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

Abstract

Empirical results concerning a freshwater snail community are interpreted using a two-species consumer model that incorporates resource structure. Behavioural-scale measurements on a guild of five species of freshwater pond snails (Mollusca: Pulmonata) indicate a trade-off between the ability to utilize a patch's resource and the ability to quickly find new resource patches. Community-level experiments demonstrate that both species richness and composition are affected by the patchiness of the environment. In particular, treatments with low patchiness are dominated by species best at exploiting local resources (diggers) whereas treatments with high patchiness are dominated by species best at finding new patches (grazers). Results from a controlled mesocosm experiment with two of the most common of these species, Helisoma trivolvis (a relative digger) and Physella gyrina (Physidae) (a relative grazer) show that the patchiness of the environment strongly influences the outcomes of interspecific competition among these two species: the digger performed much better in less patchy habitats, whereas the grazer performed better in more patchy habitats. A two-species model of diggers and grazers modified to incorporate behavioural aspects of patchiness produces this same pattern of competitive outcomes.