Predator Effects on an Assemblage of Consumers through Induced Changes in Consumer Foraging Behavior

This investigation examines how a predator-induced reduction in activity of a prey species (a ‘‘nonlethal’’ predator effect) affects the growth rate of the prey and, indirectly, the growth rate of competitors of the prey. We further determine how the magnitudes of these effects depend on density of the prey species. We develop a foraging model that predicts a predator will reduce the growth of a focal prey species at low prey density but have little effect at high prey density. The model also predicts that presence of the predator will indirectly facilitate growth of a competitor of the focal species, and that this effect will be negligible at low, and maximal at high, focal species density. Thus it is precisely where the effect of the predator on the growth of the focal species is minimum that the indirect effect on the competitor is maximum. We tested these predictions using a system of three species/size classes of anuran larvae and a larval odonate predator (Anax longipes) in cattle watering tanks. By caging the predator we isolated its effect on foraging behavior of the anuran larvae from that of density reduction. We manipulated the density of small green frog larvae (Rana clamitans) while holding density of small bullfrog (R. catesbeiana) and large green frog competitors constant. Small green frogs and bullfrogs reduce their activity in the presence of caged Anax whereas the large green frogs do not. Results were in accord with the model predictions. Caged Anax reduced small green frog and bullfrog growth at low small green frog density but had no effect at high small green frog density. In contrast, caged Anax had a positive effect on large green frog growth, but this effect was negligible at low small green frog density and maximal at high small green frog density. The results also showed that the indirect effects of the predator that arose through reduction in prey foraging activity were comparable in magnitude to those expected through reduction in prey density. These results suggest that the nonlethal effects of a predator on prey and, indirectly, on competitors of their prey, can be large in magnitude and depend strongly on relative species densities.