Maybe I'll skip the frog's legs

April 21, 2000

Why are the world's frogs disappearing at an unprecedented rate? Ayala Ochert reports on a global initiative to find the culprit.

All over the world, frogs are disappearing. From the back gardens of England to the tropical rainforests of Costa Rica, the story is the same. And while theories abound, no one really knows the cause.

The problem came to light ten years ago, at the first World Congress of Herpetology, in Canterbury, Kent. As amphibian experts from around the world gathered, it became clear that something was terribly awry.

Andrew Cunningham, of London's Institute of Zoology, was at the meeting. He already knew there was a problem in England because concerned members of the public kept calling him to ask why their ponds were full of dead frogs.

Andrew Blaustein, an ecologist at the University of Oregon, was also at the meeting. "In 1986, I was saying, it's getting harder to find amphibians around here - they used to be really common. Then a colleague in Berkeley said, 'We've got problems in the Sierras too'," Blaustein recalls. "And then, lo and behold, this conference is held and there are 15 papers on amphibian declines all over the world. It was a shock."

A recent global survey of almost 1,000 amphibian studies conducted over the past 50 years has confirmed that a rapid decline in amphibian numbers began in the 1960s, and that trend has continued. Most troubling - and most puzzling - are the dead frogs that have been turning up in isolated, pristine nature reserves.

The Monteverde Cloud Forest Preserve in Costa Rica is one such place. It would be hard to imagine a more untouched habitat, and yet of the 50 amphibian species that once lived there, fewer than 30 remain. One cherished inhabitant of the forest, the golden toad, has not been seen there since 1987 and is now presumed extinct. The same disturbing story is repeated across the Americas and in Australia, where seven species have been lost completely from nature reserves.

One theory mooted by researchers suggested that the amphibians' demise might be an early signal of a global environmental danger. Amphibians, in intimate contact with nature, are especially sensitive indicators of environmental change. They live in water as tadpoles and then on land as adults. Their eggs have no shells, and adults have thin, moist skin, which can absorb toxins from air or water.

One way or another, researchers were determined to get some answers.

Shortly after the Canterbury meeting, a Declining Amphibian Populations Task Force was set up. Headquartered at the Open University in Milton Keynes, it was to bring together the work of more than 3,000 scientists in more than 90 countries. In the past decade, the task force has managed to pinpoint several possible culprits.

Blaustein long ago suspected that the sun's ultraviolet radiation might be involved. He noticed that in Oregon's Cascades mountains something was killing frog and toad eggs and that something seemed to be hurting some species more than others. He showed that species producing plenty of photolyase - an enzyme that protects against ultraviolet radiation - were not declining, only those that produced little of the enzyme were in trouble. That was in the early 1990s. Since then the case against ultraviolet radiation has been mounting. "People have now looked at a couple of dozen species of amphibians, and it is clear that ultraviolet radiation kills the eggs of at least half of those species. It causes sub-lethal effects in many more species - in other words, it doesn't actually kill them, but it does a lot of damage to them," Blaustein says.

But in other areas, the picture has not been so clear. Amphibians that lay their eggs under stones, away from sunlight, also seem to be dropping off, so the focus has turned towards new amphibian diseases. In 1992, in an attempt to get to the bottom of the epidemic in Britain, Cunningham set up the Frog Mortality Project, and asked members of the public to report any dead frogs they found. The response was enormous, and he soon worked out that a new "iridovirus", never before seen in British amphibians, was responsible.

Five years ago, Elizabeth Davidson of Arizona State University was wondering what was killing huge numbers of salamanders in the nearby San Rafael Valley. Like Cunningham, she identified iridoviruses as the culprits. Various related iridoviruses were responsible for a mass of deaths all the way up to Saskatchewan, in Canada.

The viruses do not seem to wipe out populations altogether, but the same cannot be said for another new amphibian disease. The sudden, lethal emergence of a "chytrid" fungus in the rainforests of Australia and South and Central America has caught the attention of scientists because of its ability to wipe out amphibians completely. It seems to be what ultimately led to the demise of the golden toad and many of its neighbours. Although no one is sure how it kills, the chytrid infects the animal's skin, possibly disrupting its ability to absorb water, causing infected frogs to simply dry out.

Scientists agree that no single cause is solely responsible for the global decline in amphibian populations. But many now suspect that some global phenomenon - an increase in ultraviolet radiation, perhaps - may be compromising amphibian immune systems, making them far more susceptible to rare diseases.

"In the early days of Aids what was first noticed was Kaposi's sarcoma, which was extremely rare. Then people started getting unusual protozoan infections and bacterial infections and it became clear that patients were immune-compromised, but no one knew why until the HIV virus was isolated," Davidson says. "If something in amphibians is attacking their immune system, you could get a thing such as the chytrid that might always have been there."


As if pollution, habitat destruction, global warming and ultraviolet radiation were not enough to contend with, amphibians have suddenly been blighted with several new diseases.

In the United Kingdom, the United States and Canada, new viruses have emerged, while in Australia and Central and South America a chytrid fungus has struck with particular force.

The question is, are amphibians suddenly more susceptible to disease, or is something causing new pathogens to appear?

According to James Collins, a population biologist at Arizona State University, "We had hypotheses that went from the possible role of toxins or pathogens - which involved molecular biology - to hypotheses that included global climate change, which is at the other end of the spectrum. It became clear that we needed a vertically integrated research programme, one that could move seamlessly from the molecular level to large global scales."

Collins plans to pursue this approach for a study into the role of pathogens in the global decline of amphibians, funded by the National Science Foundation. Over the next three years, 24 investigators from research institutions across the US, Canada and Australia will attempt to form a truly interdisciplinary team to get to the root of the problem.

Members of the team include pathologists, immunologists, geneticists and ecologists.

According to Collins, it takes a special kind of person to handle this sort of interdisciplinary research.

"You need individuals who are expert in their own field, but with an open mind so that they are able to take advice from someone else. They also have to be willing to give up some autonomy, especially when it comes to publication and seeking funding. They need to be able to skip across the disciplines and pull in the essential pieces," he says.

A tall order, perhaps, but collaboration has apparently been working well so far, with labs sharing expertise.

Collins is confident that by the end of the study, they will have something important to say about how chytrids and viruses are killing the world's amphibians.

But the issue of amphibian declines is likely to remain a complex one, with many intersecting causes.

"I have no illusions about the notion of finding a smoking gun," Collins says.

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