9701120 Leibold Several factors can influence the impacts of consumers in food webs. In simple food chains, the impact of predators increases with productivity. If prey can attain a refuge from predation due to size at some point in its life history, however, the food web interactions become more complex. In this proposal, the investigators will examine how size-structured prey populations may influence the strength of interactions of predators, and how initial conditions, species composition, and environmental factors (e.g., productivity) may greatly determine the degree to which predators can control the food web. An ongoing modeling effort shows how all of these factors can influence the effects of predators in food webs, with particular reference to potential alternative stable states depending on initial conditions. The salient features of the model were developed for, and appear to be upheld in small pond benthic food webs of southern Michigan, where the snail, Helisoma trivolis, is only susceptible to predators (primarily the Hemipteran, Belostoma flumineum, leeches, and predatory beetles) when small, whereas another common snail, Physella gyrina, is susceptible at all sizes. A large mesocosm experiment will examine the predictions of the model based on prey species identity and diversity, initial densities, and productivity. Biotic and abiotic surveys of a wide range of ponds, and enclosure/exclosure experiments m situ will be employed to further elucidate these effects. Results from this study should shed light into a variety of questions in ecology concerning the strength of species interactions in relation to morphological, environmental, and initial conditions.
