negative effect on productivity. Although there is a certain degree of variability in
their responses, territories located near supplementary feeding points that are also
near to other bearded vultures breeding territories had lower productivity than
territories with less conspecific pressure.
15.5.2 Habitat Heterogeneity and Population Regulation
The above evidence suggests that, in accordance with other studies on territorial
raptors (e.g. Krüger and Lindström 2001; Sergio and Newton 2003; Carrete et al.
2006a), habitat heterogeneity plays a key role in the population regulation of
bearded vultures. As the Pyrenean bearded vultures are not individually marked, we
cannot tell whether habitat heterogeneity is a consequence of sites possessing different suitabilities for reproduction or survival (Breininger and Carter 2003;
Lambrechts et al. 2004; Carrete et al. 2006a). However, our findings that productivity declined and its variation increased as bearded vulture populations increased
from 38–91 pairs (during 1987–2002) are new and relevant to both basic and
applied ecology because they show that population regulation is not merely a result
of interference (i.e. ideal free distribution) or pre-emptive use of space. Moreover,
traditional discussions on density dependence in territorial systems are mainly
based on data obtained from populations in demographic equilibrium, where
crowding mechanisms are usually precluded. Our data, collected throughout a
period of population growth, show that when high-density situations are encouraged, demographic density dependence in territorial birds can occur because of the
combined effects of site quality (ideal despotic distribution) and crowding mechanisms (ideal free distribution). Moreover and no less importantly, we also show
that non-breeding birds can make up a significant fraction of the whole population
and that their effects on breeding individuals as scramble competitors must be taken
into account (López-Sepulcre and Kokko 2005).
Age differences could be proposed as an alternative hypothesis to explain productivity variation between territories (Forslund and Pärt 1995), where inexperienced birds occupying new territories increase their productivity through years and
senescence promotes a progressive decay of productivity in traditional sites
(Margalida et al. 2008b). Even when we were not able to test age effect on
reproduction (Bearded vultures are not individually marked), and therefore we
cannot discard it, our data are reliably showing that habitat heterogeneity and
interference play a role in productivity depression, explaining a substantial percentage of deviance.
The density of conspecific competitors has been shown to affect territory size in
several bird species negatively (see review in Newton 1998), independently of food
availability (e.g. Arcese and Smith 1988; Stamps 1990). Although we have no
information on either home range size or its change with density, our results suggest
15 Importance of Long-Term Studies to Conservation …
355
their responses, territories located near supplementary feeding points that are also
near to other bearded vultures breeding territories had lower productivity than
territories with less conspecific pressure.
15.5.2 Habitat Heterogeneity and Population Regulation
The above evidence suggests that, in accordance with other studies on territorial
raptors (e.g. Krüger and Lindström 2001; Sergio and Newton 2003; Carrete et al.
2006a), habitat heterogeneity plays a key role in the population regulation of
bearded vultures. As the Pyrenean bearded vultures are not individually marked, we
cannot tell whether habitat heterogeneity is a consequence of sites possessing different suitabilities for reproduction or survival (Breininger and Carter 2003;
Lambrechts et al. 2004; Carrete et al. 2006a). However, our findings that productivity declined and its variation increased as bearded vulture populations increased
from 38–91 pairs (during 1987–2002) are new and relevant to both basic and
applied ecology because they show that population regulation is not merely a result
of interference (i.e. ideal free distribution) or pre-emptive use of space. Moreover,
traditional discussions on density dependence in territorial systems are mainly
based on data obtained from populations in demographic equilibrium, where
crowding mechanisms are usually precluded. Our data, collected throughout a
period of population growth, show that when high-density situations are encouraged, demographic density dependence in territorial birds can occur because of the
combined effects of site quality (ideal despotic distribution) and crowding mechanisms (ideal free distribution). Moreover and no less importantly, we also show
that non-breeding birds can make up a significant fraction of the whole population
and that their effects on breeding individuals as scramble competitors must be taken
into account (López-Sepulcre and Kokko 2005).
Age differences could be proposed as an alternative hypothesis to explain productivity variation between territories (Forslund and Pärt 1995), where inexperienced birds occupying new territories increase their productivity through years and
senescence promotes a progressive decay of productivity in traditional sites
(Margalida et al. 2008b). Even when we were not able to test age effect on
reproduction (Bearded vultures are not individually marked), and therefore we
cannot discard it, our data are reliably showing that habitat heterogeneity and
interference play a role in productivity depression, explaining a substantial percentage of deviance.
The density of conspecific competitors has been shown to affect territory size in
several bird species negatively (see review in Newton 1998), independently of food
availability (e.g. Arcese and Smith 1988; Stamps 1990). Although we have no
information on either home range size or its change with density, our results suggest
15 Importance of Long-Term Studies to Conservation …
355