Mass coral bleaching events can drive reefs from being the domains of corals to becoming dominated by seaweed. But longitudinal data show that more than half of the reefs studied rebound to their former glory. See Letter p.94
A constant battle for space is fought every minute of every day on the hard substrates that provide the foundation for living coral reefs. In one corner are reef corals and the photosynthetic dinoflagellate microalgae that live in symbiosis inside them; in the other are fleshy macroalgae, better known as seaweed. On healthy reefs, corals are the clear winners and dominate reef substrates (Fig. 1a). But regime shifts to macroalgae (Fig. 1b) often occur in response to local anthropogenic drivers such as overfishing of herbivores1 or increased nutrients2 from pollution and land-use changes — two conditions more favourable for seaweed than for corals. On page 94 of this issue, Graham et al.3 provide the first unequivocal evidence that regime shifts from corals to macroalgae also occur in response to coral bleaching, and they identify aspects of reef ecology that influence the likelihood of this occurring.
Coral bleaching occurs when the coral hosts expel their symbiotic dinoflagellates, which provide much of the vibrant coloration typical of coral reefs. Corals rely on the photosynthetic symbionts for their energy provision, and if bleached corals do not rapidly regain symbionts, they die. Mass bleaching events occur over broad spatial scales and affect a large component of the reef coral community. One such episode, in 1998, is often referred to as the largest mass bleaching event on record4; in the Seychelles, more than 90% of live coral cover was lost.
Graham et al. tracked the response of coral and fish communities to this event across 21 inner Seychelles islands using a 17-year data set that started in 1994. They found that 9 of the 21 reefs underwent a regime shift to macroalgae, with the live coral cover decreasing from an average of 31% before the event to about 3% by 2011, and macroalgal cover increasing from 3% to 42% during the same period. Where these regime shifts occurred, the functional diversity of associated reef fishes shifted in concert with the changes in coral and macroalgal cover.
One of the key strengths of this study was its ability to test for predictors of ecosystem responses to the bleaching event. Graham and colleagues evaluated several potential factors: the three-dimensional structural complexity of the reef5, water depth, abundance of juvenile corals, nutrient load, density of herbivorous fish and whether the reefs were part of 'no-take' marine reserves. The first three of these drivers turned out to be the most important. Indeed, combining structural complexity with water depth correctly predicted whether or not a regime shift would occur in 98% of cases — regime shifts occurred less frequently in more structurally complex and deeper-water habitats. These correlations bode well for our ability to predict the effects of future mass bleaching events, especially in tropical regions where conservation resources are limited, because these two variables can be quickly and easily measured on most reefs.
Coral reefs are often portrayed as one of the marine ecosystems that are most vulnerable to the threats of climate change, and global warming is commonly thought to be the principal underlying driver of mass bleaching events. Although Graham and colleagues' study is groundbreaking in its attribution of coral-to-algal regime shifts to a mass bleaching event, perhaps their most striking finding is that, in most cases (12 of 21 reefs), such regime shifts did not occur. The fact that more than half of the reefs fully recovered after the bleaching event is a promising outcome for the future of coral reefs. It is also consistent with studies showing that each mass bleaching leaves many sites unaffected, with almost complete recovery of corals from the 1998 event in many parts of the world6, and that coral survivors of past bleaching events have a capacity to persist under subsequent bleaching events7. The findings also fit with experimental work suggesting that corals can quickly adapt to environmental change8. Put simply, many reef corals just might be capable of adapting fast enough to survive current rates of global environmental change9,10.
A key challenge facing reef managers around the world is how to protect coral reefs from the 'big three' human threats: overfishing, pollution and climate change. A range of specific tools is available to tackle the first two of these, which are comparatively local stressors, but there is a paucity of appropriate climate-specific responses. Given the contribution of these local stressors to the global degradation of reefs, it is crucial that their management continues. However, Graham and colleagues' delineation of reef characteristics most closely associated with regime shifts caused by mass bleaching events means that we can now take concrete steps towards managing specifically for climate change as well. For example, the authors' findings suggest that structural complexity and water depth should be explicitly incorporated into the spatial design of marine reserves, with structurally complex and deep-water habitats targeted as high-value sites that will be more resistant to mass coral bleaching than shallower sites.
The authors' finding that the design of marine protected areas in the Seychelles had no bearing on the ability of reefs to rebound from the 1998 bleaching event is unsettling, and is a case in point of the need for new design approaches. But the Seychelles are not alone — many marine reserves only target areas that are important for sustaining fisheries. Perhaps we need to think about broadening the role of marine reserves to one that includes being a refuge from regime shifts, such that their success can be gauged not only by the number of fishes they contain, but also by the degree to which they protect explicit attributes of habitat diversity. To achieve this, Graham and colleagues' messages on how to manage reefs in the face of climate change will need to be placed in a global context, and further long-term studies from reefs in other regions will be needed if we are to fully understand the drivers of regime shifts on reefs.
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