Effects of vertebrate predation on freshwater gastropods: an exclosure experiment

A field experiment with predator exclosures was performed in a pond in Southern Sweden to evaluate the effects of vertebrate predators on the diversity, density and distribution of freshwater gastropods. Ten exclosures (1 × 1 × 1.5 m) were erected in the nearshore vegetation at a waterdepth of 0.5–0.7 m in early June 1983 so that they included different amounts and species of macrophytes. The exclosures and cageless controls were sampled for macrophytes and gastropods at the end of the summer. The density of gastropods in the controls, exposed to predators, was positively correlated to the abundance of macrophytes. No such correlation existed in the exclosures. Further, the density of gastropods was significantly higher in the exclosures than predicted by the regression between gastropods and macrophytes in the controls. There was no difference between species composition of gastropods between controls and exclosures, but the number of species and the density of gastropods were lower in the macrophyte stands dominated byScirpus lacustris than in the more complex stands dominated bySparganium erectum. These results indicate that vertebrate predation is a major structuring force of benthic freshwater gastropod communities.