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An integrated empirical and modeling approach to evaluate determinants of community structure and alternate stable states dynamics on tropical reefs


Coral reefs have seen precipitous declines across the globe that are generally associated with transitions to reefs dominated by algae. Understanding the causes and dynamics of these transitions is of critical importance for the implementation of effective management strategies to protect reefs that remain healthy and to recover degraded reefs. To address these issues I evaluated the potential resilience of coral reefs at Isla Contadora, Panamá in the Easter Tropical Pacific (ETP), to different anthropogenic stresses with an integrated approach using empirical and modeling methods. Direct community response to stressors was investigated with experimental manipulations of herbivore abundance, nutrient supply and sediment loading. These experiments showed that any of the three stressors could push reefs toward algal dominance but that the effects of particular stressors were variable and highly dependent on the environmental context in which they were applied. In addition, I identified that herbivory rates and nutrient availability, two critical controls of community structure, vary in response to the local abundance of coral and algae. Because both processes are stronger in the community state they support they act as positive feedbacks pushing reefs toward divergent community states and producing patchy spatial patterning. Using these empirical results I developed a spatially explicit simulation model that incorporated and tested if the environmental conditions measured in the ETP supported alternative stable states (ASS). ASS theory is a dominant conceptual framework for understanding processes that support resilience of ecological communities in the face of anthropogenic disturbance, and, by combining empirical and modeling methods, I propose a rapid and non-destructive method to evaluate ASS in fragile habitats. Using the model I demonstrated that the presence of positive feedback are essential for ASS and the strength of those feedbacks is the critical factor that separates systems with phase shifts and ASS. With data from Isla Contadora I was also able to identify the particular conditions under which ETP reefs should display ASS and demonstrated that the reefs at Isla Contadora exist within that range suggesting they exist as ASS.

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