Foraging Substrate and Escape Substrate: Patch Use by Three Species of Gerbils

Abstract

The three Negev Desert gerbils, Gerbillus pyramidum (greater Egyptian sand gerbil), Gerbillus allenbyi (Allenby's gerbil), and Gerbillus dasyurus (Wagner's gerbil), show strong patterns of habitat selection along a gradient, from sandy to loessal to rocky habitats, respectively. Within a habitat, a gerbil must be able to harvest seeds and escape predators. To test for the habitat-specific processes governing habitat partitioning by gerbil species, we investigated the roles of escape substrate and foraging substrate in affecting patch use. In an aviary, we manipulated predatory risk using artificial illumination and the presence of Barn Owls (Tyto alba). We manipulated escape substrate and foraging substrate by creating habitat and food patches of sand, rock (a mix of sand and small rocks), and loess substrate. In response to owls, all three gerbil species foraged less and increased their giving-up densities (GUDs) in food patches. In response to foraging substrate, all three species had their lowest GUDs on sand, and their highest GUDs on loess. Gerbillus dasyurus responded less intensively to owls when loess comprised the foraging substrate. Also, each species depended more on its “home” foraging substrate than did the others for the total amount of seeds harvested. Gerbillus pyramidum in particular harvested a greater proportion from sand than did G. dasyurus. Escape substrate had no direct effects on patch use. However, G. dasyurus exhibited a foraging substrate × escape substrate interaction, as the rocky escape substrate enhanced its use of the rocky foraging substrate. Our experiments show that foraging substrate contributes to habitat use by G. allenbyi and G. pyramidum, and that both foraging substrate and escape substrate contribute to habitat use by G. dasyurus.