Habitat heterogeneity affects population growth in goshawk Accipiter gentilis

Abstract

The concept of site‐dependent population regulation combines the ideas of Ideal Free Distribution‐type of habitat settlement and density dependence in a vital rate mediated by habitat heterogeneity. The latter is also known as habitat heterogeneity hypothesis. Site‐dependent population regulation hypothesis predicts that increasing population density should lead to inhabitation of increasingly poor territories and decreasing per capita population growth rate. An alternative mechanism for population regulation in a territorial breeding system is interference competition. However, this would be expected to cause a more even decrease in individual success with increasing density than site‐dependent regulation. We tested these ideas using long‐term (1975–99) population data from a goshawk Accipiter gentilis population in Eastern Westphalia, Germany. Goshawk territory occupancy patterns and reproduction parameters support predictions of site‐dependent population regulation: territories that were occupied more often and earlier had a higher mean brood size. Fecundity did not decrease with increasing density in best territories. Using time‐series modelling, we also showed that the most parsimonious model explaining per capita population growth rate included annual mean habitat quality, weather during the chick rearing and autumn period and density as variables. This model explained 63% of the variation in per capita growth rate. The need for including habitat quality in the time‐series model provides further support for the idea of site‐dependent population regulation in goshawk.