In the coming decades, the world’s coral reefs will suffer a variety of indignities, from global threats such as warming seas and ocean acidification to local and regional problems such as overfishing and nutrient-rich runoff. If carbon dioxide emissions remain high until the end of the century, reef coverage may drop by 50 percent or more even if local threats are addressed aggressively, a new study suggests. Despite this bad news, another study provides a glimmer of hope for long-suffering reefs: In some cases, the coral ecosystems that rise to replace ones blighted by climate change may actually be more resistant to disease.
In a paper to be published in Global Change Biology, Kenneth Anthony, a marine ecologist at the University of Queensland in Brisbane, Australia and his colleagues modeled how reefs of branching corals of the genus Acropora would fare under various levels of climate change and fishing. In the team’s model simulations, ocean acidification and warming impair the growth and boost the mortality of corals, elevated nutrients in runoff fuels the growth of coral-stifling Lobophora seaweed, and more intense fishing drives down the numbers of herbivores that help to keep the seaweed under control.
Unsurprisingly, under the most extreme climate scenario — the IPCC’s A1FI scenario, in which atmospheric concentrations of carbon dioxide rise to exceed 900 parts per billion by 2100, compared to around 390 ppb today — reefs suffer the most. By the end of the century, even if fish graze about 60 percent of the seaweed each year and the nutrient content of runoff remains relatively low, the area covered by Acropora corals could drop to half today’s amount. If increased fishing drives the grazing rate below 40 percent, however, coral coverage plummets to near zero and the area occupied by seaweed rises to 40 percent or higher. A rise in nutrient-rich runoff would boost the growth of seaweed even higher, Anthony says.
The new analysis doesn’t include effects of non-nutrient pollution and coral disease, and it doesn’t account for any synergetic effects among global and local threats that would amplify known detriments from individual threats, says Anthony. “In that sense, our figures may quite possibly paint an overly optimistic picture.” Nevertheless, he adds, “coral reefs can survive climate change if they’re treated well at the local scale, but many people would consider a nearly 50 percent drop in coral coverage a significant deterioration.”
If there’s any positive aspect to be seen in the ecological havoc wrought by climate change, it’s this: The coral ecosystems that replace old-growth reefs could, in some instances, be more resistant to disease outbreaks, according to a study published online on January 17 in Proceedings of the National Academy of Sciences. In that paper, Laith Yakob and Peter Mumby of the University of Queensland in Brisbane, Australia propose a model in which climate change affects disease resistance in corals in two contradictory ways.
First, says Mumby, warming waters will render individual corals more susceptible to disease (by stressing the corals and possibly boosting the abundance or virulence of pathogens). Second, by increasing the frequency of coral bleaching events and coral mortality, warming will lead to faster turn over of coral populations within a reef, leading to an overall decline in the risk of disease. In such a scenario, disease resistance doesn’t arise due to evolution-driven immunity; instead it arises when the old reef, composed primarily of one or two dominant species of corals, is replaced by a mixture of new, fast-growing species in which a critical mass of disease-susceptible corals forms less often. The net outcome of these conflicting trends will vary from one situation and one reef to another, the researchers report.
In their paper, Yakob and Mumby used a model to better explain how reefs near Key West, Florida responded after an outbreak of a disease dubbed White Plague type II. Previous studies couldn’t duplicate the pattern of death and recovery seen there, says Mumby. he team’s study “is a refreshing look at the future of coral reefs and their potential to evolve and adapt to environmental change,” says Kim Ritchie, manager of the Marine Microbiology Program at the Mote Marine Laboratory in Summerland Key, Florida.
The analysis also points out that “we just can’t assume that coral reefs will continue to function as they have in the past,” says Alastair Harborne, a reef ecologist at the University of Exeter in the United Kingdom (now on short-term assignment to the University of Queensland, where he works with both Anthony and Mumby). Still, he notes, “the overall picture is not a good one for reefs in general.”
Image: © ISTOCKPHOTO / LEE CHIN YONG