occupied, to a non-breeding lifestyle (Newton 1998). This pre-emptive settlement
pattern, coupled with habitat heterogeneity and density-dependent changes in
demography, has been defined as site-dependent population regulation
(Rodenhouse et al. 1997). This mechanism, which complements and, in certain
kinds of species, may even preclude local crowding mechanisms, can generate
negative feedback at all population sizes, sometimes independently of local population densities (Rodenhouse et al. 1997).
The behaviour of floaters may also be a potential regulatory factor
(López-Sepulcre and Kokko 2005). These ‘‘surplus’’ individuals that form a buffer
against population fluctuations may harm breeding performance through
intraspecific conflicts. The establishment of supplementary feeding points within
the distribution area of the breeding population has been the most significant
management action in terms of time and effort undertaken to help the bearded
vulture in the Spanish Pyrenees from the 80s. These sites concentrate a substantial
fraction of the floater population. In this sense, the spatial and temporal overlap
between the breeding and non-breeding population fractions may affect the fitness
of territorial birds by increasing intraspecific interactions.
15.5.1 Density-Dependent Productivity Depression
Associated to Supplementary Feeding Sites (SFS)
From 1978 to 2002 the bearded vulture population in the Spanish Pyrenees
increased from 38 to 91 breeding pairs. During the same period, the mean annual
productivity of the population declined from 0.8 to 0.37 young/territorial pair.
Variability among territories was responsible for a large proportion of this decline.
However, this variability was not related to habitat heterogeneity per se. Habitat
quality index (i.e. an index obtained by considering only relief, altitude and distance
to villages) dropped by 13% during the study period, while the same index
including distance to the nearest breeding pair declined by 20%. Thus, while the
increase in population resulted in some pairs occupying intrinsically poorer territories, the proximity between conspecific breeding pairs seemed to be the most
important factor reducing habitat quality and, therefore, productivity.
Productivity in traditional territories (those occupied at least since 1978) was
better and more stable than in new ones (those occupied from 1988 onwards, when
the population started to increase) initially, from 1988 to 1993. However, these
differences lessened and disappeared all together in the final years (1994–2002),
when territories became more homogeneous regarding productivity. Accordingly,
we found that although the intrinsic quality of traditional territories remained relatively constant through time, a rise in the number of breeding pairs may have
increased intraspecific interactions, thereby reducing the other quality index.
Even when all territories are not equally affected, the distances to both the
nearest conspecific pair and the nearest supplementary feeding points have a
354
A. Margalida
pattern, coupled with habitat heterogeneity and density-dependent changes in
demography, has been defined as site-dependent population regulation
(Rodenhouse et al. 1997). This mechanism, which complements and, in certain
kinds of species, may even preclude local crowding mechanisms, can generate
negative feedback at all population sizes, sometimes independently of local population densities (Rodenhouse et al. 1997).
The behaviour of floaters may also be a potential regulatory factor
(López-Sepulcre and Kokko 2005). These ‘‘surplus’’ individuals that form a buffer
against population fluctuations may harm breeding performance through
intraspecific conflicts. The establishment of supplementary feeding points within
the distribution area of the breeding population has been the most significant
management action in terms of time and effort undertaken to help the bearded
vulture in the Spanish Pyrenees from the 80s. These sites concentrate a substantial
fraction of the floater population. In this sense, the spatial and temporal overlap
between the breeding and non-breeding population fractions may affect the fitness
of territorial birds by increasing intraspecific interactions.
15.5.1 Density-Dependent Productivity Depression
Associated to Supplementary Feeding Sites (SFS)
From 1978 to 2002 the bearded vulture population in the Spanish Pyrenees
increased from 38 to 91 breeding pairs. During the same period, the mean annual
productivity of the population declined from 0.8 to 0.37 young/territorial pair.
Variability among territories was responsible for a large proportion of this decline.
However, this variability was not related to habitat heterogeneity per se. Habitat
quality index (i.e. an index obtained by considering only relief, altitude and distance
to villages) dropped by 13% during the study period, while the same index
including distance to the nearest breeding pair declined by 20%. Thus, while the
increase in population resulted in some pairs occupying intrinsically poorer territories, the proximity between conspecific breeding pairs seemed to be the most
important factor reducing habitat quality and, therefore, productivity.
Productivity in traditional territories (those occupied at least since 1978) was
better and more stable than in new ones (those occupied from 1988 onwards, when
the population started to increase) initially, from 1988 to 1993. However, these
differences lessened and disappeared all together in the final years (1994–2002),
when territories became more homogeneous regarding productivity. Accordingly,
we found that although the intrinsic quality of traditional territories remained relatively constant through time, a rise in the number of breeding pairs may have
increased intraspecific interactions, thereby reducing the other quality index.
Even when all territories are not equally affected, the distances to both the
nearest conspecific pair and the nearest supplementary feeding points have a
354
A. Margalida