Joseph Maina

Live and let die. Some corals (green spots) are located in places where climatic conditions are likely to allow them to survive global warming, whereas others (red dots) are likely to die early from more adverse regional conditions.

Coral Reefs: Winners and Losers

Scientists have just made the job of those trying to save the world's coral reefs a bit easier. A new map published today pinpoints the reefs that can still be saved—and those that are likely to die. The team hopes that the data will help reef managers better focus their conservation efforts.

Today, about 19% of the world's coral reefs have died as a result of overfishing of algae-eating fish, pollution, and temperature spikes, according to the Global Coral Reef Monitoring Network (based on the opinion of 370 coral reef scientists and managers from 96 countries). Another 15% are expected to die in the next 15 years. More than 100 million people depend on them for food, as a live reef supports many edible fish and crustaceans. In addition, many more people rely on reefs as a buffer against high waves.

Reef managers can help restore coral reefs by restricting fishing and reducing pollutants in the water that then runs off land near reefs. Until now, however, they've had a hard time figuring out which reefs to focus on. Their best measure was looking at how a reef resisted past bleaching events-periods of exceptionally warm water that kill the corals and turn them white-and how fast the corals recovered from these events. But this strategy is inefficient and costly because there are not enough coral reef field biologists to study all reef locations.

To help reef managers better focus their efforts, spatial ecologist Joseph Maina of Macquarie University in Sydney, Australia, and his colleagues performed a global assessment of areas most susceptible to bleaching. They mined satellite and tidal data to determine where the water has gotten warmer and where there is the least amount of wind (wind makes waves, which cool the sun-heated top layers by mixing them with the cooler ones underneath). Differences in ultraviolet radiation were also factored in.

Then they looked at the positive factors that make a heat wave bearable: high, swift tides; big variations in temperatures (corals that are used to big swings survive warming better than those that are not); and strong, cooling currents. They also factored in one of the main direct causes of coral mortality: runoff from rivers that dumps sediment and excessive nutrients into the sea, helping algae replace corals. This is the bit that humans can do something about by regulating the use of fertilizers and soil erosion.

The team then plugged all of its data into mathematical formulas and generated a map. The map, in a paper published online today in PLoS ONE, shows that some reefs have a much brighter future than others.

La Reunion, a French island in the Pacific that recently stepped up protection, won the "Most Likely to Succeed" award. On a scale where maximum stress is 1 and minimum is 0, it scored 0.13. On the other end of the spectrum, Lombok Island, which has Indonesia's best score, rates only 0.74, a high stress value.

Florida's Key Largo scored an impressive climatic score of 0.25, but because overfishing and runoff have damaged its reefs, its total stress level is a high 0.85. That makes it a perfect candidate for stepping up protection, Maina says.

Conversely, he says, other healthy reefs in places like Bonaire off Venezuela, the Hawaiian Islands, the Great Barrier Reef, and New Caledonia are already getting good protection. This study should encourage managers to maintain their efforts there, he says.

The study yielded some surprising results. Britain's largely untouched Chagos Islands, which sit in the middle of the biggest marine reserve in the world, nearly the size of Spain, in the central Indian Ocean, scored a high climatic stress level of 0.80. The Pacific nation of Kiribati's Phoenix Islands, whose reefs are protected, scored 0.87 and nearby Palmyra, probably the most studied pristine atoll in the world, 0.83. But Takapoto Atoll in French Polynesia scored a much better 0.66.

In Southeast Asia, however, climatic conditions are so bad that most of the corals are dying and disappearing much faster than reefs in lower-stress environments. The region contains the so-called Coral Triangle that's home to 500 of the world's 700 species of coral. The remarkable biodiversity has led to the Coral Triangle Initiative convened in 2009 in which the United States, the Asian Development Bank, and others pledged more than $400 million over 5 years to protect the reefs from overfishing and pollution. That's far more than in places that the new study predict have a rosier future, despite little success in curbing a declining coral cover in the Coral Triangle over the past 30 years.

"There are a lot of reefs around the world that are going to do much better, and together they're only getting a tiny fraction of that amount," says co-author Tim McClanahan of the Wildlife Conservation Society in Mombasa, Kenya. "A lot of money is being wasted on reefs that have a dismal future," he says.

"We need to focus on the winners, not losers," McClanahan adds. "They're off in the Pacific, southern East Africa, Sri Lanka, and the northern Red Sea. They're not very well protected and they'll do much, much better if they had more fish and less sediment."

The 1982-2009 temperature data the study is based on is a good indicator of future trends, McClanahan says, because many oceanographic physical processes are stable enough to insure consistent patterns in temperature changes.

"This is a valuable contribution that helps to identify locations that will suffer the least from climate change and therefore benefit the most from local conservation measures," says Alan Friedlander, a marine biologist at the University of Hawaii, Manoa, who studies reefs.

But not everyone agrees. John Bruno, a marine ecologist at the University of North Carolina, Chapel Hill, says it's hard to tell which corals will bleach in the future. "The Bahamas bleached in 1998 but not in 2003, while the Virgin Islands bleached in 2010 but not before, and that's just in the Caribbean."

Posted in Environment