The stress-gradient hypothesis (SGH) predicts that positive interactions in ecological communities should increase, and competition decrease, with increasing environmental stress. A meta-analysis of 71 published tests of this model in marine ecosystems suggests that it has strong empirical support. These findings were consistent across fitness measures, stress types, ecosystems, and taxa. Complicating things somewhat, the SGH technically predicts that the frequency of positive interactions will change with stress level; however, in almost all cases, the response variable assessed was the relative intensity of facilitation or competition. And many of the papers discussed expected and observed variation in the relative importance of positive interactions. Given that frequency, intensity and importance are clearly different concepts that may or may not be correlated, it would be helpful to evaluate them separately. The recent development of a so-called “index of importance” may allow us to do just that, but employing this index will require a different experimental design than has been used in most tests of the SGH to date. I used this approach to test how the relative importance of facilitation versus competition might change for the California mussel, Mytilus californianus, across a gradient of wave exposure and associated thermal and desiccation stress in the rocky intertidal zone. Although mussels grew less when exposed to higher temperatures in the field, indicative of thermal stress, I found no consistent effect of neighbors on mussel growth, condition, or survival. As a consequence, the index of importance remains untested in a marine system at this time.
|Commitee:||Pernet, Bruno, Whitcraft, Christine|
|School:||California State University, Long Beach|
|School Location:||United States -- California|
|Source:||MAI 81/9(E), Masters Abstracts International|
|Keywords:||Intertidal zone, Meta-analysis, Stress gradient hypothesis|
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