Effects of Predators on Prey Growth Rate: Relative Contributions of Thinning and Reduced Activity

Abstract

Predators affect individual growth rates of surviving prey in two conflicting ways. First, predation acts to increase growth rate by thinning the density of prey populations, which releases survivors from competition. At the same time, predators intimidate prey into decreasing their feeding activity and increasing refuge use, causing prey to grow more slowly. Both processes are known to affect individual growth rates in many systems, but their relative importances and interactive effects have not been measured. We designed an experiment to estimate the separate and joint effects of thinning and activity suppression, using Rana sylvatica tadpoles reared in 1100-L outdoor artificial ponds The experiment manipulated the perceived risk of predation (using caged Anax dragonfly larvae) independently from the loss rate (by manually removing tadpoles every other day according to a predetermined mortality schedule). The presence of predators caused tadpoles to decrease time spent feeding and swimming, verifying that the conditions for behaviorally-mediated growth suppression were satisfied. During the first half of the experiment, when tadpoles were small and not yet competing for food, growth declined sharply with predation risk but was unaffected by thinning. During the second half of the experiment, when tadpoles were much larger and had presumably depleted food resources, growth rate increased under thinning but was unaffected by predation risk. Overall, there was an interaction among treatments because activity suppression was only important at low density, while thinning was especially important in the absence of predation risk. Our results suggest that the numerical effects of predators on prey will predominate in communities composed of strongly interacting species with resource depletion (e.g., communities with clear keystone predator effects), whereas growth costs of predator avoidance may quantitatively affect species interactions in communities with less severe exploitative competition.