The first worldwide assessment of amphibians--the group that includes frogs, toads, salamanders and caecilians--concludes that they are in even more trouble than mammals and birds are. The study classifies nearly a third of the 5,743 known amphibian species as threatened. And another quarter might be if scientists knew more about them. In many cases, the researchers could not explain the threat in terms of known causes, such as habitat destruction or hunting.

Five hundred and twenty scientists in more than 60 countries contributed to the Global Amphibian Assessment, coordinated by IUCN, Conservation International and NatureServe. Regional experts decided whether each species was "vulnerable," "endangered" or "critically endangered." The IUCN Red List of threatened species defines these categories using such criteria as population size, population decrease and species range reduction. A report published online yesterday by Science summarizes the results.

In all, 32 percent of amphibian species fell into one of these groups, which are collectively referred to as "threatened." By contrast, about a quarter of mammal species and one eighth of bird species are threatened. An additional 1,294 amphibian species were too poorly understood to classify. The study further determined that at least 2,468 amphibian species are in decline, 435 are worse off than they were in 1980 and as many as 122 have disappeared altogether since then. "There's never been a decline [and] extinction phenomenon in human history that's been like this," says team leader Simon Stuart of the IUCN.

Only about half of the rapid declines could be attributed to habitat reduction, human exploitation or both. Some of the remaining so-called enigmatic declines happened even in such well protected areas as Yosemite National Park in California. "Clearly there's something different going on here," Stuart asserts. One possible factor is a fungal disease called chytridiomycosis, which is particularly prevalent in the mountainous streams where most of the enigmatic declines occurred. Until the causes are better understood, Stuart notes, hundreds of amphibian species are at risk of extinction unless scientists buy time by raising them in captivity.