There and back again? Combining habitat suitabilitymodelling and connectivity analyses to assess a potentialreturn of the otter to SwitzerlandC. Cianfrani1, L. Maiorano1, A. Loy2, A. Kranz3, A. Lehmann4, R. Maggini5* & A. Guisan1

1 Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland2 Department of Science and Technology for the Environment, University of Molise, Pesche, Italy3 Alka kranz, Graz, Austria4 Institute of Environmental Sciences, University of Geneva, Carouge, Switzerland5 Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.

Keywords

species recovery; habitat suitability model;connectivity; multi-step approach; integratedmanagement framework; long-termconservation strategies.

Correspondence

Email: carmen.cianfrani@gmail.com

Editor: Res AltweggAssociate Editor: Madhu Madhusudan

*Current address: Centre of Excellence forEnvironmental Decisions (CEED), Universityof Queensland, Goddard Building 8, StLucia, Brisbane, Qld 4072, Australia.

Received 3 July 2012; accepted 29 January2013

doi:10.1111/acv.12033

AbstractAfter a steady decline in the early 20th century, several terrestrial carnivore specieshave recently recovered in Western Europe, either through reintroductions ornatural recolonization. Because of the large space requirements of these speciesand potential conflicts with human activities, ensuring their recovery requires theimplementation of conservation and management measures that address the envi-ronmental, landscape and social dimensions of the problem. Few examples exist ofsuch integrated management.

Taking the case of the otter (Lutra lutra) in Switzerland, we propose a multi-step approach that allows to (1) identify areas with potentially suitable habitat, (2)evaluate their connectivity, (3) verify the potentiality of the species recolonizationfrom populations in neighbouring countries. We showed that even though suitablehabitat is available for the species and the level of structural connectivity withinSwitzerland is satisfactory, the level of connectivity with neighbouring popula-tions is crucial to prioritize strategies that favour the species recovery in the field.This research is the first example integrating habitat suitability and connectivityassessment at different scales with other factors in a multi-step assessment forspecies recovery.

Introduction

Following a steady decline since the beginning of the 20thcentury, several species of carnivores such as the wolf, thelynx, the bear and the otter have disappeared from mostWestern European countries (Enserink & Voge, 2010).Their decline is generally attributed to a combination ofhuman persecution, habitat destruction and losses of preyspecies (Ceballos et al., 2005). However, in recent decades,carnivore populations have recovered in most of WesternEurope, partly supported by reintroduction projects butlargely as a result of natural population expansions. Forinstance, wolves returned in the Alps coming from theItalian Apennines (Enserink & Voge, 2010), while otters arerecovering in many Central European countries (Kranz,2000; Loy et al., 2010; Panzacchi, Genovesi & Loy, 2011).But some exceptions are remarkable, such as the case of theIberian lynx (Palomares et al., 2011).

The recovery of any medium to large carnivore speciesshould be closely monitored to carefully design conserva-

tion and management measures because these speciesusually require large areas and can potentially create con-flicts with human activities. This need for management isparticularly true in human-dominated landscapes, such ascentral-western Europe, where the habitat suitable for thepresence of many species may be highly fragmented (Temple& Terry, 2007) and freshwater habitats are highly impactedby introduced species, pollution, water captation and loss ofriparian vegetation.

Before planning the reintroduction of a species (Morrel,2008) or establishing actions to favour the natural expan-sion of threatened and rare species, at least three main ana-lytical steps should be undertaken: (1) identify the areaspotentially suitable (or partially suitable, as here, depend-ing on environmental data availability) for the species(Martinez-Meyer et al., 2006; Olsson & Rogers, 2009; Cook,Morgan & Marshall, 2009; Rodrigues-Soto et al., 2011); (2)evaluate whether the suitable areas in a region are suffi-ciently well connected to guarantee gene flow and survival ofa viable population (Kramer-Schadt et al., 2004; Singleton,

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Gaines & Lehmkuhl, 2004; LaRue & Nielsen, 2008; Hucket al., 2010; Carranza et al., 2012); (3) evaluate the level ofconnectivity with neighbouring areas from were the popu-lation can expand (Kramer-Schadt et al., 2004).

In this study, we illustrate these three steps using theEuropean otter (Lutra lutra) as a case study. The otter is aflagship species for freshwater ecosystems, and it suffered asevere decline during the 20th century in most Europeancountries (Foster-Turley, Macdonald & Mason, 1990;Robitaille & Laurence, 2002) because of the reduction offood supplies, increases in water pollutants, persecution byhumans and the destruction of riparian habitat (Foster-Turley et al., 1990; Kruuk, 2006). In Switzerland, the otterwent extinct in the 1980s (Foster-Turley et al., 1990) and it isstill absent, even though the species is really close to the Swissborder on the French side and is moving closer on theAustrian side (Kranz et al., 2008; Fig. 1). The challenges andopportunities for the comeback of the otter in Switzerlandhave been discussed considering two alternative options:active reintroduction projects and support to natural recolo-nization (Weber, 1990; Weber, 2004). Building on previouswork (Cianfrani et al., 2010, 2011), we assess here the feasi-bility of these two options using a framework that relies onthe most advanced spatial techniques and most robust dataavailable, and incorporating the three steps previously iden-tified. We aim particularly at using this framework to answerthe following questions: (1) are there environmentally suit-able areas for the otter in Switzerland?; here, we mainly assesspartial environmental suitability due to the lack of preyavailability and water pollution maps; (2) are the partiallysuitable areas connected with each other within Switzerland?;(3) would it be possible for the otter to naturally recolonizeSwitzerland from neighbouring countries?

Material and methodsOur multi-step approach (Fig. 2) consists of three steps. Wefirst identify potentially suitable areas using habitat suitabil-ity models (HSM; sometime referred to as species distribu-tion models; see ‘Habitat Suitability’ section) and thenevaluate the connectivity between the suitable patches (see‘Internal Connectivity’ section). Considering that there arepopulations of the species living in close proximity to theSwiss border, we also evaluate the potential for these popu-lations to recolonize suitable areas in Switzerland (see‘External Connectivity’ section).

Habitat suitability

Given that no recent observations were available to cali-brate a species distribution model in Switzerland, we used acombined approach to identify potentially suitable habitatsfor the otter in Switzerland. First of all, we fitted a purelyclimatic model at the scale of the European Alps (seeCianfrani et al., 2011), considering the largest possible cli-matic niche for the species (see Pearson, Dawson & Liu,2004), and then we combined this model with one based onlocal environmental variables (i.e. mainly topography andland use; climate excluded), calibrated at the regional scalein Austria and projected to Switzerland (using the samevariables) (see Fig. 2). Although water quality and preyavailability are important environmental determinants ofthe otter presence, these information are not available in aspatially explicit way along rivers in both study areas (seediscussion). Therefore, none of these two variables could beintegrated in the local HSM. To overcome as much aspossible these limitations, we used surrogate variables in the

Figure 1 Otter distributions in neighbouringcountries of Switzerland and map of theSwiss river catchments.

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model (e.g. elevation for fish density) and combined theexisting non-spatially explicit information later on withthe spatial predictions made with the other local predictors,as a way to improve the interpretation of the the results. Themodelling approach, species data, environmental predic-tors, model calibration and prediction’s combination aredescribed in detail in Supporting Information Appendix S1.Given the particular ecology of the species, the projection ofthe final model was restricted to a buffer of 150 m aroundmain rivers in Switzerland.

Environmental similarity between Austria

and Switzerland

Before calibrating the environmental model in Austria andprojecting it into Switzerland, we assessed the environmen-tal similarity between Austria and Switzerland to ensurethat no bias would occur when a regional model fitted in

Austria is projected in Switzerland. To perform this assess-ment, we used the Mahalanobis distance to calculate anindex of environmental similarity. In particular, we calcu-lated the Mahalanobis distance between the values of theenvironmental predictors in each pixel in Austria and theaverage of the same predictors calculated for the wholeAustrian environment (hereafter d_AUAU). Next, we cal-culated the Mahalanobis distance between the values of thesame environmental predictors for each pixel in Switzerlandand the average values for Austria (hereafter d_AUCH).The comparison between d_AUAU and d_AUCH allowedus to test for the similarity between the Austrian and Swissenvironments. Using ArcGis, we mapped d_AUAU andd_AUCH. We divided d_AUAU into 20 quantile classesand we calculated the percentage of d_AUAU pixelscontained in each of the 20 quantile classes. Following thesame procedure, we calculated the percentage of pixels ofd_AUCH contained in each quantile class. At the end of thisprocess, we obtained the percentage of pixels of d_AUAUand d_AUCH contained in each quantile class. We testedwhether these two distributions differed significantly using aKolmogorov–Smirnov test.

Internal connectivity

To assess the landscape permeability for an otter movingbetween potential suitable patches within Switzerland, weused the Circuitscape software (version 3.5.4; McRae,2006). Following Carranza et al. (2012), the permeabilitywas evaluated within the whole suitable catchments, thatis, not restricting to the 150 m buffer around the riverstretches, as we were interested in exploring the potentialmovements of otters both within (daily movements) andbetween (dispersal process) catchments. The softwaredraws on circuit theory to evaluate the total landscape con-ductance between sites based on multiple paths (McRae,2006). The inputs for Circuitscape are a raster in whicheach cell has a conductance value that corresponds to therelative probability of the otter moving through the differ-ent habitat types and a raster of focal nodes (patches suit-able for the presence of the otter according to the habitatsuitability analysis).

To generate the conductance map, we considered the landcover (Land use 25 categories; Maggini, 2011), slope (calcu-lated from ASTER GD, see Table 1) and human density(Swiss Federal Statistical Office) of the region. The conduct-ance values of the different land-use classes were assignedfollowing Loy et al. (2009) and Carranza et al. (2012;Table S3 in Supporting Information Appendix S3).

External connectivity

To assess the landscape permeability for an otter dispersingfrom populations closer to the Swiss boundaries (externalpermeability; Fig. 1), we used the same approach explainedin the previous paragraph.

Figure 2 Flow chart of the integrative approach.

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To generate the conductance map, we considered the landcover (Corine Land Cover 2000 categories), slope (calcu-lated from ASTER GD, see Table S3 in SupportingInformation Appendix S3) and human density (from Euro-pean Environmental Agency) of Austria, Alsace (Germany),and Haute-Savoie (France), for which data on otter pres-ence were available at a 1-km2 resolution. Following Loyet al. (2009) and Carranza et al. (2012), we assigned differ-ent conductance values to the different land-use classes,(Table S3 in Supporting Information Appendix S3). In thiscase, we considered the areas with stable otter presence inthe countries surrounding Switzerland as the focal nodes.These areas include the Inn, Rhine and Rhone catchments.

Results

Habitat suitability

The results showed that the distributions of Mahalanobisdistances calculated for the average environmental predic-tors in the whole Austria (hereafter d_AUAU) and the samepredictors for each pixel in Austria (d_AUCH) and Switzer-land (d_AUCH) were not significantly different (D = 0.37,P-value = 0.15), providing support to the transferability ofthe HSM from Austria to Switzerland (Fig. 3). The areaswith the highest difference in habitats between Austria andSwitzerland are located in the Swiss Alps sector (Fig. 3).

Table 1 Percentage area in the catchment, suitable area considering the catchment area, suitable area considering the entire area of Switzerland(CH), permeable area considering the catchment, permeable area considering the entire area of Switzerland (CH) in each catchment

Rivercatchment

Catchment surfacearea/Swiss surfacearea (%)

Suitable area in thecatchment/catchmentsurface area (%)

Suitable area in thecatchment/totalsuitable area in CH (%)

Permeable area in thecatchment/catchmentsurface area (%)

Permeable area in thecatchment/total permeablearea in CH (%)

Aare 26.6 33 30.7 44.0 38.5Adige 0.5 0 0 0.0 0.0Inn 6.2 0 0 12.5 1.7Limmat 7.04 43.2 10.6 26.1 4.5Mera 0.6 0 0 7.6 0.1Posh 0.7 7 1 40.1 0.7Reuss 9.12 36.9 11.8 23.1 5.8Rhine 25.9 28.6 25.9 51.1 38.6Rhone 13.15 25.6 11.8 10.1 5.2Ticino 9.7 26.9 9.1 20.8 4.9

Figure 3 The map visualizes the Mahalano-bis distances between the values of theenvironmental predictors for each pixel inSwitzerland and the average values forAustria. The graphic represents the distribu-tion of the Mahalanobis distances betweenthe values of the environmental predictorsfor each pixel in Austria and the averagevalues for Austria (hereafter d_AUAU) andbetween the values of the same environ-mental predictors for each pixel in Switzer-land and the average values for Austria(hereafter d_AUCH) (orange line). The x-axisrepresents the 20 quantiles for which theMahalanobis distance between the meanof the distribution of the Austrian environ-ment and each point in Austria has beenclassified, and the y-axis represents the per-centage of pixels included in each quantile.We performed a Kolmogorov–Smirnov testto compare the two distributions.

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Overall, all of the HSM calibrated with the different tech-niques can be considered as potentially ‘useful’ (sensu Swets,1988) since they have area under curve (AUC) values > 0.7.In particular, the bioclimatic models calibrated consideringthe entire Alpine range had a minimum AUC of 0.835 [forGeneralized Linear Model (GLM)] and a maximum of0.905 [for random forest (RF)]. Comparable results wereobtained for the models based on the environmentalvariables and calibrated in Austria, with a minimumand a maximum AUC of 0.711 (for GLM) and 0.807 (forANN), respectively (Table S2 in Supporting InformationAppendix S2).

According to our ensemble model derived from the inter-section of the bioclimatic and environmental models, 31% ofSwitzerland (considering only buffers of 150 m aroundrivers) is suitable for the otter (Figs. 4a–c; Table 1). The30.7% of suitable areas is located in the Aare River catch-ment and 25.9% in the Rhine catchment, were they aremainly concentrated in the northern part of the catchment.The remaining suitable patches occur in Rhone and Reusscatchments (11.8% each), and in the Limmat (10.6%;Fig. 4c; Table 1).

Internal connectivity

Almost 33% of Switzerland has a permeability value greaterthan the median permeability of the country (Fig. 4d;Table 1). The Aare and Rhine catchments together containalmost 80% of the most permeable areas, while the Reussand Rhone catchments are much less permeable (5.8 and5.2% of permeable areas, respectively; Table 1). Extendedpermeable areas connect the Aare and the Reuss catch-ments, the Rhone and Ticino catchments (in particularalong the Rhone Valley), the Ticino and the Reuss catch-ments, and the Limmat and the northern part of the Rhinecatchment (Figs. 4d–f). In the Alpine sector, the connectiv-ity is mostly very low because of the rugged topography(Figs. 4d–f).

External connectivity

The external permeability map clearly shows that the ottercould easily colonize Switzerland from the surroundingpopulations in the Haute-Savoie (Fig. 4f). The Rhone catch-ment in Switzerland should be the first to be recolonized,and from here, the otter could easily expand into the Aarecatchments. According to our model results colonizationfrom the Alsace to the Rhine catchment is less likely, andcolonization from Austria to the Inn catchment (where thereare permeable areas but not suitable habitats for the otter)appears very difficult but this possibility can not be excludedin the long term (Figs. 4e,f).

Integrated predictions

Our results indicate that natural recovery is most likely tooccur from populations in Haute-Savoie, potentially recolo-nizing 12% of the suitable habitat and 5% of the permeable

areas in the Rhone catchment in Switzerland (Table 1).From these areas, colonization can spread to the Aarecatchment, which is well connected to the Rhone catchment.These results suggest that, if repopulating Switzerland withotters was to become a main goal, the permeable areasbetween the Haute-Savoie region and the Rhone catchment,as well as that between the Rhone catchment and the Aarecatchments (Fig. 4e,f) should be preserved to the greatestpossible extent, and eventually restored where needed.

DiscussionAfter a severe otter’s decline in the 20th century in mostEuropean countries (Robitaille & Laurence, 2002), severalotter’s reintroduction actions started between the end of1970s and the beginning of 2000, for example, in the Neth-erlands, Alsace, Spain (Catalonia), Italy, Switzerland,Czech Republic and Sweden. In Switzerland, eight ottersfrom Bulgaria were released in 1975 in the SchwarzwasserRiver (Bern canton), but the reintroduction proved unsuc-cessful (Weber, 1990). In others countries, the reintroduc-tion proved successful in a first step, but current populationsseem not viable in the long term. This is the case, forexample, in the Netherlands, where the reintroductionstarted in 2002, with 31 otters released along 6 years. Thereintroduced populations maintained themselves so far, butare small and isolated, with dispersal and migration beingseverely limited by heavy vehicular traffic. In 2008, animportant inbreeding rate was measured in these isolatedpopulations, urging for the creation of dispersal corridors toconnect them with the larger neighbouring German popu-lations (http://www.otter.wur.nl). In such case, a more thor-ough evaluation of habitat suitability and connectivitywould have certainly helped planning a reintroductionprogram with a stronger emphasis on dispersal and migra-tion of reintroduced individuals, and associated longer termviability of reintroduced populations.

Our study does indeed illustrate that, before deciding if aspecies recovery should be supported by reintroduction orfavoured through natural expansion from neighbouringpopulations, several key aspects need to be investigated(Morrel, 2008). This process requires the type of assessmentand tools that we have proposed.

Assessing habitat suitability

The first step consisted in identifying whether suitable habi-tats were available that could be eventually repopulated andwhere they were located (Morrel, 2008). This step can bedifficult when the target species is no longer present in thearea targeted for recovery because it requires fitting a suit-ability model in a different area from which the model canbe safely transferred to the area of concern. Such transfer-ability proved not to be an issue here, as shown by ourenvironmental similarity diagnostic test, but it could becritical in other cases and should be considered carefully.Several studies have similarly assessed model transferabilityto evaluate how well a model calibrated in one situation can

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Figure 4 (a) Map of the bioclimatic suitable areas; (b) map of the environmental suitable areas; (c) map of the total suitable areas obtained byoverlapping the bioclimatic and the environmental suitability; (d) map of the connectivity between all suitable areas; (e) map of the integratedresults; the permeable areas were identified as those areas with a permeability value greater than the median permeability in Switzerland. Thepoints refer to PCBs measurements expressed in function of the effects on the otter.

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be transferred to a different context (e.g. another timeperiod after a climatic change, or another region for aninvasive species; Araujo & Guisan, 2006; Randin et al.,2006; Anderson & Raza, 2010; Maiorano et al., 2012), andour study illustrates well why testing for environmentalsimilarity is an important step to perform prior to transferenvironmental niche models.

In our example, predictions of environmental suitabilitywere restricted to a 150 m buffer around rivers, as otters arerarely found beyond 150 m from streams (Philcox, Grogan& Macdonald, 1999; Kruuk, 2006). In this case, havingpresence data of high locational accuracy for calibrating theHSM becomes particularly important. In this respect, theAustrian data were the only data with a sufficiently accuracybut also from a similar enough environment to allow rea-sonable transferability. To better capture the realized nicheof the species, we combined climatic data at large scale withother non-climatic regional environmental factors. We com-bined these data because it is known that climate mostlycontrols the distributions of species at macroscales (e.g.Cianfrani et al., 2011), whereas other abiotic or bioticfactors control species distributions on a finer spatial scale(Pearson et al., 2004; Lomba et al., 2010).

Assessing internal connectivity

If there are suitable habitats throughout an area to be even-tually repopulated, the second step consists in evaluatingwhether the suitable habitats are sufficiently connected toallow movements between populations to assure that suit-able patches can be colonized (Kramer-Schadt et al., 2004;Singleton et al., 2004; Morrel, 2008; LaRue & Nielsen, 2008;Kuemmerle et al., 2011; Fig. 2). The analyses of the internalconnectivity among the suitable habitats, allows for theidentification of the land matrix available for species move-ments within catchments and dispersal between river basins(Carranza et al., 2012). This analysis provides a generalframework to assess the species’ possibilities to move acrossthe landscape and to identify the patches that could bepossibly colonized (in our case, river catchments; Loy et al.,2009; Carranza et al., 2012). This type of analysis is essentialin Central Europe where landscapes are profoundly domi-nated by humans (Kramer-Schadt et al., 2004; Kuemmerleet al., 2011).

In response to the extensive habitat modification andfragmentation, conservation planners have developed anumber of tools to increase connectivity between coreareas of habitat to facilitate the movement of species.Among them, ecological networks, which aim to identifycore areas, species corridors and buffer zones, the integra-tion of ecological concerns into spatial land-use planningand broader approaches to increase landscape permeability(Jongman & Pungetti, 2004; Crooks & Sanjayan, 2006).Providing increased connectivity is a vitally importantaspect of mammal conservation in Europe and will providea key tool to allow species to adapt to current habitat frag-mentation and projected future climate change (Temple &Terry, 2007). The maintenance and restoration of land-

scape permeability is a particularly important issue and amain responsibility for Switzerland since it occupies acentral position in the middle of the three important Euro-pean otter subpopulations (Cianfrani et al., 2011). We rec-ognize that we only considered structural connectivity inthis analysis, while functional connectivity was only indi-rectly evaluated (Boitani et al., 2007). This is due to thefact that no data on otter movements during dispersal arecurrently available, and thus our approach could not befurther improved. In fact, the only available data refer tothe radiotracking monitoring of reintroduced otters in theNetherlands and Spain (Saavedra Benditi, 2004; Koelewijnet al., 2010), but no details are provided there on the envi-ronmental characteristics limiting or favouring otter move-ment across the landscape.

Assessing external connectivity

Considering that in recent decades carnivore populationshave recovered in most of Western Europe mainly as aresults of natural population expansions, the evaluation ofthe level of connectivity with the neighbouring populationsis an important step before deciding if a species recovery canbe supported by a reintroduction or by actions to favour thenatural expansion of the species from areas where it is cur-rently present. Investigating the possibility of a natural rec-olonization from neighbouring populations is crucial toprioritize conservation strategies and to establish where toact. As reintroductions have a number of limitations andextra costs, if a recovery from natural populations is feasi-ble, a reintroduction plan should be avoided and actionsto favour the natural expansion should be preferred(IUCN/SSC Re-introduction Specialist Group, 1998). Thisis particularly true for the Eurasian otter. In fact, otterreintroduction projects did face such a series of problemsand extra costs (Weber, 1990; Ralls, 2002). In contrast, thespecies seems to have high dispersal capabilities, as sug-gested by its fast recovery in many European countries,including Germany, Austria, Italy, France, UK and Spain(Clavero et al., 2010; Marcelli & Fusillo, 2009; Loy et al.,2010; Marcelli et al., 2012), following its overall decline inthe 1990s. In our example, considering that the species is atpresent close to the Swiss border on the French side and ismoving closer on the Austrian side, the investigation of theotter re-expansion dynamics in Austria and France wouldbe a crucial step.

In our study, we used a conductance approach to evalu-ate the connectivity of neighbouring populations of Franceand Austria with the locations of suitable habitats in Swit-zerland in order to preliminarly assess if a natural recoloni-zation (as opposed to a human reintroduction) would bepossible.

Our results, obtained from the external permeabilityanalyses, indicate that, in the short term, a recolonizationcould most likely occur from populations in Haute-Savoie,potentially colonizing 12% of the suitable habitat and 5% ofthe permeable areas in the Rhone catchment in Switzerland(Table 1). According to the model, from these areas,

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colonization can spread to the Aare and Ticino catchments,which are well connected to the Rhone catchment. This maywell be true for the Aare, but not for Ticino because of thetopography there. This highlights the risks of modellingsuitable habitats without considering connectivity, andemphasizes the need for careful interpretation. However, theresults suggest that, if repopulating Switzerland with otterswas to become a main goal, the permeable areas between theHaute-Savoie region and the Rhone catchment, as well asbetween the Rhone catchment and the Aare catchment,(Fig. 4e,f) should be preserved and monitored to the great-est possible extent.

Our results show that, according to the environmentalvariables considered, suitable habitats are available inSwitzerland and sufficiently connected between them andwith those outside the borders to allow for the return ofthe otter. On the basis of these results, we can draw somepreliminary recommendations for the prioritization ofactions required for the otter recovery in Switzerland. Themain one would certainly be the preservation of the suit-able riverine habitats in the Rhone catchment that areclosest to the Haute-Savoie populations. Also, accordingto the recent IUCN recommendations (Vasilijevic &Pezold, 2011), the development of a concerned trans-boundary conservation plan would be crucial to ensure theprotection and establishment of source populations inAustria and France.

Missing factors

Although we tried to be exhaustive in covering the mainfactors affecting the distribution and the expansion acrossthe landscape of the otter, three factors previously reportedas important causes of the otter’s extinction in Switzerlandcould not be included in the analyses. These are (1) pollu-tion by polychlorinated biphenyl (PCB)s (Foster-Turley,1992; Weber, 1990; Gutleb & Kranz, 1998; Weber, 2004),(2) food availability (Kruuk, 2006) and (3) hunting andsocial acceptance.

Data on PCBs, fish biomass and social acceptance are notavailable in a spatially explicit way for all rivers in thecountry, and available measurements or information are tooscattered to be spatialized in a reliable way and used in theHSM. Furthermore, these data were not available forAustria where the regional model was fitted. And last, envi-ronmental data for freshwater bodies – such as water tem-perature, depth, water velocity or water chemistry – do nothave a sufficient level of spatial accuracy to allow fishdensity or PCB contamination to be satisfactorily modelledacross Switzerland and be used as predictors in the ottermodels. In particular, fish distrtibution is affected by small,local variations in the river network so that interpolationalong the river network is often inaccurate and not reliable(White, McClean & Woodroffe, 2003). Furthermore, therelationship between fish density and otter distribution isdifficult to assess because this information is unevenly avail-able within an otter range and existing studies led to con-

trasting results (Thorn et al., 1997; McGarvey, Johnston &Barber, 2010). As a rough surrogate for fish assemblagesand abundance we included altitude in the model (Whiteet al., 2003; Remonti, Balestrieri & Prigioni, 2009). ForPCBs contamination, only scattered point measurementswere available for Switzerland and, because of the complex-ity of modelling pollutant fluxes in freshwater systems, thesedata could not be included in the models.

Obtaining these information in a spatially explicit wayshould constitute a priority field for future freshwaterresearch in Switzerland.

Nevertheless, available PCB data can provide a generalidea of the freshwater contamination levels in Switzerland.Data indicate that contamination by PCBs may still be toohigh (Weber, 1990; Michelot et al., 1998; Boscher et al.,2010; Fig. S4 and Table S4 in Supporting InformationAppendix S4) to allow a proper return of the otter in thecountry. Similarly, qualitative data on fish in Switzerlandreveal that fish biomass is alarmingly low in Switzerland(Fischnetz, 2004) and may further prevent the establishmentof viable populations in many river basins. These issuesrequire further investigation to allow a definitive statementfor the otter. Finally, the social acceptance information wasdifficult to include in the models because it was not availablefor Austria (calibration area) and cannot be easily madespatially explicit. In particular, this social study was prelimi-nary and only aimed at testing the overall acceptance of anotter comeback in Switzerland. A more exhaustive socialstudy should be conducted to make it more spatially explicitand then possibly be incorporable in the models.

Would the otter be welcometo Switzerland?

Concerning the hunting and social acceptance dimension,the species is now under strict protection laws. Therefore,only the question of social acceptance remains. Independentresults from a preliminary survey on the socio-economicperception of a species return in Switzerland (Mateus, 2011;see Supporting Information Appendix S5 for the details)revealed quite high percentages of favourable perceptions,even by fishermen and fish farmers, those stakeholders mostsusceptible to experiencing damaging losses from an otterrecovery. In the same survey, most stakeholders stressed theimportance of revitalizing Swiss rivers as a first step, so as toincrease fish biomass and reduce PCB pollution. For thesestakeholders, under the present conditions of low levels offish biomass and high levels of PCB contamination, areintroduction of the otter would not be viable. The deter-mination of stakeholder attitudes about a potential returnof a carnivorous species is essential to anticipate future con-flicts (Treves, 2009).

Conclusion and perspectivesOur study proposed a three-step approach to assess therecovery of a species in an area where it was formely present

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but now extirpated, based on the incorporation of connec-tivity analyses into traditional habitat suitability assess-ments. As few studies so far have considered all these facetswithin a same conservation case (Morrel, 2008), our studyproved novel in this regard. By applying it to the case of theotter in Switzerland, we reached the following conclusions:(1) From a climatic and land-use perspective, suitable habi-tats are present, reasonably well interconnected and con-nected with existing populations outside Switzerland.(2) However, some key factors like water contaminationand food resources could not be included in our analysesand would need to be carefully considered in addition to ourpredictions in any future otter recovery plan.(3) Despite the fact that the otter would be socio-economically welcome and that suitable and well-connectedhabitats could support its return, current levels of PCB andfish density seem still too critical for a full recovery of theotter in Switzerland.(4) Future studies should aim at improving existing spatialdata on fish denstity and water quality for freshwater bodiesin Switzerland, and use these to improve estimates of habitatsuitability and connectivity.

AcknowledgementsW. Hordik helped with the elaboration of the ensemblemodels; S. Matteus performed the human dimension study;H. Satizábal provided useful comments on the paper. Thisstudy was supported by the Swiss Federal Office, MAVAFoundation and ECOCHANGE project (EU 6th Frame-work Programme; contract number FP6-036866).

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Supporting informationAdditional Supporting Information may be found in theonline version of this article at the publisher’s web-site:

Appendix S1 Habitat SuitabilityAppendix S2 Model evaluationAppendix S3 Conductance matrixAppendix S4 Water contamination (PCB)Appendix S5 Social perception of the otter’s returnAppendix S6 (a) Otter occurrence for the Alps sector, at a1-km resolution, which were used to develop the bioclimaticsuitability model. (b) Otter occurrence data in Austriathat were used to develop the environmental suitabilitymodel.

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