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The world's first census of amphibians has found that nearly one in three species -- 32.5 per cent -- is under threat of extinction, says a new study published yesterday in the on-line edition of the journal Science.

The findings are dramatic proof that the rate of global extinctions is proceeding thousands of times faster than expected, said Simon Stuart, the paper's lead author and a senior adviser on biodiversity assessment for the Swiss-based World Conservation Union.

"I think it's very worrying," he said. "This is outside our frame of reference."

Many of the sharp declines in the number of amphibians have occurred since 1980. While some extinctions are the result of disease linked to drought induced by climate change, or air and water pollution, others are happening for no apparent reason -- a phenomenon scientists say they have never seen to such a degree. It means that there may also be no known way of keeping a threatened species alive.

The findings are particularly significant because scientists consider them early warning signs about the health of the planet, as amphibians breathe through their skins and often rely on fresh water.

"As things get more inhospitable on the planet, we can expect many more species to feel the impact," Dr. Stuart said.

Scientists have long feared that the world is on the cusp of the sixth major extinction spasm in Earth's history, and the most severe since the dinosaurs vanished 65 million years ago.

The new study shows that the ancient class of amphibians -- the first animals with backbones to develop legs instead of fins and to move to dry land -- is more endangered than birds or mammals. By comparison, 12 per cent of birds and 23 per cent of mammals are at risk of extinction, according to the World Conservation Union.

"It's a kind of terrifying story," said Janice Chanson, an author of the study and a program officer for the biodiversity assessment unit of the World Conservation Union and Washington-based Conservation International.

"Twenty-five years ago, we could never have predicted that this would happen with amphibians."

The concern is that if unexplained declines are happening with amphibians -- a group of more than 5,700 species that includes frogs, toads, newts, salamanders and legless, worm-like creatures called caecilians -- similar types of declines could eventually happen with other classes of animals, such as mammals and birds.

Some amphibians are disappearing even in protected areas that offer prime living conditions, said Ms. Chanson, who worked on the study for more than three years. This means that scientific techniques for keeping creatures alive in the wild, including setting aside nature preserves and limiting capture, may not be helpful.

Scientists started becoming concerned about numbers of amphibians in the 1970s, when they noticed that frogs in the western United States were vanishing. The trend spread to Puerto Rico, through Central America and South America and finally to Australia and New Zealand, Ms. Chanson said. So far, amphibian populations in Africa and Asia have not been affected, but she said there is no reason to think declines will not happen there as well.

The study also found that many amphibians not on the endangered list are not doing well. Apart from the 32.5 per cent of species that are globally threatened, another 43.2 per cent are in decline. Only 28 species are thriving, while 1,552 are stable, the study says.