Nonconsumptive predator effects (NCEs) have the potential to exert great influence on ecosystem structure and function, but the strength of NCEs is strongly influenced by abiotic and biotic factors. Studies which investigate biogeographic patterns in the emergence of NCEs across natural environmental and ecological gradients will improve predictions of the role of predators in community regulation. We investigated the effects of a biotic factor (trophic structure) on NCE emergence in basal prey species (oysters Crassostrea virginica) across a natural range in abiotic conditions (tidal prism). Artificial oyster reefs were constructed across 1000 km of the South Atlantic Bight and seeded with basal species (bivalves); basal and intermediate species (mud crabs and oyster drills); or basal, intermediate and top predator species (blue crabs and toadfish). We measured oyster recruitment, shell strength, survival, and growth to determine 1) to what extent NCEs on oysters occur under varying trophic complexity and 2) to what extent NCEs on oysters emerge on reefs across an environmental gradient.
Results/Conclusions
Results/Conclusions</p> </b><p> Oyster recruitment correlated strongly with tidal prism and the greatest variation in oyster characteristics (shell strength, survival, growth) manifested as strong unimodal patterns which reflected this gradient of recruitment. Intense recruitment where the tidal prism was large likely acted as a strong source of bottom up pressure on oyster reef communities, fostering strong competition which would be expected to reduce survival and growth, consistent with our results. In contrast, responses to trophic treatments were few and limited to locations where the tidal prism was weakest, supporting the hypothesis that abiotic forces often overwhelm predator effects in natural systems. Patterns in response to trophic treatment varied between sites, but the limited frequency and intensity of responses constrained our ability to generalize results. These results suggestion strong predator effects may be more limited in natural systems than is currently thought and highlight the degree to which further investigation is needed to inform accurate prediction of community regulation in natural systems.