COS 225-6 - Coral halo patterns: geometry & consumer-resource interactions

Abstract: Coral halos, or grazing halos, are an absence of vegetation surrounding coral heads. Ecologists have noted this phenomenon since the 1960s and more recent research has demonstrated that halos are present in reefs across the globe. Empirical evidence indicates that grazing pressure declines as distance from patch reefs increases, suggesting that the landscape of fear hypothesis may apply to halo formation. The landscape of fear is a hypothesis that non-consumptive effects of predators can significantly alter the population dynamics and spatial patterning of prey through changes in behavior. If true, the presence of halos could indicate healthy reefs where predator populations keep risk-adverse herbivores from consuming algae too far from coral patches. Though there is some evidence to support the landscape of fear hypothesis in coral reefs, halos are not present in all healthy reefs and appear to have fixed widths regardless of predator pressure. Alternative mechanisms have been proposed for halo pattern formation, yet no unifying theory has been formulated. The goal of this study is to explore if and to what extent coral halos can be explained through simple geometric rules alone or whether consumer-resource interactions are necessary to explain halo patterns. To address these questions, we develop models for coral halo patterns based on the geometry of halos, observed spatial clustering patterns of corals, and higher order consumer-resource interactions. Model results are compared with data from satellite imagery of Heron Island, Australia. We find that geometric rules can explain an absence of halos in highly dispersed reefs, with clustered reefs most likely to have halos. However, predator-prey interactions can help explain higher variance in algae coverage found in clustered reefs. Our results suggest that the landscape of fear is not necessary to explain halo patterns in all reefs. This could explain discordance between theory and observations of halos across reefs with varying spatial distributions of coral. In conclusion, coral halos offer a unique window for understanding how self-organized landscape patterns emerge from a combination of biophysical and consumer-resource interactions.