15.3 Long-Term Changes in Nest-Site Selection
and Distribution
15.3.1 Importance of Nest Selection Studies
for Conservation
Nest-site selection studies are a valuable tool that provides information about the
suitability and limitations for the future population growth and geographic
expansion of a species. Explanatory and predictive models are powerful instruments
in the study of spatial distributions of populations, and their use is growing
exponentially in animal ecology (Guisan and Zimmermann 2000; Guisan et al.
2002, 2013; Rushton et al. 2004; Oppel et al. 2012). Models establish statistical
relationships between a response variable, such as the presence or absence of the
species, and a set of explanatory variables that usually quantify environmental
characteristics such as climate, landscape features and degree of human influence
(e.g. Bustamante and Seoane 2004; Guisan and Thuiller 2005; Gavashelishvili and
McGrady 2006; Addison et al. 2013; Guisan et al. 2013). This procedure has been
regularly employed in wildlife conservation, mainly to determine the probability of
future site occupation for expanding populations (Mladenoff et al. 1995; Buckland
et al. 1996; Jerina et al. 2003; Hirzel et al. 2004) and to examine habitat suitability
for released populations within reintroduction programmes (Bustamante 1998). The
accuracy of predictive models is strongly dependent on population equilibrium,
biotic interactions and stochastic local events. While such complexity frequently
limits the generality of the results (Guisan and Zimmermann 2000; Guisan et al.
2002, 2013), an increasing number of studies take this into account (e.g. Fielding
and Haworth 1995; Lindenmayer et al. 1995; Rodriguez and Andrén 1999).
Predictive models are a potentially valuable tool to detect long-term trends in those
key variables capable of explaining the distribution of organisms. In the case of the
bearded vulture, the restricted distribution of the isolated Pyrenean region was a
concern from a conservation perspective by making the species more vulnerable to
extinction threats. As a consequence, we examined the long-term variation in how
individuals select nesting cliffs all over the Spanish Pyrenees (see details in
Margalida et al. 2008a). We focussed on variables indicating a high probability of
cliff occupancy as determined by a previously published model (Table 15.1, see
Donázar et al. 1993; Margalida et al. 2008a).
15.3.2 The Compacting Process on Pyrenean Bearded
Vultures
Although the breeding population increased from 53 to 93 territories between 1991
and 2002, the breeding range surprisingly expanded only slightly. New and old
nesting cliffs had similar habitat features in relation to topography, altitude and
346
A. Margalida
and Distribution
15.3.1 Importance of Nest Selection Studies
for Conservation
Nest-site selection studies are a valuable tool that provides information about the
suitability and limitations for the future population growth and geographic
expansion of a species. Explanatory and predictive models are powerful instruments
in the study of spatial distributions of populations, and their use is growing
exponentially in animal ecology (Guisan and Zimmermann 2000; Guisan et al.
2002, 2013; Rushton et al. 2004; Oppel et al. 2012). Models establish statistical
relationships between a response variable, such as the presence or absence of the
species, and a set of explanatory variables that usually quantify environmental
characteristics such as climate, landscape features and degree of human influence
(e.g. Bustamante and Seoane 2004; Guisan and Thuiller 2005; Gavashelishvili and
McGrady 2006; Addison et al. 2013; Guisan et al. 2013). This procedure has been
regularly employed in wildlife conservation, mainly to determine the probability of
future site occupation for expanding populations (Mladenoff et al. 1995; Buckland
et al. 1996; Jerina et al. 2003; Hirzel et al. 2004) and to examine habitat suitability
for released populations within reintroduction programmes (Bustamante 1998). The
accuracy of predictive models is strongly dependent on population equilibrium,
biotic interactions and stochastic local events. While such complexity frequently
limits the generality of the results (Guisan and Zimmermann 2000; Guisan et al.
2002, 2013), an increasing number of studies take this into account (e.g. Fielding
and Haworth 1995; Lindenmayer et al. 1995; Rodriguez and Andrén 1999).
Predictive models are a potentially valuable tool to detect long-term trends in those
key variables capable of explaining the distribution of organisms. In the case of the
bearded vulture, the restricted distribution of the isolated Pyrenean region was a
concern from a conservation perspective by making the species more vulnerable to
extinction threats. As a consequence, we examined the long-term variation in how
individuals select nesting cliffs all over the Spanish Pyrenees (see details in
Margalida et al. 2008a). We focussed on variables indicating a high probability of
cliff occupancy as determined by a previously published model (Table 15.1, see
Donázar et al. 1993; Margalida et al. 2008a).
15.3.2 The Compacting Process on Pyrenean Bearded
Vultures
Although the breeding population increased from 53 to 93 territories between 1991
and 2002, the breeding range surprisingly expanded only slightly. New and old
nesting cliffs had similar habitat features in relation to topography, altitude and
346
A. Margalida