Exploitation of Seed Aggregates by Merriam's Kangaroo Rat: Harvesting Rates and Predatory Risk

Abstract

The simple cost—benefit model of foraging and patch usage of Gilliam and Fraser (1987) was tested for Dipodomys merriami, using data from the laboratory and field. Costs are modeled as the risk of being preyed upon while foraging from a particular patch and benefits as the amount of energy harvested per unit time. The model predicts that foraging rodents should choose patches where the risk: gain ratio is minimal. Field tests of the model using three types of 1—g seed samples (i.e., pure milo, pure millet, and a mixed sample of the two) showed that the extent to which seed aggregates were exploited (an estimate of patch residency time) increased with the rate at which seeds are harvested (determined through laboratory trials) and decreased with increasing predatory risk as apparently affected by microhabitat and nocturnal illumination. The patch encounter rate for single seeds and 1—g seed aggregates did not differ, but rodents consistently removed a greater proportion of seeds from the latter. These results suggest that foraging decisions involve a balancing of energy gain with the perceived risk of being eaten, that these are evaluated at the scale of individual resource patches, and that D. merriami accepts proportionately more risk at higher resource levels. While many studies have inferred the importance of both predation and resources to the population and community ecology of desert rodents, this study is one of the first to examine the interaction of responses to predation and patch quality. It is noted that predators may concentrate their prey in refugia, thereby increasing competition for resources among prey species. Hence, awareness of interactions both within and between trophic levels may be required to understand how communities are organized.