Distribution of freshwater snails: spatial scale and the relative importance of physicochemical and biotic factors
Traditionally, freshwater snail distributions have been explained as the result of physicochemical factors, especially calcium concentration. Tet factors operating on different spatial and temporal scales rarely have been stated explicitly and alternate explanations have not been explored thoroughly. In the following conceptual model, we suggest that different factors govern snail species composition and abundance across different spatial scales. Across biographic boundaries, water chemistry screens potential colonists, with some species not persisting where calcium levels are less than about 5 mg per liter. Given adequate calcium, abundance and distribution of species among and within water bodies within a region are determined by available habitats and food, if levels of disturbance, competition, and predation are low. In temporary ponds, disturbance lowers species richness and competition. Predators such as fish and crayfish determine snail abundance and species composition among and within most permanent lakes. In support of this perspective, we provide preliminary data from three geographic areas on two spatial scales, among and within lakes, to document the importance of disturbance, competition, food selection, and predation in structuring freshwater snail assemblages. In northern Indiana, disturbance and predation seem most important in determining snail assemblages across lake types. Within a permanent pond in southern England, snail distribution depends on disturbance and food selection. Finally, distribution and abundance of snails and predators in a large permanent lake in northern Wisconsin suggest the importance of habitat-mediated predation by sunfish, crayfish, and leeches. We are now testing the predictions of this conceptual model using laboratory selection experiments, field cage studies, and extensive lake surveys.