Behavioral and demographic responses of tungara frogs to variation in pond density

Population responses to variable environments can result from individual behavior (i.e., dispersal and habitat selection) or demography (i.e., changes in birth and death rates). Distinguishing behavioral from demographic responses to environmental variability has important implications for interpretation of spatial patterns and projections of population persistence. I studied the behavioral and demographic responses of tungara frogs, Physalaemus pustulosus, to experimental variation in the density of breeding ponds over two years. At natural spatiotemporal scales of variation in pond density, behavioral responses predominated over demographic responses. In the first year of the experiment, when plots varied in pond density, tungara frogs were proportionally more abundant in plots that contained higher densities of ponds. This behavioral response appeared to be due to increased ability to find plots with high pond densities and reduced effects of pond saturation (i.e., availability of unused ponds) within these plots. In the second year of the experiment, when plots had equal pond densities, there was a detectable demographic response to breeding-pond density early in the season, with fewer frogs in plots that had contained no ponds in the previous year. This response was short-lived, however, and by the end of the second year there was no detectable demographic effect. These results demonstrate that behavioral responses may predominate over demographic responses even on relatively large scales. Thus, studies demonstrating changes in patterns of abundance and distribution must be cautious in interpreting these results in the context of metapopulation dynamics without a detailed understanding of species-specific dispersal behavior.