Establishment and persistence of macroalgal phase shifts

Coral reefs are among the most diverse and productive marine ecosystems, and are currently threatened by an increasing frequency and magnitude of natural and anthropogenic perturbations. Natural disturbances such as predator outbreaks and tropical storms can facilitate community shifts from coral to macroalgae. Such shifts have received considerable attention since macroalgae are known to inhibit coral recruitment, and thus hinder or prevent recovery to the coral-dominated state. It is critical, then, to understand mechanisms that permit the establishment and persistence of macroalgae. I addressed these issues using field experiments and herbivore assays in the lagoons of Moorea, French Polynesia. Recent disturbances to the reef have reduced the cover of live coral, while the cover of macroalgae (specifically the chemically and structurally defended species Turbinaria ornata) has increased in some areas. Using feeding assays with different sized individuals, I tested the hypothesis that size-related vulnerability to herbivory may facilitate establishment of Turbinaria. Results suggest that consumption of Turbinaria generally decreases with size, which can lead to the establishment of Turbinaria if the alga survives the brief period when it is especially vulnerable to herbivores. Once established, a phase shift to macroalgae may be reinforced by additional positive feedback mechanisms. I tested whether vulnerable Turbinaria recruits experience a refuge from herbivory when they are associated with unpalatable adults.  This mechanism would promote persistence of a Turbinaria population once mature individuals become established. My results show that survival of recruits was higher when they were associated with Turbinaria adults than when alone. These feedbacks can limit the capacity of herbivores to control macroalgae and hence reverse the phase shift.