Predicting climate-driven regime shifts versus rebound potential in coral reefs

An analysis of 21 coral reefs in the Indian Ocean using data across 17 years that spanned a major climatic disturbance reveals factors that predispose a coral reef to recovery or regime shift from hard corals to macroalgae; these results could foreshadow the likely outcomes of tropical coral reefs to the effects of climate change, informing management and adaptation plans. Coral reef adaptation to change When coral reefs are damaged, their ecosystem can change so radically that a new stable state is reached. This process, known as regime shift, is occurring globally: previously super-diverse reefs are becoming dominated by macroalgae instead of coral, losing animal biodiversity and potentially ecosystem services as a result. Regime shift is not ubiquitous however, and perturbed reefs can also recover to their coral-dominated state. Nicholas Graham and colleagues used long-term data from 21 perturbed reefs in the Indo-Pacific region to examine the factors predisposing a reef to recovery or regime shift. By way of this natural experiment, they identify thresholds for characteristics such as structural complexity, water depth and fish density that predict reef responses to an extreme weather event. These results improve our understanding of one of the greatest threats to marine biodiversity and could enable pre-emptive action to mitigate climate change effects on tropical coral reefs. Climate-induced coral bleaching is among the greatest current threats to coral reefs, causing widespread loss of live coral cover 1 . Conditions under which reefs bounce back from bleaching events or shift from coral to algal dominance are unknown, making it difficult to predict and plan for differing reef responses under climate change 2 . Here we document and predict long-term reef responses to a major climate-induced coral bleaching event that caused unprecedented region-wide mortality of Indo-Pacific corals. Following loss of >90% live coral cover, 12 of 21 reefs recovered towards pre-disturbance live coral states, while nine reefs underwent regime shifts to fleshy macroalgae. Functional diversity of associated reef fish communities shifted substantially following bleaching, returning towards pre-disturbance structure on recovering reefs, while becoming progressively altered on regime shifting reefs. We identified threshold values for a range of factors that accurately predicted ecosystem response to the bleaching event. Recovery was favoured when reefs were structurally com