Resilience as a community-level property in coral reefs

Background/Question/Methods:

Whether a community shifts in response to disturbance depends on properties such as resistance, resilience, and stability.  Individual species within a community typically display tradeoffs between these properties.  For example, in coral reefs where disturbances such as bleaching events and hurricanes hold the potential to shift communities from coral-dominated to algal-dominated states, corals with massive morphology might be more resistant as they typically experience lower mortality during disturbance but less resilient because of their slow growth.  On the other hand, corals with branching morphology might be less resistant because of their susceptibility to disturbance but more resilient because of their fast growth.  Multiple types comprise communities, where the negative effect of competition might act antagonistically, or the positive effect of both reduced effects of and more rapid recovery from disturbance might act synergistically, to determine community-level resilience.  To determine the combined effect of competing resistant and resilient types in a community, we model two corals and macroalgae dynamics under bleaching disturbances.  We measure resistance in terms of the relationship between mortality and disturbance magnitude, stability as rate of return to a coral-dominated state, and resilience as the amount of loss of coral cover that will lead to a macroalgae-dominated state.  

Results/Conclusions:

Based on a sensitivity analysis of our stability and resilience metrics to all parameter values given a single coral in competition with macroalgae, we characterize our resilient coral by fast growth, low baseline mortality, and high resistance to macroalgae overgrowth but high mortality under disturbance, and the resistant coral as the opposite.  Given these two corals in combination, the effect of disturbance depends on its frequency: under low disturbance the negative effect of competition dominates and the total coral cover can be less than what would occur under each alone, with increased likelihood of loss of the resilient coral.  However, under high disturbance, the positive effect of combined resistance and resilience dominates, with the resistant coral cover benefiting from the presence of the resilient coral, even when the resilient coral is lost subsequent to a disturbance event.  Adding external recruitment as a disturbance-recovery component of resilience enhances the positive effect of combined resistance and resilient corals, where both corals benefits from the presence of the other under high disturbance.  Therefore, the potential for enhanced community-level resilience from resistant and resilient types in combination depends on what characteristics drive species-level resilience as well as the disturbance frequency.