Print PageVariation in body size has important effects on individual physiology and behavior, including metabolism and foraging gain. A large body of theoretical and empirical research has shown that net energetic gain of aquatic filter feeders is a hump-shaped function of body size. However, non-physiological variables like predation risk and food levels can also affect foraging activity of individuals at different sizes. Thus, assuming constant predation risk throughout different sizes, foraging reduction due to predation risk will be most costly at the convex point of the curve, i.e., at an intermediate size. Therefore, it is predicted that foraging activity will decrease least at this point. I examined how predation risk and food levels can affect foraging activity of prey individuals at different sizes. I used different size classes of larval Rana clamitans, and late instar Anax junius as prey and predator species, respectively. Laboratory experiments were executed to determine the activity level and net growth of tadpoles of varying sizes. To create non-lethal predation risk, tadpoles were exposed to predator cues obtained from tadpole-fed water. Food levels were based on a percent body weight for each size class.
Results/Conclusions
Contrary to the predictions, I found that tadpoles of intermediate-size decreased foraging activity the most. Smaller and larger tadpoles were more active in the presence of predation risk than intermediate sized individuals. This result suggests that the monotonic decrease in activity over size due to size-dependent predation risk does not adequately predict the foraging activity mediated by the growth-predation risk. Incorporating an accurate prediction of size-dependent variation in foraging activity into structural population dynamic modeling may improve the accuracy of our predictions in size-structured community dynamics.
