Caribbean Corals in Dire Trouble, Study Finds

John Roach
for National Geographic News
July 22, 2003
Corals are rapidly disappearing from reefs in the Caribbean and unless conservation actions are taken immediately the trend may prove irreversible, according to British scientists who performed the first ever basin-wide survey of coral reef decline.

"We all knew it was bad, but not this bad," said Isabelle Côté, a biologist at the University of East Anglia in Norwich, England.

Côté and colleague Toby Gardner together with Jennifer Gill, Alastair Grant, and Andrew Watkinson from the Tyndall Centre for Climate Change Research compiled and analyzed data from 263 separate sites across the Caribbean and found that hard coral cover on the reefs has dropped from approximately 50 percent to 10 percent over the last three decades.

Researchers have generally assumed that the rate of coral decline on Caribbean reefs was dramatic, but prior to this study the magnitude and geographic extent of the problem was unknown, say the researchers. Previous studies were small-scale and site-specific.

John McManus, director of the National Center for Caribbean Coral Reef Research at the University of Miami in Florida, said that the results of this study are consistent with the extrapolations he and others have made in recent years, but he nevertheless finds this study's results and those of others depressing.

"I set up and ran ReefBase for a while—the global database on coral reefs. I felt like I was something like an editor of obituaries," he said.

The study by Côté, Gardner, and colleagues was posted to the Science Express website on July 17. It will also be published in an upcoming issue of the journal Science.

Cause of Decline

The researchers report that there is "no convincing evidence" that the rate of coral decline on Caribbean reefs can be attributed to global warming, but that it is rather the result of several local natural and human factors.

"Coral bleaching—related mainly to rising sea temperature—has affected other parts of the world to a much greater extent than the Caribbean so far," said Côté. "However, the threat of climate change…remains a serious concern for the future."

The major factors for coral decline in the Caribbean include overfishing, which removes fish species that eat algae. Without predators, the algae grow more quickly than the corals and smother them, especially young corals looking to get a foothold on the reef.

Other causes of the decline are sedimentation from deforestation and land development, pollution, disease, and storms such as Hurricane Allen in 1980 which combined with a few other stressors nearly wiped out corals in Jamaica.

Another event specific to the Caribbean was the sudden die-off of the sea urchin Diadema antillarum in 1983. This urchin ate algae, so when it died—for reasons that to this day remain unknown, noted McManus—the algae took over the reefs.

McManus said that the greatest worry is that coral reefs worldwide seem to be losing their ecological resilience. They no longer bounce back like they did for hundreds of thousands of years from periodic hurricanes, typhoons, floods, and tidal waves.

"In the last decade or two we've seen many, many cases where coral reefs have been damaged and instead of coming back as coral-dominated systems, they develop a heavy cover of seaweed," he said.

Ebbing Trend?

The rates of decline appear to be slowing in Florida, the Virgin Islands, Jamaica, and south Central America, but Côté and colleagues say that the slowing is likely because the most susceptible coral species were lost in the 1980s. What remains are hardier species.

"In all of the places where the rates of decline have slowed there is no good evidence of recovery, therefore the coral cover through the '90s has remained largely unchanged at very low levels," said Côté.

In addition, the composition of coral species has changed over the past 25 years. Many of the new coral growth comes from species known as non-framework builders that do not contribute to the growth of reef structure.

"If this is a widespread phenomenon, then there is serious concern about the capacity of Caribbean reefs to cope with rising sea levels," said Côté. Many scientists believe that sea levels will rise in the Caribbean as a result of global warming.

Reef Conservation

To reverse the trend of coral decline in the Caribbean, Côté and colleagues said that the known human causes for decline must be addressed through legislation and strict enforcement.

Specific measures the researchers suggest include strengthening the network of marine protected areas via enforcement of laws and the creation of more protected areas. As well, the researchers recommend pollution-control measures such as mandatory treatment of sewage water and strict controls on coastal development.

"They are all realistic propositions and there are already many new and promising initiatives in this direction," said Côté.

McManus said that what the coral reefs need more than anything is solid, long-term management that takes into account the socio-economic impact of reef conservation measures as well as the reef ecology.

"Funding mechanisms have to be established for really serious, long-term, high-interdisciplinary projects where we compare coral reefs around the world," he said.

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