Biogeography, Climate and the Role of Direct Positive Interactions in Rocky Shoreline Communities

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In comparison to competitive and consumer processes, the role played by direct positive interactions in marine communities has been overlooked. While direct positive interactions are widely recognized to occur, they have received little experimental attention by community ecologists and have been largely ignored as general community organizing forces. This study explicitly examines the role of habitat ameliorating positive interactions as organizing forces in rocky intertidal communities and specifically will explore the role of biogeography and climate in dictating the importance of these interactions. Specifically, this project (1) tests the hypothesis that the importance of habitat ameliorating, intraspecific group benefits in setting the upper intertidal limits of primary spaceholders increases with decreasing latitude and is more important in bays and estuaries than in open coast habitats due to predictable variation in the intensity of solar radiation. And (2) tests the hypothesis that, by limiting physical stresses on associated organisms, high intertidal algal canopies play a keystone role in structuring high intertidal communities, and that, like intraspecific group benefits, the importance of these effects are also strongly mediated by biogeography and climate. By focusing on the predictability and consequences of positive interactions in the otherwise well studied New England rocky intertidal system, this work will begin to incorporate positive interactions into our understanding of rocky shorelines and other marine and terrestrial systems. Studies suxh as this one that directly quantify how climate can influence process and pattern in natural assemblages may also be one of the best tools available for exploring the potential consequences of global climate change on communities.