than road verges (4.0 ± 0.4m), we captured even less S. coronatus.

Roadside verges as barrier

The significant effect of the distance to the next adjacent natural habitat at the same side of the road (DC) on the relative abundance of S. coronatus, and the absence of a significant effect of distance to the next natural habitat at the opposite side of road (DX), might suggest a barrier effect of the roads for shrews. This result should be confirmed by more direct methods, e.g. based on capture-recapture or radio-tracking data. However, this result fits with other studies that used a variety of methodological approaches but always concluded that the roadway prevents movement of small mammals from one side to the other of the road because of their particularly unfriendly habitat [START_REF] Oxley | The effects of roads on populations of small mammals[END_REF][START_REF] Mader | Animal habitat isolation by roads an agricultural field[END_REF][START_REF] Swihart | Road crossing in Sigmondin hispidius and Microtus ochrogaster[END_REF][START_REF] Merriam | Barriers as boundaries for metapopulations and demes of Peromyscus leucopus in farm landscapes[END_REF][START_REF] Kirby | Habitat fragmentation and infrastructure: problem and research[END_REF][START_REF] Brehme | Response of small terrestrial vertebrates to roads in a coastal sage scrub ecosystem[END_REF][START_REF] Rytwinski | Do species life history traits explain population responses to roads? A meta-analysis[END_REF]. This barrier effect could be a driver of small mammal assemblages' differences between natural habitat patches isolated by urban matrix (see [START_REF] Fernández | Small mammal assemblages in fragmented shrublands of urban areas of Central Chile[END_REF]

Management policies and small mammals

In the European Common Agricultural Policy implemented from 2002, environmental objectives to preserve biodiversity were introduced into the process of agrarian subsidies: farmers have to choose between different environmental practices to obtain financial helps.

One of those practices is the establishment of new margins around crop fields, with an expected increase of biodiversity within agricultural landscapes. Such policies should be strongly encouraged because they may also increased ecosystem services by maintaining populations of agriculture pest predators, such as shrews, which feed mainly on beetles, caterpillars and slugs [START_REF] Pernetta | Diet of the shrew Sorex araneus and Sorex minutus in Wytham grassland[END_REF][START_REF] Churchfield | Food availability and the diet of the common shrew Sorex araneus in Britain[END_REF][START_REF] Bellocq | Arthropods preferred as food by Sorex cinereus (masked shrew) and Peromyscus mariculatus (deer mouse): an experimental approach[END_REF]Maisonneuve and Riou, 2001) and stabilising vole dynamics by avoiding cycling outbreaks [START_REF] Delattre | Vole outbreaks in a landscape landcape context: Evidence from a six year study of Microtus arvalis[END_REF]Redon (de) et al. 2010).

Because (i) our result suggest that roads might play a role of barriers for small mammals, because (ii) we found that shrews densities are highly correlated to the distance to adjacent natural habitats, because (iii) culverts have strong effects on shrews dispersal between the two sides of the road [START_REF] Yanes | Permeability of roads and railways to vertebrates: The importance of culverts[END_REF] and because (iv) they limit small mammal road casualties [START_REF] Lodé | Effetcts of a mortorway on mortality and isolation of wildlife populations[END_REF], we also recommend to purchase the implementation of underground passages in order to help maintaining shrew communities in verges.

According to [START_REF] Meunier | Effects of landscape type and extensive management on use of motorway roadsides by small mammals[END_REF] and because verges could represent important conservation areas for small mammals in intensive agrarian landscapes, it should be also interesting to encourage their extensive management. Finally, this paper deals with the role of roads in agricultural areas, but these are not the only ecosystems where roads may play an ecological role, suburban, open, and forested landscapes are shown to hold markedly different road effects on species, habitat, water, soil, and atmosphere (Forman & Hersperger 1999, Forman 2000).

Figure 1 .Fig. 2 .Fig. 3 .Fig. 4 .

 1234 Figure 1. A, Map of site locations, showing the 176 sites sampled during one month in May

Table 1 .

 1 Mammal species captured in traps settled in the different habitats.

						Mice		Voles			Shrews	Estimat
				Trap	Traps								ed
	Zones	Habitats Sites			Apodemus	Apodemus	Clethrionomys	Microtus	Microtus	Crossidura	Sorex	Sorex
				by site	recovered								Richnes
						flavicollis	sylvaticus	glareolus	agrestis	arvalis	russula	coronatus	minutus
													s
	Natural												4.65 ±
		Woods	32	1	32	0	1	2	2	0	2	6	0
	areas												0.49
		Highway											4.27 ±
			31	5	155	0	4	0	0	37	13	98	6
		verges											0.69
	Margin	Road											4.55 ±
			48	5	240	5	8	0	4	71	22	63	0
	al areas	verges											0.84
		Field											3.44 ±
			65	3	187	0	8	0	0	18	5	10	0
		margins											0.71
		Fields											2.51 ±
			65	2	110	0	5	0	0	9	1	1	0
		25m											0.66
	Fields
		Fields											1.75 ±
			56	2	89	0	3	0	0	6	0	0	0
		50m											0.43
	Total		297		813	5	29	2	6	141	43	178	6

Table 2 :

 2 Independent contribution of each variable, assess by the Hierarchical Partitioning of variance.

	M. arvalis	S. coronatus

Acknowledgements

We thank the Institute for Urban Planning and Developpement of the Paris Ile-de-France Region (I.A.U.R.I.F.) and the Equipement Department Agency (D.D.E. 77) who provided GIS data and technical support. We thank also François Hervieu from the Agriculture Minister and the SRPV -Service Régional de Protection des Végétaux Ile de France -for provided us field authorisation. We thank Jeremy CASTELLI, Elise CONTAN and Rose-Line PREUD'HOMME who help us during field working. We also thank Pavel Kindlmann and two anonymous reviewers for their helpful comments on the manuscript. The research was supported financially by Cofiroute, the Conseil général de Seine-et-Marne (Seine-et-Marne Department Assembly, C.G. 77) and the Direction Générale des Routes (Roads General Direction, D.G.R.) of French Minister for Ecology, Energy, Sustainable Developpement and Spacial Planning,(M.E.E.D.A.D.).

Electronic Supplementary Material -Appendix A

Electronic Supplementary Material -Appendix B: Details on small mammal sampling.

Traps

The traps were Barber pitfall traps, i.e. 450 mL plastic pots (10.2 cm height & 7.5 cm diameter) inserted into the ground, containing a 155 mL solution composed with 75 mL of water, 75 mL of conservative (monopropylene glycol), 5 mL of surfactant (dishware soap)

and 15 g of NaCl, and protected from rain by plastic transparent covers (15 cm×15 cm)

supported by wood sticks 10 cm above the ground. For this study, we used the data of the small mammals incidentally fallen in the traps with the hypothesis that their abundance in the traps was correlated to their abundance in the verges. This hypothesis is supported by the fact that the solution inside the pots is neither attractive nor repulsive. It is not the most common trapping method for small mammal sampling but was shown relevant to highlight some distribution patterns and trends (see de Redon et al. 2010).

Potential bias in trap line

Given that captured both the individuals cannot be recaptured in another trap, we tested the independence of trap. Using similar modelling of abundance at the trap scale we test if trap identity could influence abundance (GLMM, with an autocov distance function, an error poisson distribution and a random effect on site). We did not detected any obvious bias, so, we can reasonably consider the hypothesis of independence respected. Small mammals are not attracted or avoid the traps based on the presence of other congeners, or when an individual is captured by the first trap in the trapline it does not affect the following traps.

Potential attractiveness of collected invertebrates on small mammals captures

We can hypothesize that small mammals can be attracted by the presence of invertebrates fallen to the bottom of the trap: the traps with many invertebrates could be more attractive.

We test the potential effect of the number and the biomass of invertebrates on the abundance of small mammals through the comparisons of captures of mammals and invertebrates in 51 traps. Biomass (B) was approximated as follow:

Where ai is the abundance of species i and si is the size of the specie i This subsample corresponds to traps for which we had information on both invertebrates and small mammals captured. We assess this potential effect using a GLM (with Poisson error distribution according to the nature of data: count). We did not detect any obvious correlations between invertebrate number /biomass and small mammals (Table B1).

References:

De Redon, L., Machon N., Kerbiriou C., Jiguet, F. 2010. Possible effects of roadside verges on vole outbreaks in an intensive agrarian landscape. Mammalian Biology 75, 92-94.

Variation of invertebrate biomass among habitat

Invertebrate biomass (potential preys of shrew) is higher in margins (FM, HW, RV) than in Fields (F25 and F50) (F2,48=11.121; P<0.0001). and Microtus arvalis (B) using Generalized Additive Model (GAM, package mgcv).

Margins

Relative abundance of

S. coronatus

Relative abundance of

M. arvalis

Margin width (m)

Margin width (m)