Species interactions, environmental variation and the balance of production and consumption in marine benthic communities

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Variation in interactions between trophic levels can determine whether productive or consumptive processes dominate community structure (ie, "bottom-up" vs. "top-down" control), and thus it is imperative that we understand which mechanisms modulate key species interactions and how they operate. Here I propose to examine the interaction between benthic algae and their invertebrate herbivores in two marine communities that, by the nature of their low diversity, provide a relative simple system in which to address these complex questions. The general approach is to examine variation in production and consumption along environment gradients to identify where transitions between production-dominated and consumption-dominated conditions occur, and then conduct additional observations and experiments to determine how variation in abiotic factors can shift the balance toward one community state or the other. This approach will first be developed in an intertidal system where filamentous green algae can form extensive algal mats when not controlled by herbivorous snails, and then extended to a subtidal community where consumption by urchins can create barren grounds that lack conspicuous kelp. In both systems we will focus on how variability in contrasting abiotic factors (nutrients and water motion) can permit early vulnerable stages to escape control by consumers. The results will not only contribute to general ecological theory, but will be critical for understanding the functioning of coastal ecosystems as well as addressing applied problems (eg, eutrophication).