Top-down and bottom-up community regulation in marine rocky intertidal habitats

Strong top-down control by consumers has been demonstrated in rocky interti dal communities around the world. In contrast, the role of bottom-up effect s (nutrients and productivity), known to have important influences in terre strial and particularly freshwater ecosystems, is poorly known in marine ha rd-bottom communities. Recent studies in South Africa, New England, Oregon and New Zealand suggest that bottom-up processes can have important effects on rocky intertidal community structure. A significant aspect of all of th ese studies was the incorporation of processes varying on larger spatial sc ales than previously considered (10's to 1000's of km). In all four regions , variation in oceanographic factors (currents, upwelling, nutrients, rates of particle flux) was associated with different magnitudes of algal and/or phytoplankton abundance, availability of particulate food, and rates of re cruitment. These processes led to differences in prey abundance and growth, secondary production, consumer growth, and consumer impact on prey resourc es. Oceanographic conditions therefore may vary on scales that generate eco logically significant variability in populations at the bottom of the food chain, and through upward-flowing food chain effects, lead to variation in top-down trophic effects. I conclude that top-down and bottom-up processes can be important joint determinants of community structure in rocky interti dal habitats, and predict that such effects will occur generally wherever o ceanographic 'discontinuities' lie adjacent to rocky coastlines. I further argue that increased attention by researchers and of funding agencies to su ch benthic-pelagic coupling would dramatically enhance our understanding of the dynamics of marine ecosystems. (C) 2000 Elsevier Science B.V. All righ ts reserved.