Foraging trade-offs and resource patchiness: theory and experiments with afreshwater snail community

Empirical results concerning a freshwater snail community are interpreted u sing a two-species consumer model that incorporates resource structure. Beh avioural-scale measurements on a guild of five species of freshwater pond s nails (Mollusca: Pulmonata) indicate a trade-off between the ability to uti lize a patch's resource and the ability to quickly find new resource patche s. Community-level experiments demonstrate that both species richness and c omposition are affected by the patchiness of the environment. In particular , treatments with low patchiness are dominated by species best at exploitin g local resources (diggers) whereas treatments with high patchiness are dom inated by species best at finding new patches (grazers). Results from a con trolled mesocosm experiment with two of the most common of these species, H elisoma trivolvis (a relative digger) and Physella gyrina (Physidae) (a rel ative grazer) show that the patchiness of the environment strongly influenc es the outcomes of interspecific competition among these two species: the d igger performed much better in less patchy habitats, whereas the grazer per formed better in more patchy habitats. A two-species model of diggers and g razers modified to incorporate behavioural aspects of patchiness produces t his same pattern of competitive outcomes.