Sometimes important things happen, right under our eyes, without anyone noticing. That thought occurred to David Bradford as he stood looking at a quiet lake high in the Sierra mountains of California in the summer of 1988. Bradford, a biologist, had hiked all day to get to the lake, and when he got there his worst fears were confirmed. The lake was on a list of mountain lakes that Bradford had been visiting that summer in Sequoia-Kings Canyon National Parks while looking for a little frog with yellow legs. The frog’s scientific name was Rana muscosa, and it had lived in the lakes of the parks for as long as anyone had kept records. But this silent summer evening the little frog was gone. The last major census of the frog’s populations within the parks had been taken in the mid-1970s, and R. mucosa had been everywhere, a common inhabitant of the many freshwater ponds and lakes within the parks. Now, for some reason Bradford did not understand, the frogs had disappeared from 98 percent of the ponds that had been their homes.
After Bradford reported this puzzling disappearance to other biologists, an alarming pattern soon became evident. Throughout the world, local population of amphibians (frogs, toads, and salamanders) were becoming extinct. Waves of extinction have swept through high elevation amphibian populations in the Western United States, and have also cut through the frogs and salamanders of Central America and coastal Australia.
Amphibians have been around for 350 million years, since long before the dinosaurs. Their sudden disappearance from so many of their natural homes sounded an alarm among biologists. What are we doing to our world? If amphibians cannot survive the world we are making, can we?
In 1998 the U.S. National Research Council brought scientists together from many disciplines in a serious attempt to address the problem. After two years of intensive investigation, they have begun to sort out the reasons for the global decline in amphibians. Like many important questions, this one does not have a simple answer.
Four factors seem to be contributing in a major way to the worldwide amphibian decline: (1) habitat deterioration and destruction, particularly clear-cutting of forests, which drastically lowers the humidity (water in the air) which amphibians require, (2) the introduction of exotic species that outcompete local amphibian populations, (3) chemical pollutants that are toxic to amphibians, and (4) fatal infections by parasites.
Infection by parasites appears have played a particularly important role in the Western United States and Australia. Speaking at Washington University last week, amphibian ecology expert James Collins of Arizona State University reported one clear instance of infection leading to amphibian decline. When Collins examined populations of salamanders living on the Kaibeb Plateau along the Grand Canyon rim, he found many sick salamanders. Their skin was covered with white pustules, and most infected ones died, their hearts and spleens collapsed. The infectious agent proved to be a virus common in fish called a ranavirus. Virus isolated by Collins from one sick salamander would cause the disease in a healthy salamander, so there was no doubt that ranavirus was the culprit responsible for the salamander decline on the Kaibeb Plateau.
Ranavirus outbreaks eliminate small populations, but in larger ones a few individuals survive infection, sloughing off their pustule-laden skin. These populations slowly recover.
A second kind of infection, very common in Australia but also seen in the United states, is the real species-killer. Populations infected with this microbe, a kind of fungus called a chytrid (pronounced “kit-rid”), do not recover. Usually a harmless soil fungus that decomposes plant material, this particular chytrid (with the Latin name of Batrachochytrium dendrobatidis) is far from harmless to amphibians. It dissolves and absorbs the chitin mouthparts of amphibian larvae, killing them.
This killer chytrid was introduced to Australia near Melbourne in the early 1980s. Now almost all of Australia is affected. How did the disease spread so rapidly? Apparently it travelled by truck. Infected frogs move all across Australia in wooden boxes with bunches of bananas. In one year, 5000 frogs were collected from banana crates in Melbourne market alone.
In other parts of the world, infection does not seem to play as important a role as habitat loss, exotic species, and toxins. This complex pattern of cause and effect only serves to emphasize the take home lesson — worldwide amphibian decline has no one culprit. Instead, all four factors play important roles. It is their total impact that has shifted the worldwide balance towards extinction.
To reverse the trend toward extinction, we must work to lessen the impact of all four factors. It is important that we not get discouraged at the size of the job, however. Any progress we make on any one factor will help shift the balance back toward survival. Extinction is only inevitable if we let it be.
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