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Two of the most powerful ways in which humans have altered ecosystems are by increasing productivity and changing the densities of important consumers. The bottom-up effects of productivity and the top-down effects of consumers have been identified as primary determinants of biological diversity, though the links between them remain unclear. Understanding how consumers and productivity act and interact to yield differences in diversity is of both conceptual and pragmatic importance. Here, I describe three experiments designed to examine the links between grazing, productivity, and diversity in rocky intertidal systems in Oregon and New Zealand. In two experiments I used fully-factorial designs in which I manipulated both grazing and nutrients. Both experiments revealed the primary importance of grazing as a structuring force of algal assemblages. In the Oregon experiment, I also document an interaction between grazing and productivity, with nutrient enrichment decreasing algal diversity at low grazer densities and increasing algal diversity at high grazer densities. This interaction was not apparent in the New Zealand
experiment. In the absence of grazers, nutrient addition led to increased abundance of foliose algae at this site but had no net effect on algal diversity. In the third experiment, also conducted in New Zealand, I used natural variation in nearshore productivity as a backdrop against which I manipulated the access of grazers. In this experiment, I found that grazers had negative impacts on benthic algal diversity and abundance at sites with lower productivity and negligible impacts on benthic algal assemblages at sites with higher productivity. Overall these three studies suggest that in these intertidal grazer-dominated communities, the strong top-down effects often documented in such systems can be modified by more subtle bottom-up effects. Together, results from these experiments elucidate factors that determine algal diversity in these systems and underscore the importance of the evolutionary context in which experiments are conducted. Finally, I conclude with a synthesis of the literature in which I put these and other findings to work by exploring the ways in which basic marine ecological research can inform the management of human activities that affect the marine environment. |
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