Predation can structure ecological communities and also alter ecosystem function. But because predator effects also tend to vary widely across physical and biological gradients that are common at biogeographic scales, extrapolating results from one biogeographic region to others¾or even from one site to another site within a region¾is often problematic. Moreover, this mismatch between the scale of ecological studies and the biogeographic range at which species interactions occur inhibits our understanding of linkages between community ecology and ecosystem processes. We examined the ecology of oyster reefs along 1000 km of the southeastern U.S. as a model system (1) to address whether the consumptive and non-consumptive effects of a well-documented trophic cascade operate similarly across space; (2) to evaluate whether the trophic cascade affects an end-product of benthic-pelagic coupling (sediment organic matter) on oyster reefs and whether such a linkage is consistent across space; and (3) to determine whether the experimental trophic cascade helps explain broad spatial patterns in food-chain length, trophic distribution of biomass, and ecosystem functioning.
Results/Conclusions
Across our study region, predators (toadfish and blue crabs) consistently created a weak consumptive effect on consumer (mud crab) density within experimental oyster reefs. In contrast, the nonconsumptive effects of predators on the foraging behavior of mud crabs and their indirect influences on its prey (oysters) differed in space. The outcome of the trophic cascade also influenced benthic-pelagic coupling, but the strength of this linkage also varied across space. We also documented spatial variation in trophic structure and ecosystem processes on natural oyster reefs. For instance, biomass of predatory fishes that consume mud crabs increased from north to south, and mud crab biomass correspondingly decreased from north to south. In addition, geographic variation in tidal amplitude predictably increased food-chain length by creating deeper water on reefs that in turn promoted reef access by predatory sharks. Finally, geographic variation in local tidal sediment delivery predicted whether oyster reefs increased sediment organic matter relative to that of a paired non-reef location. This large-scale, multi-disciplinary study demonstrated that nonconsumptive predator effects can influence benthic-pelagic coupling on oyster reefs, and that these linkages are highly context-dependent along environmental gradients. However, biogeographic patterns in trophic structure and ecosystem functioning appear to be driven more by predictable gradients in tidal amplitude, predator biomass, and sediment loading.