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Other reports from the series:
Vanishing Frogs of the Panamanian Rainforests
Frog Deformities May Warn of Human Risk

Frogs and toads have lived on earth for more than 100 million years. They survived whatever extinguished the dinosaurs, yet in our age, they seem to be vanishing. In a three-part series beginning today, Mary Losure examines this scientific mystery. It's a detective story in which the victims are frogs, not people - but people may have a big stake in the mystery's solution.

Amphibians are sensitive indicators of environmental problems. If we can find out what's killing frogs, we may also learn if it will someday harm us.

TO BEGIN WITH, IT'S IMPORTANT TO UNDERSTAND ONE THING - the main reason many frogs are in trouble is not a mystery. Biologists call it "habitat loss." What they mean is that the places where frogs live are being destroyed or altered by people.

Madeline Link: A frog just becomes a splat on the asphalt; it's very hard to count them. Well, there's one that's not too old. That was a fairly large frog that didn't make it.

This road west of Minneapolis cuts right through the Baker Park Reserve. Every spring and fall, thousands of leopard frogs have to cross the heavily travelled two-lane highway during their migration.

Every year, Madeline Link of the Hennepin County park department and her volunteer helpers save many of the frogs from becoming roadkill. But to do it, they have to trap them in buckets, then carry each one across the road. The road is a problem that the frogs' long history on this planet has not prepared them for. Link picks a captured frog out of one of the buckets, and it cowers in her hand, shielding its eyes with its two front feet in a startlingly human gesture.

Link: I've seen them on the road when a truck is coming and they actually squat there and do this. Cover their eyes. And they make themselves big and puff up; that's their response, for example, if a snake were going to approach them, to make themselves look bigger. Obviously, it doesn't work with a truck, but they still go through the motions.

What's happening to the leopard frogs outside of Minneapolis is being repeated all over the country and all over the world. New roads, drained wetlands, logged forests, and other human-caused disruption are the major reasons why not just frogs, but many other species, are dying out at unprecedented rates. By some estimates, one-half of the world's species will be gone within the next 60 years - a rate of extinction unequalled in human history.

That much is well understood. But something is happening to frogs that baffles scientists. Frogs are also disappearing from some of the most beautiful, untouched places in the world; from remote, seemingly pristine wilderness, where their habitat has been protected, not destroyed or cut into pieces.

Field biologist Martha Crump of Northern Arizona University first visited the rainforest preserve above Monteverde, Costa Rica in 1979. There she heard about the golden toad.

Listen to a golden toad call.

The golden toad's voice wasn't much, but its story read like a fairy tale. It lived only in the high mountains, among stunted tropical trees in a place called an elfin cloud forest. The toad appeared for just a week or so every spring in the rainy season, to breed in little puddles among the gnarled tree roots. Then, every year, it vanished back underground until the next spring. Monteverde was the only place in the world the golden toad was known. Crump first saw the animals in 1987, in a far area of the reserve.

Crump: It was early morning, and at one point we rounded a bend, and it was the most incredible sight that I think I've ever seen as a field biologist. There must have been a hundred male golden toads. They were this brilliant golden orange, just sitting there like little statues.

Crump was captivated by the toads. That year she saw more than 1500 of them during their short breeding season, until they all disappeared underground again. Crump decided to come back to Costa Rica the next year to study the toads further, but when she did, the Golden Toads were gone. So she tried again the next spring, in 1989.

Crump: I started off the trip being optimistic, thinking, well, maybe 1988 was just a bad year, maybe the toads for whatever reason just stayed underground and skipped a breeding season.... That certainly happens with amphibians. We again went up and searched all day, every day, day after day, and the toads just didn't come out. We did see one individual that year.

1989 would be the last year Crump - or anyone else - ever saw a golden toad. Biologists and others have searched for it every year since, but the animal is now believed to be extinct.

No one knew why the toads died. Crump worried she had caused the deaths herself, perhaps by bringing in a disease. But later that same year she went to a worldwide meeting of amphibian biologists and there she heard story after story, each oddly similar to hers.

Scientists who had once studied thriving populations in remote mountain sites in California reported being unable to find a single animal. Others told of frogs disappearing from the highlands of Brazil. Biologists from Australia remembered the disappearance a few years earlier of a curious and newly discovered species there.

At first some scientists were skeptical that such anecedotes really represented anything new, or were part of a worldwide pattern. But University of California researcher David Wake says declines documented since that first meeting have eliminated such doubts.

Wake: Around the world there are examples of some really stunning declines and disappearances. These are highly unexpected, highy unanticipated events that are leading not just declines in amphibian populations, but extinctions of species.

Scientists trying to explain these global disappearances have looked for some global cause. They've considered higher-than-normal levels of ultraviolet light seeping through the earth's thinning ozone layer, global warming, and pollution carried by winds and rain. Now various studies have linked declines in specific places to one or more of these causes. But culprits that fit one case don't always work for others. Frog eggs in the mountains of Oregon are being killed by ultraviolet light, but that can't be what happened to the golden toad, since it lived its whole life in deep forest shade.

Wake: I would say that all of these issues still remain in the background, as contributors to the decline, but I think it's also fair to say that we don't think there is a single, unifying cause.

Wake says many scientists are beginning to suspect another, more unsettling explanation: that stress caused by the overall environmental degradation of the natural world is weakening frogs, so that many different causes can pick them off. And that, says Wake, is a message humans should pay attention to.

Wake: We use organisms as surrogates for humans; we don't do experiments on humans. We try to read from other organisms something about what's going on in the world, and interpret it to our own ends. What is happening to amphibians is particularly important because they are vertebrates. We're not talking about bacteria or fruit flies, we're talking about kin.

And while frogs are similar to us in many ways, they are also especially sensitive to environmental pollutants that may someday affect humans.

Wake: Their skins are moist and exposed to the air, as if our lungs were drawn out over the outside of our bodies; you might expect they would feel, they would reflect these stresses earlier. On the other hand, these are very ancient organisms... they've been here for far longer than mammals, and very very much longer than primates or humans, and if they are checking out now, I think it's the responsibility of all of us as biologists or citizens to understand why.

It's difficult to know - much less prove - that the vanishing of frogs is a sign that the web of life that sustains them is slowly unravelling. But we do know it's the same web that sustains us all.

Funding for our series on vanishing frogs was provided by the listeners of Minnesota Public Radio, and by a grant from the Society of Environmental Journalists. Special thanks to researcher Karen Lips and field assistant Liese Greensfelder for their help with the Panama segment. The golden toad call was provided by the Cornell Library of Natural Sounds in Ithaca, New York.