| Many studies show that the removal of large predators can have strong and cascading impacts in simple food webs. In contrast, few studies have found these same impacts in species-rich food webs following similar removals of large predators. Why this is so remains uncertain, but we predict that this discrepancy is due more to extremely patchy distributions of large predators than to an overall absence of strong top down effects on lower order consumers in diverse systems. To test this prediction, we compared the impacts of variation in piscivore density and species assemblage on the composition, density and feeding rates of invertivores between two back reefs (piscivore-poor settings) and fore reefs (piscivore-rich settings) in the Florida Keys. Visual census methods were used to document piscivore impacts on invertivore density and composition. Underwater video recordings, in addition to two forms of tethering experiments, were used to document impacts of larger predators on invertivore predation rates. The results showed that when piscivores are abundant they significantly impact small invertivore abundance and overall invertivore feeding rates. Our findings also illustrate that the intensity of trophic cascades can vary greatly over very small spatial scales. |
