Js. Brown et al., FORAGING UNDER PREDATION - A COMPARISON OF ENERGETIC AND PREDATION COSTS IN RODENT COMMUNITIES OF THE NEGEV AND SONORAN DESERTS, Australian journal of zoology, 42(4), 1994, pp. 435-448
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.