By Barbara Liston

ORLANDO, Florida (Reuters) - A 13-year study of coral reefs spontaneously recovering in the Cayman Islands offers hope of refuting often doomsday forecasts about the worldwide decline of the colorful marine habitat.

Scientists monitoring the Cayman reefs noted a 40 percent decline in live coral cover between 1999 and 2004 during a period of warmer seas in the Caribbean.

However, seven years later, the amount, size and density of the live coral had returned to 1999 levels as sea temperatures eased, according to Tom Frazer, professor of aquatic ecology at the University of Florida and part of the research team.

"People have said these systems don't have a chance," Frazer told Reuters. "What we are saying is: 'Hey, this is evidence they do have a chance.'"

Coral reefs account for 0.01 percent of the marine environment. They harbor up to 25 percent of the different species of marine organisms and generate millions of dollars for the fishing and tourism industries, the report states.

"They're kind of like the rain forest of the sea," Frazer said.

The reefs are dying around the world. In 2012, the Australian Institute of Marine Science reported that coral coverage of the Great Barrier Reef had declined by half over the previous 27 years.

The Cayman Islands study, conducted with the Central Caribbean Marine Institute there, was published in the November online issue of the San Francisco-based Public Library of Science and highlighted in last month's issue of the Science journal.

Global Coral Reef Alliance, a nonprofit organization dedicated to maintaining reefs, considers high temperatures caused by global warming to be the biggest threat to them.

Frazer said human activity such as pollution, overfishing, dropped anchors and sediment kicked up by dredging also causes chronic stress to the reefs.

Researchers studied the Cayman reefs, which are 80 miles south of Cuba and surrounded by deep ocean water, in part because of their remoteness and negligible impact from a small nearby human population, Frazer said.

"The take home message is, it's worth our time to alleviate all of what we might call those lesser stressors," said Frazer, who is also director of the University of Florida's School of Natural Resources and Environment.

"So when these bigger stressors hit," he added, "the corals are going to be more healthy and essentially be able to recover from the disturbance event."

(Editing by David Adams and Lisa Von Ahn)