Safety from predators or competitors? Interference competition leads to apparent predation risk

Abstract

Prey often react to predation risk by foraging preferentially in safe, rather than risky, patches. Yet prey also must deal with the negative effects of competition for the same patches. The pattern of patch use can mimic that created by predation risk if dominant competitors cause subordinates to increase their use of safe foraging patches. I develop a simple theory of this form of apparent predation risk that describes patch use by an optimal forager confronted with interference from individuals of a dominant, competing species. The theory predicts that predation risk for subordinate individuals increases with the density of nearby dominants. I tested the theory’s prediction with meadow voles and southern red-backed voles foraging in pairs of safe and risky patches in four adjacent sub-grids in an old-field enclosure. I used dyadic encounters to confirm that meadow voles are dominant over red-backed voles. I estimated the density of meadow voles likely to be encountered by foraging red-backed voles as the number of uniquely marked meadow voles using at least one of a pair of safe and risky patches. Subordinate red-backed voles foraged indifferently between safe and risky patches when few meadow voles were encountered. Red-backed voles increased their use of both safe and risky patches as the number of nearby meadow voles increased. Giving-up densities were lower and harvesting efficiency higher in safe patches when the number of nearby meadow voles was high. These results document competition between the two species and suggest that competition increases the benefits of foraging more efficiently in safe than risky patches. Experiments using foraging behaviour to assess predation risk might misinterpret its effect unless they first account for competition among foraging individuals.