Foraging substrate and escape substrate: Patch use by three species of gerbils

The three Negev Desert gerbils, Gerbillus pyramidum (greater Egyptian sand gerbil), Gerbillus allenbyi (Allenby's gerbil), and Gerbillus dasyurus (Wag ner'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 fur the habitat-specific processes governing habitat partitioning by gerbil species , we investigated the roles of escape substrate and foraging substrate in a ffecting patch use. In an aviary, we manipulated predatory risk using artif icial illumination and the presence of Barn Owls (Tyto alba). We manipulate d escape substrate and foraging substrate by creating habitat and food patc hes 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 substr ate, all three species had their lowest GUDs on sand, and their highest GUD s on loess. Gerbillus dasyurus responded less intensively to owls when loes s comprised the foraging substrate. Also, each species depended more on its "home" foraging substrate than did the others for the total amount of seed s harvested. Gerbillus pyramidum in particular harvested a greater proporti on from sand than did G. dasyurus. Escape substrate had no direct effects o n patch use. However, G. dasyurus exhibited a foraging substrate x escape s ubstrate interaction, as the rocky escape substrate enhanced its use of the rocky foraging substrate. Our experiments show that foraging substrate con tributes to habitat use by G. allenbyi and G. pyramidum, and that both fora ging substrate and escape substrate contribute to habitat use by G. dasyuru s.