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Thresholds and the resilience of Caribbean coral reefs

Abstract

The deteriorating health of the world’s coral reefs threatens global biodiversity, ecosystem function, and the livelihoods of millions of people living in tropical coastal regions1. Reefs in the Caribbean are among the most heavily affected2,3, having experienced mass disease-induced mortality of the herbivorous urchin Diadema antillarum4 in 1983 and two framework-building species of coral5. Declining reef health is characterized by increases in macroalgae. A critical question is whether the observed macroalgal bloom on Caribbean reefs is easily reversible. To answer this question, we must resolve whether algal-dominated reefs are an alternative stable state of the ecosystem or simply the readily reversible result of a phase change along a gradient of some environmental or ecological parameter6. Here, using a fully parameterized simulation model in combination with a simple analytical model, we show that Caribbean reefs became susceptible to alternative stable states once the urchin mortality event of 1983 confined the majority of grazing to parrotfishes. We reveal dramatic hysteresis in a natural system7,8 and define critical thresholds of grazing and coral cover beyond which resilience is lost. Most grazing thresholds lie near the upper level observed for parrotfishes in nature, suggesting that reefs are highly sensitive to parrotfish exploitation. Ecosystem thresholds can be combined with stochastic models of disturbance to identify targets for the restoration of ecosystem processes. We illustrate this principle by estimating the relationship between current reef state (coral cover and grazing) and the probability that the reef will withstand moderate hurricane intensity for two decades without becoming entrained in a shift towards a stable macroalgal-dominated state. Such targets may help reef managers face the challenge of addressing global disturbance at local scales.

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Figure 1: Comparison between model predictions and Hughes’ empirical data for the trajectory of structurally complex forereefs in Jamaica at a depth of 10 m.
Figure 2: Stable and unstable equilibria for Caribbean coral reefs at two levels of algal-coral overgrowth.
Figure 3: Phase plane showing equilibrium covers of macroalgae and corals and trajectories over time.
Figure 4: Probability that reefs of given initial state will remain above the unstable equilibrium during a 25-year period.

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Acknowledgements

This study was funded by grants from the Environmental Protection Agency, The Royal Society, the Natural Environment Research Council and the National Science Foundation. We thank P. Armsworth and H. Possingham for discussions and comments on the manuscript, respectively.

Author Contributions P.J.M. jointly conceived the study with A.H., designed and implemented the simulation model, and prepared the manuscript; A.H. created the analytic model with contributions from H.J.E. and P.J.M, supervised its analysis and edited the manuscript; H.J.E. analysed and described the analytic model, carried out the stochastic implementation of the simulation model, and edited the manuscript.

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Correspondence to Peter J. Mumby.

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Mumby, P., Hastings, A. & Edwards, H. Thresholds and the resilience of Caribbean coral reefs. Nature 450, 98–101 (2007). https://doi.org/10.1038/nature06252

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