The differential effects of increasing magnitude and frequency of extreme events on coral populations

Background/Question/Methods

Global climate change is expected to have substantial effects on ecological systems, motivating accurate ecological forecasts for future climate scenarios. Ecological forecasts typically focus on the effects of changes in mean environmental variables, but changes at the extremes may also push ecological systems beyond their tolerances. While static models suggest the distributional effects of changing extremes, disentangling the relative effects of changing frequency and magnitude of extreme events on population persistence requires a consideration of dynamical populations. Scleractinian corals present an opportunity to test the relative effects of changes in frequency and magnitude, as extreme temperatures cause coral bleaching and mass mortality. In this study, we coupled climate projections and a dynamic population model to explore the effects of increasing bleaching frequencies and magnitudes on coral populations. First, we quantified how coral persistence and cover depended on bleaching frequency and magnitude. Second, we determined whether the relative importance of coral recovery- and survival-related traits depended on bleaching regime, as well as whether traits contributed differentially to coral persistence or cover. Finally, we tested whether the existence of stable macroalgae dominance could influence the effect of bleaching regime and different coral traits.

Results/Conclusions

Our results demonstrate that increases in the frequency and magnitude of extreme events can have disparate effects on ecological communities. Specifically, our model predicted that severe bleaching is more detrimental to Scleractinian corals than frequent bleaching. Survival-related attributes were more important than recovery-related attributes across all bleaching regimes, but the overall and relative importance of specific coral traits depended on the bleaching regime. Corals with low adult mortality and high growth rates were successful when bleaching was mild, but bleaching resistance was necessary for persistence when bleaching was severe, regardless of frequency. The existence of macroalgae-dominated stable states reduced coral persistence and changed the relative importance of coral traits. Building on previous studies, our results demonstrate that management efforts may need to prioritize protection of "weaker" corals with high adult mortality when bleaching is mild, and protection of "stronger" corals with high bleaching resistance when bleaching is severe.  In summary, ecological forecasts and conservation targets depend on multiple aspects of environmental change and biodiversity.