SYDNEY: Massive declines in amphibian populations are due to a combination of factors linked to changing climate, not a single issue, according to a new study.

Frogs, newts and relatives have been unable to adapt swiftly enough to cope with today's unprecedented rate of global warming, said zoologist Andrew Blaustein of the Oregon State University (OSU) in Corvallis, U.S., and this is driving them toward extinction.

Over the last 30 years, amphibian have been diminishing worldwide. Of the 5,743 species, 43 per cent are in decline, 32 per cent are under threat, and 168 have become extinct. Infectious disease, U.V. light, habitat destruction and pollution have all been cited as causes of the rapidly plummeting numbers.

The infectious chytrid skin fungus has had a particularly devastating impact on frog populations, especially in the tropics.

Climate linked

Now, Blaustein and OSU colleague Betsy Bancroft, argue that these causes are all linked to global climate change, and the inability of amphibians to cope with so many different pressures simultaneously. They detail their findings, the result of an extensive literature review, in the journal Bioscience.

"There have always been threats, and [amphibians] have been some of the most adaptive and successful vertebrates on Earth," said Blaustein. "They were around before the dinosaurs, have lived in periods with very different climates, and continued to thrive while many other species went extinct. But right now, they just can't keep up."

Amphibians are considered by experts to be a 'canary in the coalmine' of environmental damage, due to a physiology that makes them highly sensitive to changes in their habitats, said Blaustein.

Due to their exposed and permeable skin, complex life cycle, and the fact that their eggs have no shells, they are particularly susceptible to changes in temperature and moisture. And while these characteristics have played a role in their evolutionary success, under present conditions, they seem to be contributing to their downfall, he said.

"Emerging disease"

Though global warming is a factor, this study underestimates the role of infectious disease, commented Michael Mahony, a conservation biologist at the University of Newcastle in New South Wales, Australia. He argues that there hasn't yet been a great enough rise in global temperatures to have such a rapid impact on populations.

The chytrid fungus, which was first identified in Australia, is highly infectious and highly transmissible among frog populations, said Mahony, and he has seen first hand how it can ravage amphibian species. "I have the dubious honour of being the only biologist to describe a new frog species and see it become extinct within two years," he said.

After a million years, amphibians are survivors, added Mahony. "The bottom line is that around 1980, something catastrophic happened which led to the rapid extinction of hundreds of species of frogs in 30 years, and it wasn't global environmental change. It was the release of [the chytrid fungus,] a new emerging disease which had never been seen before."