Research article Giving-up densities of foraging gerbils: the effect of interspecific competition on patch use YARON ZIV 1 and BURT P. KOTLER 2, * 1 Department of Life Sciences, Bcn-Gurion University, Beer Sheva 84105. Israel; 2 Mitrani Department for Desert Ecology, Jacob Blaustein Institute for Desert Research, Ben-Gurion University, Sde-Boqer Campus 84993, Israel (*author for correspondence, e-mail: kotler@bgumail.bgu.ac.il) Received 14 May 2002; accepted 30 April 2003 Abstract. We studied the effect of a dominant species, Gerbillus pyramidum (Egyptian sand gerbil), on the patch use of its subordinate competitor, G. andersoni allenbyi (Allenby’s gerbil), to better understand interspecific competition between the two species. We used manipulated resource patches (seed trays) covered with cages with two adjustable species-specific gates (either opened or closed to the bigger-dominant species, but always opened to the subordinate one). We recorded species tracks around and on the seed trays and giving-up densities (GUDs) of seeds in the trays after each night of foraging. G. a. allenbyi depleted seed patches to a lower level whenever G. pyramidum was given the opportunity to forage on the seed trays (i.e., present on the grid). This result held regardless of whether G. pyramidum was actually present at a particular station. We suggest that competition from G. pyramidum occurs both directly by interference, in which G. a. allenbyi is forced to be active in the late part of the night, and indirectly by exploitation via resource depletion by G. pyramidum in the early part of the night. The results suggest that interspecific competition from G. pyramidum reduces seed availability and the richness of the environment for G. a. allenbyi enough to affect the marginal value of energy for G. a. allenbyi individuals and cause them to experience lower costs of predation and manifest lower GUDs. Key words: gerbils, giving-up density, interspecific competition, Israel, optimal-patch use Introduction Interspecific competition is known to have a major role in desert rodent communities (e.g., Brown, 1975; Rosenzweig, 1977; Price, 1986; Kotler and Brown, 1988). Interspecific competition is often divided into exploitation and interference (Krebs, 1972; Begon et al., 1986). In the first, individuals of the two species do not interact directly with each other, but have indirect effects via the utilization of a shared resource in short supply. In the second, individuals of a dominant species directly prevent individuals of a subordinate species from utilizing the shared resource by, for example, monopolizing food patches. While exploitative competition does not require a direct interaction between individuals, interference must always have an impact on the way the shared Evolutionary Ecology 17: 333–347, 2003. Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands.