Modeling Habitat Quality for Bearded Vultures (Gypaetus barbatus)

in co-operation with W. Arnold, H. Winkler and R. Zink

Bearded Vultures became extinct in the Alps 110 years ago and have been successfully reintroduced the last 25 years. Within an international monitoring program for the released birds, all alpine observations are recorded centrally in a huge data base in Vienna. One of the most important aims is to protect potential breeding grounds. However, there is a lack in understanding age-dependent dispersal and habitat use. The thesis of R. Zink aims to answer the following questions:

•  Can the distribution of observed bearded vultures in Austria be explained predominantly from the availability of food?

•  Is the ecological niche occupied by bearded vultures dependent on their age?

•  Can the study Area (Vorarlberg, Tyrol, Carinthia, and Salzburg) provide sufficient areas (quantitatively and qualitatively) to support a permanent population of bearded vultures?

Data: study area has a size of 31,872 km² and for this investigation a grain size of 100 x 100m = 1 ha has been used. One information layer for the study area contains approx. 3.2 million cells of interest. 70 layers have been prepared for this investigation which gives a sum of approx. 220 million single information. The data set contains on one hand ecogeographical variables describing the study area concerning topography, geology, climate, land cover, and food availability (see Fig. 1a-e) and on the other hand locations where bearded vultures (broken down into 4 classes of age and the class unknown) have been observed between 1998-2002 (N=817).

Special problem: As it is common for most field observations of different species, the presence of a species is acquired for a certain location at a specific date and time. It is not guaranteed at all that observations are made in the whole area of species distribution and that areas without observations are really not used (missing absence data) which is a condition for most statistical methods. As bearded vultures are extremely mobile and have a huge home range it is very likely that they can be observed flying over unsuitable habitat like woodland.

Method: Ecological Niche factor Analysis (Hirzel et al., 2002)

Preliminary results: Computation of a habitat suitability index (see Fig. 1f and Fig.2). Food availability is important for the distribution of bearded vultures, there is a kind of age dependent habitat selection yet it is still unclear and the study are offers about 4,500 km² potential habitat for bearded vultures which should be sufficient for 15-20 breeding pairs. Maybe you want to have a look at our
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Fig 1: Ecogeographical variables. a) Height above sea level b) Number of days with more than 1mm precipitation c) Distance from karsted areas d) Distance relation from areas a priori estimated as suitable breeding sites for bearded vultures e) Log of body mass grazing sheep f) Habitat suitability index for bearded vultures

Fig 2: Habitat suitability map for bearded vultures draped over a digital terrain model, view from west to east.