The ideal free antelope: foraging dispersions

The theory of ideal free distributions seeks to predict the dispersions of organisms given heterogeneous resource landscapes and density dependent fitness. Behavioral ecologists usually test this theory by examining outcomes, particularly whether the equilibrium fraction of animals in each alternative site equals the fraction of resource there (proportional settlement). Population ecologists instead focus on the relevant process: how does dispersion relate to resource distributions and is it density dependent? Isodar analysis is a process-focused method that assesses the relative roles of quantitative and qualitative factors in shaping density dependent settlement. Here, we apply isodar analysis to the dispersions of wild Thomson’s gazelles foraging on natural swards in Kenya. Frequent and detailed mapping of resource levels allowed us to delineate rich and poor regions, and to record the densities of foraging females in each region throughout an annual cycle. Whereas quantitative differences attracted more foragers into the rich region at low-ambient densities, the poor region became increasingly favored at higher densities, implying the competing influence of a qualitative factor. Additional regressions suggested that this factor involved predation risk through the following scenario. Intakes were found to increase with proximity to ruminating conspecifics. The latter favored poor regions perhaps for improved predator detection. High wet season food abundance both increased gazelle densities and decreased the costs of foraging in the poor region next to vigilant ruminators. Low dry season food levels resulted in lower gazelle densities, and higher costs of foraging in the poor region despite shared vigilance burdens.