Influence of density dependence on predator–prey seabird interactions at large spatio-temporal scales

Abstract

Theoretical investigations of competitive dynamics have noted that numbers of predator and prey influence each other. However, few empirical studies have demonstrated how a life-history trait of the prey (such as fecundity) can be affected simultaneously by its own density and the density of predators. For instance, density dependence can reduce fecundity with increasing number of prey, while inverse density dependence or Allee effects may occur especially when the prey is a social organism. Here we analysed an intraguild predator-prey system of two seabird species at a large spatio-temporal scale. As expected, we found that fecundity of prey was negatively affected by predator density. Nevertheless, fecundity of prey also increased nonlinearly with its own density and strikingly with the prey-predator ratio. Small groups of prey were probably not able to defend their nests especially against large number of predators. At the highest prey densities (i.e. when anti-predator strategies should be most efficient), prey fecundity also lowered, suggesting the appearance of density dependence mediated by food competition. Allee effects and density dependence occurred across a broad range of population sizes of both the prey and the predator at several local populations facing different ecological environments.

Changes in the timing of egg-laying of a colonial seabird in relation to population size and environmental conditions

“…Previous studies have found evidence that social stimulation from an increased number of conspecifics can facilitate earlier onset of breeding (Coulson 2002), so we a priori predicted that, if laying date is related to breeding density, the effect will be positive. Although population size may have negative as well a positive effects on demographic parameters (Greene & Stamps 2001, Oro et al 2006, because of the highly social nature of guillemot colonies, we considered only the positive effects of population size.…”

Density-dependent parameters and demographic equilibrium in open populations

“…As third covariate we consider the ratio of fish catches to the total number of breeding pairs of Audouin's gulls and of the sympatric yellow-legged gulls, L. michahellis (Oro et al 2006). The two species compete for similar resources as shown by the high degree of overlap in the diet and breeding habitat (Martínez-Abraín et al 2003, Oro et al 2006. The fourth covariate consider was the North Atlantic oscillation index (NAO hereafter, Hurrell 1995) taken as a measure of winter conditions.…”

Research priorities for seabirds: improving conservation and management in the 21st century

“…Large opportunistic species (e.g. gulls, giant petrels) can be facultative predators on smaller seabird species within the community, and population increases of the former following the appearance of large amounts of food from human activities may increase the predation rates on vulnerable species (Regehr & Montevecchi 1997, Votier et al 2004, Oro et al 2006a. Seabirds may also more or less share parasites and pathogens that are prevalent in their breeding habitat, although the interactions involved are likely complex.…”

“…Although predatory interactions among seabird species are less well studied than competitive interactions, they do occur (Gilchrist 1999, Votier et al 2004, Oro et al 2006a, Oro & Martínez-Abraín 2007. Large opportunistic species (e.g.…”