Persistent Fungus Key to Sierra Nevada Frog Deaths

BERKELEY, California, August 9, 2007 (ENS) A deadly fungus that has wiped out mountain yellow-legged frogs across California's Sierra Nevada can spread by sexual reproduction, complicating efforts to save the frogs from extinction, finds a new genetic analysis from the University of California-Berkeley.

Once the most abundant amphibian in the Sierra Nevada, tens of thousands of mountain yellow-legged frogs have died at hundreds of sites over the past 30 years.

Mountain yellow-legged frog in California's Sierra Nevada. (Photo by Gary Nafis courtesy CaliforniaHerps)
Scientists have attributed these deaths to chytridiomycosis, a quickly spreading disease caused by the waterborne fungus, Batrachochytrium dendrobatidis. But the deaths have also been blamed on the introduction of non-native predatory fish.

Set for publication next week, the study suggests that the fungus played a bigger role than the fish because of its ability to spread over long distances and persist in the environment through sexual reproduction.

"This group of fungi, when it reproduces sexually, can create spores that can last for a decade," said John Taylor, UC Berkeley professor of plant and microbial biology and principal investigator of the study. "That could make this pathogen a harder problem to defeat."

The fungus can be spread by people who transfer the spores around the world in dirt on shoes or car tires, warns the study's lead author Jess Morgan. Spores also can be carried across mountain ranges by birds.

Exactly how this fungus kills the frogs it infects is still unclear, but most scientists believe it disrupts the skin's ability to absorb water.

As the U.S. Fish and Wildlife Service considers listing the mountain yellow-legged frog as endangered, biologists are racing to stop the spreading fungus.

First identified in 1998, the fungus has moved across the Sierra Nevada at about a mile per year.

The UC-Berkeley study could help explain the global spread of this fungus, which has been found in South America, Australia, Europe and Africa even in remote, pristine areas.

Mountain yellow-legged frogs (Photo UC Berkeley)
"Up until now, people thought the movement of this pathogen was mainly via infected frogs, so such measures as restrictions on the pet trade were put in place," said Morgan, a UC-Berkeley researcher during the study and now a government scientist in Queensland, Australia.

Study co-author Roland Knapp, an ecologist at UC Santa Barbara's Sierra Nevada Aquatic Research Laboratory, has tried to reintroduce mountain yellow-legged frogs in remote lakes in Sequoia and Kings Canyon national parks and the John Muir Wilderness where previous frog populations had been wiped out.

Out of 10 reintroduction attempts, seven have failed.

"Within two years, the healthy frogs we introduced would become infected with the fungus and die," said Knapp. "It's a stunning thing to see. One year, there is no obvious evidence of the disease, the next year, we'd come back to see hundreds of dead or dying frogs, and then the following year, they'd all be gone."

"Garter snakes that used to prey on these frogs are now declining," said Knapp. "A high-elevation ecosystem is unraveling."

The study appears in next week's issue of "Proceedings of the National Academy of Sciences."

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