INTERACTION BETWEEN FOOD AVAILABILITY AND PREDATION MORTALITY MEDIATED BY ADAPTIVE-BEHAVIOR

Increased activity rates in larval anurans are associated with both hi gher growth rates and higher predation mortality. Models of adaptive f oraging 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 manipul ated the resources available to Rana catesbeiana tadpoles and then mea sured the activity of tadpoles in the presence of caged dragonfly larv ae and the mortality rate of the tadpoles when the dragonflies were fr ee to forage. At low food levels the tadpoles moved more often and mor e quickly. Similarly, at low food levels the tadpoles suffered higher predation mortality. The dependence of predation mortality on resource s available to prey underlines the futility of characterizing populati on regulation as being due to predation or resources. Adaptive variati on in behavior responds to both pressures simultaneously. These result s also suggest the possibility that adaptive variation in behavior may lead to density-dependent population regulation. Density-dependent de pletion of resources by prey should lead to increased activity levels, which will result in higher per capita predation rates. The generalit y 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 popula tion dynamic modelling may improve our ability to predict the outcome of interactions within ecological communities.