Coral halos: Turing patterns and a sign of healthy reefs?

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Luojun Yang, Ecology and Evolutionary Biology, Princeton university, Princeton, NJ, Lisa C. McManus, University of Hawaii, HI, Vítor V. Vasconcelos, University of Amsterdam, Amsterdam, Netherlands, Alana M. Danieu, Dartmouth College and Theresa Wei Ying Ong, Environmental Studies, Dartmouth College, Hanover, NH

Background/Question/Methods
Coral reefs provide food, income and coastal protection for more than 500 million people worldwide. Unfortunately, anthropogenic stressors including climate change and overfishing are impacting their productivity and survival. Developing low-cost and easily accessible indicators of reef health are therefore critical for effective management of these systems. Spatial features on reefs that are visible in satellite images are coral halos, or coral patches that are separated from surrounding algae or seagrass by a ring of bare substrate. Recent studies have found that these features may signal the presence of a robust reef community that supports herbivores and higher trophic levels. However, the underlying biological and physical mechanisms that produce these halos remain underexplored, especially interactions between coral spatial clustering and herbivore dynamics. Using a combination of mathematical modeling and satellite data, we ask the following: (1) what types of coral spatial patterns support halo formation? and (2) can geometric rules and reaction-diffusion processes sufficiently describe the observed patterns or are higher-order trophic considerations (i.e., predators) necessary?
Results/Conclusions
We show that our model based on simple reaction-diffusion and geometric rules can describe quantitatively the expected algal coverage of a healthy reef. Incorporating predator dynamics with spatial information implicitly from the geometric model, we found oscillatory dynamics of the coral halos, with the period and amplitude reflecting the strength of trophic interactions. Stability analysis suggests that the existence of coral halos depends collectively on the rate of algal expansion at the border and the spatial clustering of corals. Examples of coral reefs taken from satellite imagery reflect a range of our theoretical results. Our results could explain why coral halos dynamics range from stable to transient to non-existing in different reef areas and provide a theoretical framework for when and how to use spatial patterns as indicators of coral reef health.