The effect of temporal scale on the outcome of trophic cascade experiments.

The outcome of experimental manipulations in ecology should depend on the duration of the experiment. We tested this proposition by examining the results of published trophic cascade experiments in freshwater communities and the results of our own previously unpublished pond experiment. In particular, we analyze the results of 90 published trophic cascade experiments to test the prediction that the outcome of freshwater trophic cascade experiments, in which zooplanktivorous fish are either present or absent, depends on the duration of the experiment. We present evidence that there are quantitative differences in the strength of trophic cascades among different venue types (enclosure, mesocosm, pond, and lake), but that the strength of trophic cascades does not diminish with increasing experiment duration. Despite the large number of studies, there have been few replicated studies of trophic cascades for longer than a summer field season, and none for the time required to estimate the long-term result of press perturbations. We therefore present the results of a 4-year study of trophic cascades in experimental ponds, to test the predictions that the addition of the top predator results in a sustained increase in the phytoplankton biomass. We found that, as predicted by our literature review, there was no decline in the strength of the trophic cascade as the experiment progressed.