We use ecological models to explore the conditions under which reefs move from one state to another. In general, trajectories towards the ‘high coral’ state represent recovery whereas trajectories towards the ‘coral depleted’ state represent degradation. A series of thresholds distinguish these two forms of trajectory and we are interested in knowing what controls these thresholds and predicting their locations.
Thresholds are influenced by the local physical environment, which in turn influences the productivity of the ecosystem. Other factors such as coral recruitment rate, coral growth rates, and so on also influence the potential recovery of the system.
For convenience, we often plot these thresholds as a function of grazing which is often depleted artificially by fishing or disease. Models of disturbance are then used to ask whether a reef in a given state today are likely to be pushed beyond a threshold. If reefs are pushed from the ‘recovery’ phase to the ‘degradation’ phase then natural processes will tend to reinforce the decline and reef health will continue to deteriorate. Clearly, this is a situation that managers wish to avoid so we look for ways to reduce the chances of this eventuality. So far, we’ve investigated a few impacts on resilience. These include fishing herbivorous parrotfish, removing mangrove nursery habitats (which also influences the number of herbivores), reducing nutrient runoff into the watershed, and mitigating the effects of ocean acidification.