Interaction Between Food Availability and Predation Mortality Mediated by Adaptive Behavior

Increased activity rates in larval anurans rates are associated with both higher growth rates and higher predation mortality. Models of adaptive foraging behavior in the face of predation risk predict that at higher resource levels, foraging activity should be reduced. Thus, at higher resource levels predation mortality should also be reduced. We manipulated the resources available to Rana catesbeiana tadpoles and then measured the activity of tadpoles in the presence of caged dragonfly larvae and the mortality rate of the tadpoles when the dragonflies were free to forage. At low food levels the tadpoles moved more often and more quickly. Similarly, at low food levels the tadpoles suffered higher predation mortality. The dependence of predation mortality on resources available to prey underlines the futility of characterizing population regulation as being due to predation or resources. Adaptive variation in behavior responds to both pressures simultaneously. These results suggest the possibility that adaptive variation in behavior may lead to density—dependent population regulation. Density—dependent depletion of resources by prey should lead to increased activity levels, which will result in higher per capita predation rates. The generality of the trade—off between growth rate and mortality rate argues that this mechanism may be widespread. If adaptive variation in behavior is as widespread as it appears, incorporating this variation into population dynamic modelling may improve our ability to predict the outcome of interactions within ecological communities.