Deteriorating Coral Reefs Adapting to Global Warming, say Scientists
By David McAlary
The world's endangered coral reefs, under stress because of global warming, are displaying resilience. U.S. scientists report that the reefs are adapting to higher temperatures by allying themselves with an organism that can tolerate warmer waters. But, David McAlary tell us that scientists say coral adaptation may go only so far.
Scientific evidence indicates that coral reefs have deteriorated rapidly around the world, in the last few decades. According to a recent report by the Pew Center on Global Climate Change, climate warming and human activities such as pollution and overfishing are to blame. A co-author of the report, Joan Kleypas of the U.S. National Center for Atmospheric Research, says there has been significant coral degradation from disease and bleaching, a condition where higher water temperatures kill the protective algae that cover the reefs.
"Currently, I think the estimate is that about a quarter of the reefs are severely damaged on the planet," she said. "Most people agree that we're probably going to see extinctions of some species and that the composition of coral communities is going to change. And that in the end is going to change the way coral reefs function and provide things that are valuable to humans."
Against this gloomy backdrop stand two separate reports in the journal Nature revealing signs that the corals are beginning to evolve their way out of trouble. They show that corals in the Pacific and Indian Oceans have adapted to higher ocean temperatures by playing host to a different kind of protective algae than they are normally associated with, one that tolerates warmer water.
The leader of one of the studies is Andrew Baker of Columbia University in New York City.
"We concluded that what is actually happening is the ability of corals to flexibly associate with different types of algae actually gives them a rather cunning mechanism by which they can respond to climate change by adjusting their physiologies rather quickly, and much more quickly than if they were forced to do this by conventional evolutionary means," he said.
Mr. Baker began surveying coral reefs in 1995. He says corals in Panama, the Persian Gulf, and Kenya that had been bleached during the ocean warming event called El Nino in the late 1990s were covered by more of the heat tolerant algae in 2001 than they were six years earlier. In contrast, corals in the Red Sea and Mauritius that did not suffer bleaching had only a tiny percentage of this same algae.
The second study by University of Guam scientist Rob Rowan analyzed the chemistry of this algae and concludes that it is a high-temperature specialist. He writes that if other coral species can host it or similar types of algae, they also might adapt to warmer habitats relatively easily.
But Andrew Baker at Columbia University says there is an environmental drawback for the coral that host them.
"The corals that contain these heat tolerant algae actually end up growing more slowly than corals that don't," he said. "So what this means is that while these corals are recovering from the affects of global climate change, they may not be resistant to a whole bunch of other stresses, things like overfishing and habitat destruction and nutrient pollution."
And just how resistant to climate change can the coral become? Although the studies show they can survive increases of two to three degrees Celsius with the heat tolerant algae, Joan Kleypas says there might be a limit on how much more adjustment they can make, but no one knows for sure.
"The reason we can't make predictions this is going to save the corals is because the temperature changes that we are seeing are likely to be much greater than what corals have experienced over the last few thousand years at least," she said. "And so their capacity to adapt probably has its own limits. Just because they are showing adaptive response, it doesn’t mean that we don't know how much it's going to help them adapt."
Nevertheless, says Ms. Kleypas, every bit of adaptation helps the endangered coral reefs in their time of stress.
David McAlary, VOA News, Washington.
注释：
endangered 濒临灭绝的
coral reef 珊瑚礁
global 全球的
adapt to 适应
organism 有机器官
indicate 显示
deteriorate 恶化或降低（质量，性质或价值）
Pew Center on Global Climate Change 全球气候变化排忧中心
U.S. National Center for Atmospheric Research 美国国家大气研究中心
degradation 退化
algae 海藻
gloomy 令人伤心的
Columbia University 哥伦比亚大学
Persian Gulf 波斯湾
El Nino 厄尔尼诺
Mauritius 毛里求斯(非洲岛国）
resistant 有抵抗力的