INTERACTIONS BETWEEN PREDATION RISK AND COMPETITION - A FIELD-STUDY OF KANGAROO RATS AND SNAKES

The effects of predation risk from snakes on microhabitat use of kangaroo rats (Dipodomys deserti and D. merriami) was studied in the Mojave Desert. I concentrated on the effects of the predator on the foraging behavior of the rodents and eliminated effects of prey capture on prey population size. Foraging stations contained three seed trays, one in each of three microhabitats (Open, Bush, Grass). The amount of seed left in each tray after one night of foraging was used to assess the responses to predation risk and other foraging costs; additional data were collected during direct observations and by trapping. To investigate the effect of snakes on foraging and microhabitat use of kangaroo rats I manipulated snake presence at the stations. I studied the interactions between predation risk from snakes and moonlight by conducting experiments near full and new moon nights. Both species of kangaroo rats preferred to forage in the Open and avoided the Bush. This preference is opposite to the preference of the main rodent-eating snake at the study site, the sidewinder (Crotalus cerastes), which prefers the Bush over the Open. At stations with snakes, D. deserti reduced its foraging and avoided the Bush more than in control plots. However, D. merriami foraged more at stations with snakes. D. merriami thus foraged at the trays that were avoided by D. deserti and it reduced the risk of interference from the dominant D. deserti. In contrast to studies by other investigators, moonlight (which is associated with increased risk from owls) did not reduce the foraging or affect the microhabitat use of kangaroo rats in summer; moonlight effect was seen only in the fall, when snakes were not active. Unlike risk from owls, risk from snakes is high under bushes and during dark nights. The activity of rodents in summer is a combined reaction to the different predation risks posed by snakes and by owls.