10 years ago, it would have been a rare thing to hear the call of a Sierra Nevada yellow-legged frog in the mountain lakes of Yosemite National Park.
This spotty member of the Ranidae family was once the most common amphibian in the high elevations of the California's Sierra Nevada range, until a deadly fungal disease known as chytridiomycosis (chytrid) arrived, all but wiping it out. The silence that followed took its toll on the wider ecosystem – and conservationists knew they had to act.
Now, after years of dedicated work, the frogs are making a remarkable comeback, breathing new life into Yosemite’s wilderness.
A remarkable recovery
Led by biologist Roland Knapp from the University of California, Santa Barbara (UCSB), a pioneering study has chronicled a 17-year journey – which included 24 reintroductions across 12 lakes in Yosemite – to restore the Sierra Nevada yellow-legged frog (Rana sierrae).
Their findings, published in Nature Communications, provide a rare success story in amphibian conservation.
At the core of this recovery is a remarkable development: the frogs have begun to exhibit resistance to Batrachochytrium dendrobatidis (Bd), a pathogenic chytrid fungi responsible for devastating amphibian populations globally.
“Going back to some of these lakes where frogs are now recovering, and just sitting and watching, reminds you of what has been accomplished,” says Knapp, who is based at UCSB’s Sierra Nevada Aquatic Research Laboratory.
“You sit on the bank, and you have tadpoles all around you in the water and adult frogs sitting next to you on the shore. You have birds flying in and feeding on them, and snakes that are feeding on them. You have a lake that’s alive again.”
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From extirpation to hope
The impact of Bd has been catastrophic. Once common in the Sierra Nevada mountains, the Sierra Nevada yellow-legged frog has vanished from over 90% of its historic range and it is now listed as endangered under the Endangered Species Act. Its plight mirrors that of amphibians worldwide, with more than 40% of species facing extinction.
Knapp’s team achieved what many thought impossible: re-establishing breeding populations by translocating Bd-resistant frogs to habitats where the species had been wiped out. Monitoring the translocated populations over several years, the researchers observed the arrival of new adult frogs, indicating successful recovery. Meanwhile, viability modelling suggested that many reintroduced populations have a strong chance of persisting for at least 50 years.
“It’s mind-blowing considering where we were 10 or 15 years ago,” says Knapp. “We weren’t sure if we were going to have this frog on the landscape anymore. To see how things are turning around is incredible.”
Knapp explains how this incredible comeback required meticulous planning and relentless fieldwork. Identifying reintroduction sites involved balancing factors such as elevation, winter severity and predation risks. Monitoring the frogs, meanwhile, demanded intensive efforts, with researchers trekking into remote areas to track the health and growth of translocated populations.
“These results provide a rare example of how reintroduction of resistant individuals can allow the landscape-scale recovery of disease-impacted species,” says Knapp. “It’s a beacon of hope not just for amphibians, but for many other species threatened by novel pathogens.”
A model for conservation worldwide
The reintroduction efforts didn’t just bring back the frogs; they revitalised entire ecosystems. By observing the successful recruitment of new adult frogs, Knapp and his team confirmed the recovery of natural processes within these habitats. This success story is a testament to the value of conserving protected areas and restoring ecological balance, say the team.
Knapp sees the Yosemite project as more than a local success. “We now have a proven strategy that is working in Sierra Nevada yellow-legged frogs,” he says. “This frog that has been pushed to the verge of extinction by this pathogen is now becoming an example of how we might recover amphibians all around the world.”
For conservationists, the recovery of the mountain yellow-legged frog is a symbol of resilience and hope, and a reminder of what can be achieved when science, determination and collaboration converge. In the shadow of Yosemite’s peaks, the lakeshores are alive again, their amphibian chorus a triumphant anthem of survival.
Find out more about the study: Reintroduction of resistant frogs facilitates landscape-scale recovery in the presence of a lethal fungal disease
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