The Global Collapse of Amphibians

Exploring an amphibian epidemic UC team believes deadly fungus may be killing frogs, toads, salamanders in Sierra

David Perlman, Chronicle Science Editor

Tuesday, August 7, 2007


For decades, the mass disappearance of frogs and other amphibians from the High Sierra has puzzled biologists, fishermen, hikers and even motorists from the city who pause by roadside streams and lakeshores in vain attempts to glimpse whatever's there.

The creatures are vanishing all over the world, too - a major environmental disaster, as a UC science team calls it - and now it appears that sexual reproduction in a single fungus species that produces hardy, long-lived spores may be primarily to blame.

Dead bodies of countless species of frogs, toads and salamanders have been found on every continent except Antarctica (where amphibians don't exist), and the California scientists have zeroed in on a remote group of lakes and streams on the eastern side of the Sierra where they have watched as red-legged and yellow-legged frogs and Yosemite toads became extinct throughout the study area.

Voracious hatchery trout introduced into the mountain lakes have been blamed in the past for eating the tadpoles of the vanishing frogs; so have pollutants like pesticides and toxic dust clouding up over the Sierra from the Central Valley. Now there's evidence that climate change too could be involved, possibly raising temperatures, and solar ultraviolet radiation has also been implicated.

But a new report published today in the Proceedings of the National Academy of Sciences focuses on the population genetics of the widely known frog-killing chytrid fungus with the forbidding name of Batrachochytrium dendrobatidis and the manner in which it reproduces and infects the amphibians.

Tough, long-lasting disease-causing spores created as the fungi reproduce may well be a major cause of the widespread amphibian die-offs, says the report.

The gene study at UC Berkeley was led by Jess Morgan, a postdoctoral researcher who worked in the microbial biology lab of Professor John Taylor before she returned to her native Australia, where she is now at the government's animal research institute at Moorooka near Brisbane.

Among the major Sierra study's other scientists are biologists Vance Vredenburg of UC Berkeley and Roland Knapp of UC Santa Barbara's Sierra Nevada Aquatic Research Laboratory in Mammoth Lakes - both of whom have spent years studying the fish and amphibians in nearly 10,000 of the High Sierra's most remote lakes and streams.

Taylor is a leading expert on the deadly fungi, and he noted that his studies with Morgan are the first to examine just how they reproduce. They may most often clone themselves, Taylor said, in which case there would be no genetic change within the species. But they may also reproduce sexually and produce the tough spores that could live for a decade before exploding to spread the fungal genes throughout the nearby environment.

"We haven't found the spores yet," Taylor said, "but we can't reject the possibility that in fact they do form - but their basic biology is still so poorly known that we can't be sure."

Most fungi, he said, produce spores by the millions and even by the billions, and the frog-killing fungi may well be no exception.

Knapp, in a phone call from his lab at Mammoth Lakes, conceded that he has failed to find any evidence so far of fungus spores in hatchery fish, and he is now seeking the spores in nearby waters. "The story gets more and more complex," he said.

In an e-mail from her lab in Australia, Morgan said: "Our results suggest that the fungus is a recently introduced pest to the Sierra Nevada, but that it is now adapting at a local scale. At some sites the equivalent of fungal sex is evident and sex may result in resistant spores, providing a mechanism for rapid disease spread and fungal persistence. If resistant, long-lived spores exist, then global control of the pathogen may be greatly complicated."

In another message to UC Berkeley she added: "If, in fact, this fungus produced resistant spores, people could be unwittingly transferring this pathogen around the world from dirt on our shoes or car tires, but spores could also hitchhike on the feathers of birds for quick transport across mountain ranges."

Global control of the killer is badly needed. Only a year ago, 50 of the world's leading experts on amphibians, including David and Marvalee Wake of UC Berkeley, warned that nearly a third of the 5,743 known amphibian species around the world are now threatened, with perhaps 122 of them already extinct since 1980.

The scientists blamed the deadly infectious disease caused by the same fungus that Morgan, Taylor and their team are studying, as well as land-use changes in many nations, commercial over-exploitation and global climate change that may encourage the spread of the fungus.

Also last year, another international team of amphibian experts headed by A. Alan Pounds at the Tropical Science Center in Costa Rica reported in the journal Nature that the Central American region was seeing the mass extinction of harlequin frogs and golden toads due to the same chytrid fungus. The fungus, they said, is growing most widely as temperatures rise in the Costa Rican highlands, and they concluded that "large-scale warming is a key factor in the disappearance (of the amphibians)."

"If it's global warming that's involved in the fungus spread," said Taylor, "then, wow!"