A Model for Evaluating the ‘Habitat Potential’ of a Landscape for Capercaillie Tetrao urogallus: A Tool for Conservation Planning
Auteurs : Braunisch (Veronika) et Suchant (Rudi)
Année de publication : 2007
Publication : Wildlife Biology
Volume :
13
Fascicule : 1
Pagination : 21-33
Résumé :
Most habitat models developed for defining priority conservation sites target areas currently exhibiting suitable habitat conditions. For species whose habitats have been altered by land use practices, these models may fail to identify sites with the potential of producing suitable habitats, if management practices were modified. Using capercaillie Tetrao urogallus as an example, we propose a model for evaluating the potential of ecological conditions at the landscape level to provide suitable habitat at the local scale. Initially, we evaluated the influence of selected landscape parameters on the structural characteristics of vegetation relevant to capercaillie. Then we used capercaillie presence data and an ecological niche factor analysis (ENFA) to identify landscape and land use variables relevant to capercaillie habitat selection. We also studied the effect of scale on predictive model quality. Despite high variance, correlations between landscape variables and forest structure were detected. The greatest influence on forest structure was recorded for climate and soil conditions, which were also found to be the best predictors of capercaillie habitat selection in the ENFA. The final model, retaining only two landscape variables (soil conditions and days with snow) and three land use variables (proportion of forest, distance to roads and forest-agricultural borders), explained a high degree of capercaillie habitat selection, even before considering patch size and connectivity. By restricting the analyses to areas with stable subpopulations and a set of relatively stable landscape variables capable of explaining habitat quality at a local scale, we were able to identify areas with long-term relevance to conservation of capercaillie. Given the limited area within central Europe suitable as wildlife habitat, and the resultant need to integrate wildlife ecology concerns into landscape planning, it is important to ask: "Which areas are relevant to an endangered species?" Yet, how should priority areas for conservation measures be defined? Considering only areas where an endangered species is currently present is obviously not sufficient, although this is still common practice in nature conservation and landscape planning. The development of increasingly powerful geographic information systems and the growing availability of digital landscape data in recent years have promoted the creation of large-scale habitat models for the identification of species-relevant sites (e.g. Mladenoff & Sickley 1998, Kobler & Adamic 2000, Schadt et al. 2002,Graf et al. 2005,Zajec et al. 2005). Most of these models are based on a comparison between currently inhabited and uninhabited areas and the identification of relevant habitat parameters, followed by the delimitation of potentially suitable habitats. However, for declining species whose habitats are massively influenced by anthropogenic land use and silvicultural practices, such approaches may be of limited use, as they analyse and extrapolate only a momentary state in an ongoing dynamic process. For example, no decision can be made as to whether the areas with currently suitable habitat conditions are the outcome of past or present management or whether the habitat conditions are a natural result of the prevailing ecological conditions (e.g. climate, soil conditions and topography) and thus have a high potential to remain suitable in the future. Because of its broad spatial and specific habitat requirements (e.g. Klaus et al. 1989, Storch 1993a,b,1995), and owing to its function as an umbrella species (e.g. Suter et al. 2002), the capercaillie Tetrao urogallus is a popular model species for the analysis of species-habitat interrelationships. This tetraonid is indicative of structurally rich and continuous boreal and montane forest habitats (Scherzinger 1989, 1991,Boag & Rolstad 1991, Storch 1993b,1995, Cas & Adamic 1998, Simberloff 1998). Within these forested habitats, the geographic distribution of the capercaillie corresponds largely to the spatial pattern of forest fragmentation and the prevailing climatic conditions. The Black Forest in southwestern Germany holds the largest capercaillie population in Central Europe outside the Alps. Up until the end of the 19th century, the capercaillie was present down into the lowlands. There, secondary habitats had been created by intensive use of forests, including grazing, which opened the forests, and raking of litter for use in stables, which removed nutrients (especially nitrogen) from the soil and led to a change in tree species composition (Suchant 2002, Gatter 2004). Partly as a result of the deterioration of these secondary habitats (Gatter 2004), the capercaillie population has continually declined over the last 100 years and is now limited to higher altitudes (Suchant & Braunisch 2004). The habitat requirements of capercaillie have been investigated in the past, primarily at the local level (e.g. Gjerde et al. 1989, Rolstad 1989, Helle et al. 1990, Storch 1993a,b, 1995, Schroth 1995, Wegge et al. 1995). In view of the capercaillie's spatial requirements, variables occurring at the landscape level have recently been considered (Ménoni 1994, Sachot 2002, Storch 2002, Suchant 2002, Graf et al. 2005). The purpose of our study was to identify the ecological conditions at a landscape scale that define the preconditions for the development of suitable capercaillie habitat at a local scale. Therefore, we focused on indirect variables (Austin & Smith 1989, Austin 2002), which may replace a combination of different resources in a simple manner and which are more stable and less susceptible to anthropogenic influences than resource variables (Guisan et al. 1999). We hypothesised that a habitat model restricted to a small set of such landscape variables, analysed in areas where subpopulations of capercaillie were stable, would allow us to identify areas with a long-term potential for producing and maintaining suitable habitat. We chose different methodological steps to answer the following questions: 1) which landscape variables support the development of suitable capercaillie habitat at a local scale, 2) which landscape and land use variables are relevant to habitat selection by capercaillie and on which spatial scale must they be considered, and 3) to what extent can a model, based on the resultant variables, explain the distribution of capercaillie?