The polluted planet: Alarm as global study finds one-third of amphibians face extinction

They were the first animals with backbones to walk on land. They witnessed the rise and fall of the dinosaurs and were present at the birth of a bipedal ape who went on to become the most destructive species the planet has ever known.

They were the first animals with backbones to walk on land. They witnessed the rise and fall of the dinosaurs and were present at the birth of a bipedal ape who went on to become the most destructive species the planet has ever known.

Amphibians - frogs, toads, newts and salamanders - are among the longest surviving animals on earth, yet something dramatic now threatens that longevity. And mankind is responsible.

A global study revealed yesterday that almost a third of amphibians face extinction - and pollution is cited as the biggest cause. The three-year survey, involving 500 scientists from more than 60 countries, has found that a third of the 5,743 known species are threatened with being wiped out and at least 427 are so critically endangered that they could disappear tomorrow.

The animals are so sensitive to the man-made environment that scientists have likened them to the canary in a coal mine - songbirds that fell silent, killed in the presence of odourless gas. The latest and most comprehensive study of amphibians around the world has shown that for many species of frogs and their nearest relatives the singing has suddenly and inexplicably stopped - and the same bipedal ape is almost certainly responsible.

"This is a problem way outside what we know," said Simon Stuart of the World Conservation Union and leader of the study published in the online version of the journal Science.

Dr Stuart said: "This level of decline is ... extraordinary and serious because amphibians represent a very important part of the overall diversity of life. Since most amphibians feel the effects of pollution before many other forms of life, their rapid decline tells us that one of earth's most critical life support systems is breaking down."

The figures in the survey are almost certainly underestimates because more than 22 per cent of the known amphibian species are too poorly understood for the researchers to reach a reliable conclusion about what is happening to them.

Populations of almost half of the known amphibian species are in decline. While 32 per cent of amphibians are threatened with extinction, only 12 per cent of birds and 23 per cent of mammals are in the same position. The latest study estimates that up to 122 species have gone extinct since 1980.

Dr Stuart said that all animal groups undergo a natural "background" rate of extinction but, in the case of amphibians, the actual loss of species is equivalent to the total number of background extinctions for many tens of thousands of years being squeezed into a single century.

"The bottom line is that there's almost no evidence of recovery and no known techniques for saving mysteriously declining species in the wild. It leaves conservation biologists in a quandary," Dr Stuart said.

Amphibians are considered uniquely sensitive to man-made changes in the environment. Their moist, porous skins are vulnerable to water-borne toxins and infections, and their reliance on two habitats - freshwater and land - means they cannot survive properly without both.

Scientists have suggested many possible reasons for the decline. Pollution of both water and the atmosphere, human exploitation for food and medicine and habitat destruction all pose serious threats.

But it is clear that amphibians are also disappearing from what appear to be pristine habitats. At one protected site in Costa Rica, for instance, some 40 per cent of amphibians disappeared over a short period in the late 1980s. Other losses occurred almost simultaneously in Costa Rica, Ecuador and Venezuela.

It is this so-called "enigmatic decline" that poses the biggest problem for conservationists simply because they have little idea about what needs to be done to address the problem.

The authors of the report say: "Enigmatic decline species present the greatest challenge for conservation because there are no known techniques for ensuring their survival in the wild. Most enigmatic declines have been recorded from the Americas south to Ecuador and Brazil, Australia and New Zealand, but they are spreading, for instance to Peru, Chile, Dominica, Spain and Tanzania."

Many of these mysterious disappearances seem to take place in tropical habitats involving amphibians living in mountain streams. Some studies suggest they may be linked with the global spread of a fungus called chytridiomycosis, which may be exacerbated by global warming. What is most worrying is that the decline in amphibians is occurring across the world.

Bruce Young, a zoologist who took part in the global amphibian assessment, said: "We already knew amphibians were in trouble, but this assessment removes any doubt about the scale of the problem." Dr Achim Steiner, director general of the World Conservation Union, said: "The fact that one third of amphibians are in a precipitous decline tells us that we are rapidly moving towards a potentially epidemic number of extinctions."

Russell Mittermeier, president of Conservation International, said: "Amphibians are one of nature's best indicators of overall environmental health. Their catastrophic decline serves as a warning that we are in a period of significant environmental degradation."