Kermit the Frog once opined that “it’s not easy being green.” His fellow frogs would probably agree.
Amphibians are among the most endangered group of vertebrates in the world, their numbers depleted as swamps are drained, forests are logged and voracious predators are introduced. Then, in the late 1900s, came what for some species was the final blow. A fungus, Batrachochytrium dendrobatidis (Bd), began wiping out frog populations around the world.
But now there is a glimmer of hope, in the form of a frog no bigger than a computer mouse. High in California’s Yosemite National Park, scientists have discovered Sierra Nevada yellow-legged frogs that not only survived the plague, but are able, with a little help, to repopulate disease-decimated ponds.
“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,” said Roland Knapp, a biologist at the University of California, Santa Barbara who leads a study of the frogs that has gone on for 17 years.
The Sierra Nevada yellow-legged frog could be a posterfrog for what ails the world’s amphibians. Its habitat has been destroyed by dams, grazing, roads, and logging. Ravenous trout introduced by people to fish-free alpine lakes feast on the frogs. When Bd arrived, it emptied many ponds. Over the last century, its numbers have fallen by more than 90%. In 2014, the army-green creature with dark brown spots was listed as “endangered” under the Endangered Species Act.
Its salvation could hinge on a handful of hardy survivors. Knapp and his fellow scientists have found at least three ponds in Yosemite where yellow-legged frogs withstood Bd infections, possible due to some genetic trait.
The key was to see whether these frogs, and their disease resistance, could spread beyond these last refuges. To see, the researchers have spent years transplanting some of the surviving frogs into a dozen ponds rendered frogless by the fungus, then watching for signs of recovery.
In 9 of those ponds, tadpoles or juvenile frogs appeared in following years, a sign that these new migrants were reproducing. As researchers tracked the fate of the transplants and their offspring, they plugged the results into a computer model designed to forecast how the changes in population sizes would play out in the future. The results showed that at least half of these pioneer frog communities would probably be around in half a century, the scientists reported last month in Nature Communications.
“It’s mind-blowing considering where we were 10 or 15 years ago, when we weren’t sure if we were going to have this frog on the landscape anymore, to see how things are turning around,” said Knapp.
Scientists don’t know exactly what is enabling these frogs to thrive where others failed, but there are clues. While Bd was present in all the ponds, the fungal load in the survivor frogs was relatively low, below the threshold that would trigger the lethal condition known as chytridiomycosis. That low-level remained even when the frogs were transplanted to new ponds. The next generation of frogs had similar or higher survival rates. That, the scientists wrote, suggests a heritable trait that enables the frog’s immune systems to keep the fungus from running rampant.
But that doesn’t mean everything is rosy. The scientists found higher fungus-related deaths among frogs that had just emerged from their tadpole phase, known as metamorphs. Frogs at that stage are known to have less developed immune systems. The toll could slow recovery.
There’s also questions around how well this success will translate to other species. For each of the more than 350 amphibian species known to be affected by Bd, conservationists would need to find a population that is resistant to the fungus. Then there’s the challenge of successfully transplanting frogs to recolonize habitat.
But for the moment, Knapp is celebrating the beginning of the revival of a linchpin of Sierra ecosystems. “You sit on the bank (of a lake) 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.”
Knapp, et. al. “Reintroduction of resistant frogs facilitates landscape-scale recovery in the presence of a lethal fungal disease.” Nature Communications. Nov. 14, 2024.
Photo by Roland Knapp
