Components of breeding performance in two competing species: habitat heterogeneity, individual quality and density‐dependence

Abstract

Components of breeding performance in two competing species: habitat heterogeneity, individual quality and density-dependence. Á/ Oikos 112: 680 Á/690.Density-dependent breeding performance due to habitat heterogeneity has been shown to regulate populations of territorial species, since the progressive occupation of low quality territories as breeding density increases may cause a decline in the mean per capita fecundity of a population while variation in fecundity increases. Although the preemptive use of sites may relegate low quality individuals to sites of progressively lower suitability, few studies on density dependence have tried to separate the effects of territory quality from individual quality, and none have simultaneously considered the effects of heterospecific competitors. Using two long-term monitored populations, we assessed the relative contribution of habitat heterogeneity and bird quality (in terms of age) on the productivity of sympatric golden Aquila chrysaetos and Bonelli's eagles Hieraaetus fasciatus under different scenarios of intra-and inter-specific competition. Productivity (number of offspring fledged) varied among territories and average annual productivity was negatively related to its variability in both species and populations, thus giving some support to the habitat heterogeneity hypothesis. However, the effect of habitat heterogeneity on productivity became non-significant when parental age and local density estimators were included in multivariate analyses. Therefore, temporal changes in bird quality (age) combined with intra-and interspecific competition explained variability in territory productivity rather than habitat heterogeneity among territories per se. The recruitment of subadult breeders, a surrogate of mortality in eagles, strongly varied among territories. Habitat heterogeneity in productivity may thus arise not because sites differ in suitability for reproduction but because of differences in factors affecting survival. Territories associated with high mortality risks have a higher probability of being occupied by young birds, whose lower quality, interacting with the density competitors, leads to a reduction of productivity. Sitedependent variability in adult survival and interspecific competition may be extensive, but so far largely overlooked, factors to be seriously considered for the site-dependent population regulation framework.

Markov models of territory occupancy: implications for the management and conservation of competing species

“…Both species are large-sized, predominantly cliV-nesting raptors, with overlapping trophic niches (Del Hoyo et al 1994;Gil-Sánchez et al 1994). It has often been assumed, mainly due to their larger body mass (around 4 kg vs. around 2.5 kg for Bonelli's) (Ferguson-Lees and Christie 2001), that golden eagles have superior competitive ability, reducing Bonelli's eagles' reproductive performance in areas of high densities of both species (Carrete et al 2006), or precluding Bonelli's eagles access to otherwise vacant territories (Gil-Sánchez et al 2004;Ontiveros et al 2004). Although competition for both food and territory have been put forward (Fernández and Insausti 1990), in most cases food availability seems unlikely to be limiting, and competition is mostly expected with regards to nesting sites (Gil-Sánchez et al 2004).…”

“…These include decreasing perturbations (e.g. direct persecution) in the areas that are abandoned or remain vacant (Carrete et al 2002a(Carrete et al , b, 2005(Carrete et al , 2006, controlling golden eagles populations (Carrete et al 2002a, b), enhancing nesting site suitability for breeding, including increasing food supply (Carrete et al 2002a, b;Gil-Sánchez et al 2004;Ontiveros et al 2004Ontiveros et al , 2005, reducing adult mortality (Carrete et al 2002a, b;Gil-Sánchez et al 2004), and decreasing the mortality of dispersing individuals (Ontiveros and Pleguezuelos 2000;Carrete et al 2002aCarrete et al , 2005Soutullo et al 2008). These measures can be grouped into two main management strategies: (a) those aimed at decreasing juvenile mortality, and thus encouraging Bonelli's eagle colonization of new territories, and (b) those aimed at decreasing adult mortality and the abandonment of occupied territories.…”

Effects of rearing conditions on natal dispersal processes in a long‐lived predator bird

“…Spatial environmental heterogeneity may also be a consequence of how local sites differ in their suitability for survival and reproduction (Carrete et al., 2006). Interestingly, the productivity of territories may also shift over time as territories vary due to changes in environmental conditions (i.e., quantity/quality of prey, inter‐ and intraspecific competition, climate change, etc.…”

“…This means that Bonelli’s eagles rarely recruit into a new territory. Under this scenario, the recruitment of nonadult individuals could be enhanced by a sudden rise in adult mortality, which will increase the probability that juvenile floaters find and/or occupy a vacant territory or mate with the remaining owner of the territory (Carrascal & Seoane, 2009; Carrete et al., 2006; Penteriani et al., 2003). This phenomenon may explain why males recruit at a younger age than female Bonelli’s eagles, as mortality rates are higher in males than in females in the studied population (Hernández‐Matías, Real, Pradel, Ravayrol, et al., 2011).…”

Mortality at wind-farms is positively related to large-scale distribution and aggregation in griffon vultures

“…Moreover, this method can be also validated using species-specific information on mortality and aggregation of other species of interest. In the case of territorial species, distance to territories as well as aggregation (computed by using a modified version proposed by Carrete et al (2006)) should be considered, as differences in social systems can change their relative importance as mortality predictors. As the limits imposed to wind-farm locations can be changed depending on the risk that could be assumed (which also depend on the species considered), this seems a straightforward and applicable criterion to be used in many different regions, situations and to a variety of species of conservation concern.…”