Endangered Yosemite frogs making comeback, scientists say
By David Perlman
Updated 4:16 pm PDT, Monday, October 3, 2016
Yellow-legged frogs are rebounding in High Sierra lakes after suffering losses to nonnative trout, pesticides and a fungus.
Photo: Brian Melley, Associated Press
Yosemite’s yellow-legged frogs, in trouble for more than a century and thought to be doomed to extinction from a worldwide epidemic of fungus disease, are on their way to recovery, biologists report.
Surveys of hundreds of High Sierra lakes have found that frog populations have increased sevenfold over the past 20 years, and are continuing to rise by more than 10 percent a year, the biologists say.
“They’re recovering from what’s been one catastrophe after another,” said Vance T. Vredenburg, a San Francisco State University biologist who has followed the fate of the amphibians during his 15-year career and led the laboratory work reported in the study.
The scientists are puzzled by the recovery, but suspect the yellow-legged frogs may no longer be as susceptible as they have been to their most recent nemesis: the worldwide epidemic of chytrid fungal disease that has hit frogs and other amphibians.
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The Yosemite frogs were listed as an endangered species by the U.S. Fish and Wildlife Service in 2013, but life hasn’t been easy for the species since the 19th century.
Trouble began for the Rana sierrae in the 1880s when fishermen’s organizations began stocking many High Sierra lakes with nonnative trout. California’s Department of Fish and Game continued the stocking effort to encourage recreational fishing until reports showed that the trout were killing off the frogs as well as other native lake creatures.
The state stopped stocking the Yosemite lakes in 1991.
Then came reports that various pesticides from Central Valley farms, drifting wind-blown over the Sierra, were threatening the yellow-legged frogs and several wildlife species of other animals. Although studies showed the chemicals were lodging in the bodies of the Sierra frogs, their effects on their populations have not been proven.
And then came the fungus, which has been ravaging frog populations worldwide.
The fungus began infecting the Yosemite frogs in 1960 and quickly caused “precipitous decreases” in their numbers in the park, said Roland A. Knapp, a biologist at the University of California’s Sierra Nevada Aquatic Research Laboratory in Mammoth Lakes.
The epidemic raged over the years, and by 2005, one survey by Vredenberg and his San Francisco State team found that of 146 High Sierra sites where the frogs had been abundant 10 years earlier, only 11 sites now contained living frogs.
In the latest study, Knapp and his colleagues reported on more than 7,000 surveys of frog populations in more than 2,000 lakes, ponds, streams and shorelines throughout Yosemite.
“They’re recovering at an unexpectedly rapid rate,” Knapp said of the frogs. But the precise reasons are still unclear.
At San Francisco State, Vredenburg is leading a laboratory study of the chytrid fungus in fresh detail. It may be that some of the Yosemite frogs have developed an immunity to the fungus or that the poisonous quality of the fungus has moderated, he suggested in an interview. It may be that microscopic organisms — known as the microbiota — on the skins of some frogs are protecting them against the disease, he said.
“And maybe the Yosemite infection alone has become less deadly,” he said.
The chytrid infection has been killing amphibian species, including salamanders and frogs of every species, all over the world, but there is also new evidence of a recovery in Australia, where the epidemic is also particularly virulent.
In a report in the international journal Plos One, Australian biologist David A. Newell of Southern Cross University in New South Wales has reported that a hard-hit species of Australian barred frog appears to be recovering from the same chytrid infection.
Newell and his colleagues have surveyed two Australian streams over seven years and showed that the number of frogs in those streams increased three- to 10-fold, and all of them have survived to old age, even though the infection is continuing.
“I suspect immunity,” Newell said in an email.
The report on “large-scale recovery” of the endangered yellow-legged frogs was published this week in the Proceedings of the National Academy of Sciences.