FORT LAUDERDALE, Fla. — Nearly one-third of the small animals that build the most massive and elaborate structures in coral reefs face elevated risk of extinction from global warming and various local problems, an international group of scientists reported today.

The worldwide assessment of more than 700 species of corals showed that 32.8 percent are threatened with extinction, especially those that form large mounds or intricate branches resembling antlers.

Coral reefs provide hiding places and habitat for a quarter of all marine life and are a major source of food for the poor and of tourist revenue in tropical countries.

Some of the threats are global, including elevated ocean temperatures that have stressed corals so much that they are "bleached" bone-white. A massive bleaching brought on by warmer waters in the 1998 El Nino resulted in a vast decline of the world's reefs.

Corals also face excessive and destructive fishing and polluted runoff that buries them under sediment or bathes them in nutrients that fuel out-of-control growth of algae and bacteria. Compounding the problem are various diseases that kill corals when they are under stress.

Using criteria established by the International Union for the Conservation of Nature, the team of scientists determined that loss of reefs and mounting threats have nudged them into the "critically endangered," "endangered" or "vulnerable" categories, leapfrogging over other groups of animals threatened with extinction.

"That makes corals the most threatened animals on Earth," said Greta Aeby, a coral disease expert with the University of Hawaii's Hawaii Institute of Marine Biology. Corals, as a group, are followed closely by frogs and related amphibians, which have also been on steep decline in recent decades due to pollution, loss of habitat and climate change.

The results, released online Thursday by the journal Science, were presented at the International Coral Reef Symposium in Fort Lauderdale, where nearly 3,000 scientists and managers have congregated to learn about the latest scientific discoveries and figure out ways to save the world's reefs.

Kent Carpenter, director of the international union's Global Marine Species Assessment and lead author of the Science article, emphasized the importance of coral reefs beyond their beauty to tourists.

"Corals make up the very framework of the coral reef ecosystem," said Aeby, one of the 38 scientists who collaborated on the study. If they disappear, she said, "we can expect to lose the fish and crabs and other critters that depend on these corals."

Loss of coral reefs could have a profound effect on more than 500 million poor, subsistence fishermen in the tropics who rely on coral reef ecosystems to feed themselves and their families, said David Obura, a marine biologist and East Africa coordinator for the Coastal Oceans Research and Development in the Indian Ocean.

"People rely on coral reefs every day," said Obura, another collaborator on the study. "In places like the Indian Ocean," he said, "we need to work with fishermen and help people decide not to fish in a destructive way."

The decline in reef-building corals has been led by the loss of the two major branching corals in the Caribbean in recent decades. William F. Precht, manager of damage assessment and restoration for the Florida Keys National Marine Sanctuary, noted that 95 percent to 98 percent of the elkhorn and staghorn corals in the Keys and elsewhere in the region have been lost to disease, toppled by hurricanes or crowded out by thick mats of algae and bacteria.

Both of these species are also listed as "threatened" with extinction under the U.S. Endangered Species Act.