Between-habitat distributions of pond tadpoles and their insect predators in response to fish presence

Between-habitat distributions of prey shared by multiple predators depend on habitat use by the predators, whose own distributions may interact. We used a large-scale, whole-system natural experiment to examine distributions of anuran tadpoles and insect predators between pond microhabitats with contrasting complexity (open water vs emergent littoral vegetation) in drainable ponds that were either kept fishless or stocked with fish. Total relative densities of insect predators did not significantly differ with respect to the fish status of the ponds. Individual anuran taxa responded variously, but only fish-tolerant Bufo bufo densities were higher in the presence of fish. The densities of both insect predators and tadpoles showed positive interactions between fish presence and the use of complex littoral habitat. The habitat shift to littoral vegetation could be indirectly amplified by fish adverse impact on submerged macrophytes, the main structured microhabitat in open-water areas. Irrespective of mechanisms of fish effects (direct consumption, behavioral deterrence or alteration of habitat conditions), aggregation of both tadpoles and insect predators in littoral vegetation may put tadpoles at greater risk of predation by insects, a potentially important factor of amphibian mortality in waters containing fish.