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Intermediate levels of disturbance promote diversity on a coral reef platform.

Madin, Elizabeth M.*,1, Madin, Joshua2, 1 University of California - Santa Barbara, Santa Barbara, CA, United States2 National Center for Ecological Analysis and Synthesis, Santa Barbara, CA, United States

ABSTRACT- A number of mechanisms have been offered to explain the diversity of species found on coral reefs. One hypothesis, the intermediate disturbance hypothesis (IDH), proposes that disturbance is a key process structuring coral assemblages. The IDH suggests that high coral species richness at intermediate levels of disturbance is maintained in a nonequilibrium state which reflects an unstable balance between disturbance and interspecific competitive displacement. Empirical studies of the predictions of, and mechanisms proposed by, the IDH have yielded equivocal results. The present study utilizes a unique system to test these predictions and mechanisms. Wave action on a hydrodynamically-exposed coral reef platform is greatest at the reef crest. The force generated by this wave action rapidly attenuates as waves travel over the reef, creating a continuous gradient of hydrodynamically-induced physical disturbance. Throughout the Indo-Pacific, a competitive dominant is also found in this habitat (Acropora hyacinthus). This competitor's delicate tabular growth form renders it particularly vulnerable to hydrodynamic force. The present study empirically tests the predictions set forth by the IDH by using this system as a means of assessing the effect(s) of disturbance on coral assemblage diversity. Indeed, our results demonstrate that the highest level of coral assemblage diversity is found at an intermediate region of this gradient. To test the specific mechanism(s) driving this pattern, we examined the relative abundance, size structure and percent over of both the competitive dominant (A. hyacinthus) and the remaining species in the scleractinian coral assemblage. Taken together, our results provide strong evidence that intermediate levels of disturbance promote diversity on a coral reef platform.

Key words: Coral reef, Disturbance, Diversity, Intermediate disturbance hypothesis

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