The key drivers of anthropogenic coral mortality and loss are nearly all regional- to global-scale stressors, including ocean warming and acidification, and coral predator and disease outbreaks. Yet some scientists hope to mitigate these threats locally through fisheries regulations, such as the implementation of Marine Protected Area (MPAs) designed to increase “reef resilience”. By limiting or preventing fishing and other extractive activities, MPAs have been relatively successful in restoring populations of overharvested fish and invertebrates. MPAs could also, in theory, benefit corals by restoring coral reef food webs and more directly by preventing destructive fishing practices and anchor damage. But can MPAs mitigate the effects of climate change?
In a paper recently published in the open access journal PloS One, Sandin et al. (2008), argue that the answer is “Yes”. Co-author Nancy Knowlton stated “These remote healthy reefs clearly show that local protection can make reefs resilient to the impacts of global change”. And lead author Stuart Sandin said “the healthier reefs showed the capacity to recover from climate change events…when the ecosystem structure is intact, the corals appear to bounce back better from previous warm water events that have killed coral.”
The study described a multifacited survey of four reefs in the northern Line Islands. Reefs differed considerably along a gradient of proximity to people; more remote reefs had more large predators, fewer herbivores and higher coral cover. The positive relationship between coral cover and predator biomass (in the non-statistical sense that the reef with the most fish had the most coral) led to the conclusion that “protection from overfishing and pollution appears to increase the resilience of reef ecosystems to the effects of global warming.”
If true this would be a remarkable finding. For a variety of other reasons we clearly need to get a handle on greenhouse emissions and climate change. But until we do, perhaps MPAs could preserve reef ecosystems, or at least minimize reef degradation. However, nearly all of my colleagues that I have spoken to about this study and the potential of MPAs remain skeptical, mainly because MPAs cannot directly regulate or eliminate the primary culprits of anthropogenic coral loss.
In an op-ed describing the impact of the new the Papahanaumokuakea Marine National Monument in the Northwest Hawaiian Islands, Enric Sala (the Line Islands expedition leader) argued, “A national monument can protect against the decimation of sharks, groupers and jacks by fishing, but it cannot protect against global threats to marine life such as global warming and marine debris…Increased temperatures and currents do not respect national monument boundaries.” William Precht, a coral reef geologist and restoration specialist for the Florida Keys National Marine Sanctuary, added “Data from throughout the Caribbean and western Atlantic indicate that no form of local stewardship or management could have protected coral populations from their major sources of mortality (pandemic diseases, regional coral bleaching, and severe storms) or changed the overall trajectory of coral loss observed during the past few decades.”
The Line Islands study could have been a nice natural experiment, testing the efficacy of MPAs in mitigating climate change, had nature cooperated. Unfortunately, it didn’t, and the temperature stress gradient and the fishing intensity gradient were positively correlated, confounding the test and any interpretation of the mechanisms underlying the observed variability in coral cover. The reef with virtually no fishing and the most predators (Kingman) also has not experienced any significant warming or warm periods over the last decade. Was the high coral cover caused by the lack of fishing or the lack of bleaching? And could the high coral cover be in part responsible for the plentiful fish populations on Kingman reef? Further study and a second expedition seem warranted. I hereby place my name on the top of the volunteer list.
In my view, the strength and novel contribution of the study is the comprehensive assessment of a pristine marine ecosystem. As a community ecologist who is far more interested in food webs than microbes, the thing that I found fascinating about the Sandin et al. study was the inverted trophic pyramid at Kingman Atoll; the biomass of top predators was far greater than that of their prey. Herbivorous fish were scarce and frightened, which makes me wonder why macroalgal cover was so low. I suspect this was due to grazing by urchins, which were most abundant at Kingman, probably because their predators were being suppressed by higher level consumers. Despite it’s limitations, the Sandin et al. study demonstrates a powerful macroecologial approach that could be used to test a key hypothesis in coral reef ecology and conservation.