Density-Dependent Habitat and Patch use in Gerbils: Consequences of Safety in Numbers?

Abstract

Habitat selection by individuals is often a density-dependent process: as density increases, the quality of the habitat decreases due to resource competition and interference. These same changes in habitat quality should likewise affect the foraging efforts of individuals at the patch scale. We used density manipulations, fenced enclosures in the field, to examine patch use in two species of gerbils, Gerbillus andersoni allenbyi and G. pyramidum. At the same time, we used sand tracking to measure activity densities and confirm that gerbils show density-dependent habitat selection across the stabilized and semistabilized sand habitats. We then quantified individual patch use, using giving-up densities (GUD; the amount of food left behind in a resource patch following exploitation) in artificial food patches. Gerbils used resource patches in a density-dependent manner, showing lower GUDs at higher activity densities. Interestingly, this was not the case in both habitats. In the stabilized sand habitat, GUDs for both gerbils were high, and decreased with increasing activity density, but in the semistabilized dune habitat, GUDs remained constantly low. This led GUDs in the two habitats to converge at high activity densities. For these surprising findings we suggest two mechanistically different explanations, one that relies on the ideal despotic distribution and the other, on safety in numbers. Evidence supports the second hypothesis, in which safety is pumped from the semistabilized to the stabilized habitat with increasing activity density.

Keywords:

• Negev Desert gerbils
• density-dependent habitat selection
• isodar
• giving-up densities
• ideal free distribution
• safety in numbers
• risk pump