Coral reefs and algal states: phase shifts and feedbacks in a changing world

In recent years, coral reefs have been subject to immense stress due to a host of global change factors (Wilkinson 1996). These stressors have lead to widespread phase shifts within reef communities. In intact communities, herbivores exert top-down control on macroalgae (Fung et al. 2011, McClanahan et al. 2011, Smith et al. 2010, Woodley 1999). However, in the absence of herbivores, reefs can switch from a coral state to an algal dominated state. Recent modeling studies have indicated that reversal is possible (Mumby 2009), but the threshold for reversal is often determined by feedback loops associated with species-interactions and disturbance regimes. Most of these are positive feedback loops which reinforce the persistence of algae, including density dependent fish behavioral feedbacks, and coral and algae competition. In addition, the global change disturbance factors that set these feedback loops in motion, including ocean warming, overfishing, and physical disturbance, will only lessen the impact of top-down control on reef systems. The variation in these locally mediated feedback loops determines the threshold for reversal, and thus the likelihood of coral reef recovery