Biologists find first evidence frogs can recover from deadly fungus

A Sierra Nevada yellow-legged frog submerged in a Yosemite lake.

The Sierra Nevada yellow-legged frog, seen above, is one of many amphibian species that have been decimated by a deadly fungus. Now, for the first time, scientists are seeing evidence that recovery is possible. Photo: Roland Knapp

SF State biologist joins UCSB-led study documenting Yosemite frog population increase

For years, scientists have watched as a deadly fungus has wiped out hundreds of amphibian species worldwide. Now, for the first time, they’ve found evidence that large-scale recovery is possible.

The population of Sierra Nevada yellow-legged frogs in Yosemite National Park has increased seven-fold over the past 20 years, a team of biologists led by the University of California, Santa Barbara’s Roland Knapp report in an article to be published this week in the Proceedings of the National Academy of Sciences. It is the first documented large-scale recovery of amphibians affected by chytridiomycosis, a deadly disease caused by the fungus Batrachochytrium dendrobatidis, also known as Bd.

The finding indicates that, given the availability of high-quality habitat, the animals may be able to adapt to the disease and recover, said Vance Vredenburg, a professor of biology at San Francisco State University and co-author of the study who has monitored the spread of Bd for more than a decade and is the co-founder of the online database AmphibiaWeb.

“This frog used to be one of the most abundant frogs in California, and now it’s the most threatened,” said Vredenburg. “Until this study, there has been mostly bad news. This is the first time we are seeing a turnaround at a scale that really matters.”

Bd dates back at least to the 1880s and has driven more than 200 amphibian species to extinction or near-extinction, making it the most devastating infectious wildlife disease ever recorded. Since 1993, Knapp and United States Geological Survey researchers Gary Fellers and Patrick Kleeman have conducted more than 7,000 population surveys at hundreds of lakes within Yosemite. Based on those frog counts, the team reported that the decline of yellow-legged frogs within Yosemite has reversed and the frog population is now increasing at a rate of roughly 11 percent per year. It’s the first-ever evidence of a recovery across multiple populations spread throughout a large geographical area.

What caused the recovery? The availability of high-quality habitat, and time to adapt, say the paper’s authors. The park’s frogs were probably first exposed to Bd more than four decades ago, and it is believed that a small number of populations survived and have since developed some resistance to the fungus. And it is no surprise that Yosemite’s frogs are the first in the Sierra Nevada to recover, Vredenburg said, since park officials have taken positive steps to ensure that the frogs have the available habitat to survive. Yosemite banned the stocking of introduced trout in 1991, and as a result numerous water bodies reverted to their natural fishless condition. The results of the new study show that many of these lakes were recolonized by frogs during the last two decades, indicating that the change in fish-stocking policy facilitated frog recovery.

The field work was coupled with trials run at SF State in Vredenburg’s lab that showed that frogs from Yosemite were less susceptible to chytridiomycosis than those from populations outside Yosemite that remain naïve to Bd.  

Future research will look at exactly how amphibians are able to adapt to and survive the fungus.

“This recovery by an endangered frog is an important reminder that amphibians are remarkably resilient,” Knapp said. “However, we still have a ways to go before frogs achieve anything close to their former abundance. That will require active fish removal from critical habitats and reintroduction of frogs into portions of Yosemite from which they are still absent.”

“Large-scale recovery of an endangered amphibian despite ongoing exposure to multiple stressors,” by Roland A. Knapp, Gary M. Fellers, Patrick M. Kleeman, David A. W. Miller, Vance T. Vredenburg, Erica Bree Rosenblum and Cheryl Briggs, will be published the week of Oct. 3 in the Proceedings of the National Academy of Sciences.