FORAGING UNDER PREDATION - A COMPARISON OF ENERGETIC AND PREDATION COSTS IN RODENT COMMUNITIES OF THE NEGEV AND SONORAN DESERTS

We used patch-use theory, giving-up densities in experimental food pat ches, and harvest-rate measurements within these patches to determine the relative contributions of predation risk and energy to foraging co sts in four species of rodents from communities in the Sonoran and Neg ev deserts. To partition costs into components of energy and predation , we converted field measurements of giving-up densities into harvest rates (J min(-1)), used these harvest rates as an estimate of total fo raging costs, estimated energetic foraging costs from published physio logical measurements of activity and thermoregulatory costs, and assum ed that missed opportunity costs were either zero or negative. Our res ults showed that predation costs predominate. Energetic costs represen ted only 24%, 19%, 16% and 13% of the foraging costs for Merriam's kan garoo rat (Dipodornys merriami; Sonoran), the round-tailed ground squi rrel (Spermophilus tereticaudus; Sonoran), the greater Egyptian sand g erbil (Gerbillus pyramidum; Negev), and Allenby's gerbil (G. allenbyi; Negev), respectively. Equally important were predation-risk differenc es between bush and open microhabitats; the microhabitat differences i n predation cost were often 2-4 times larger than the animals' energet ic costs. Seasonal patterns in foraging costs also were predominantly influenced by predation rather than energetic costs. Predation costs a ppear to be greater in the Negev Desert, but rodents of the Sonoran de sert experience greater seasonal and microhabitat variability in preda tion costs. As a result, predation risk may contribute more towards sp ecies coexistence in the community of the Sonoran Desert than that of the Negev Desert.