Freshwater snail diversity: effects of pond area, habitat heterogeneity and isolation.

A large number of eutrophic ponds were surveyed for the presence of freshwater gastropods. Factors thought to influence the distribution of the snails were evaluated. As the investigated area has a homogeneous geological background physicochemical factors probably have a low effect on the local distribution of snails. There was a significant, positive regression between pond area and the number of gastropod species in the pond, but the regression only explained a minor part of the variation in species numbers. Multiple regression of an extended number of variables, associated with habitat complexity and dispersal, indicated that, in addition to area, macrophyte diversity and the mean number of gastropods in the 5 closest ponds (S5) were important in explaining the distribution of gastropods. An increase in the number of macrophyte species increases the number of available microhabitats and refuges from predators. S5 probably influences the dispersal rates between ponds. The gastropods in this area are thought to have additional dispersal modes, besides aerial dispersal with birds, and this probably increases the immigration rates and/or decreases the extinction rate.