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Frogs in danger of croaking: Fighting a fungal epidemic

Mountain yellow-legged frogs used to be bountiful, hopping out from under hikers feet anywhere there was water. Now they’re critically endangered by both non-native predators and a deadly fungal disease that's killed over a third of the world’s amphibian species. In the summer of 2014, the Oakland Zoo gathered a bunch of tadpoles from the mountains, and, as those tadpoles grew, zoo technicians inoculated them against the disease. The following June, those same frogs were returned to their native environment, ready to procreate and regenerate the mountain yellow-legged frog species.

This story first aired in June, 2015.

The room is filled with stacked rows of 50 gallon fish tanks, surrounded by gurgling water pumps. But this isn’t a fish store. Conservation biologist Jessie Bushell is in this building way behind the scenes of the San Francisco Zoo,giving me the lowdown on the tadpoles’ menus.

“These guys eat primarily algae,” says Bushell, “so we give them a variety of algae. We give them the gel that we make here--plate that out, and then we also have a different type of algae gel that we plate for their evening meal.”

These tanks are a far cry from these tadpoles' natural environment, but they're thriving and are about to become froglets.

Yeah, that’s the technical term. Froglets.

“We’ve been having metamorph mania in here,” she says, “just two weeks ago we had 55, and now we’re down to about 25 in this tank.”

These tadpoles are turning into Mountain Yellow Legged Frogs (Rana sierrae). It’s a species that’s native only to the High Sierra Nevada. Imagine ice cold snowmelt ponds, and creeks surrounded by lodgepole pines. So what are they doing here?

“Just this summer we collected eggs and brought them in. They hatched, and this is our last remaining group,” says Bushell. She's raising these tadpoles from egg to adult; part of a huge effort to ensure the species’ survival.

What was once one of the most common sounds of spring in the High Sierra -- their weird buzzing frog call -- is now one of the rarest. That’s why for conservation biologists, these tadpoles represent an important investment in the future.

Vance Vredenburg, a researcher from San Francisco State University, spent decades following Mountain Yellow Legged Frog populations, but in 2005 he was greeted with a terrible surprise.

“As I got to the shore of the lake and I looked around I started seeing a few here, a dozen there, a hundred there; by the time I walked the shoreline there were literally thousands of dead frogs floating around in the water," says Vredenburg.


"Upside down, bloated on the shoreline, dried up… it was one of the worst sights I’ve ever seen in my entire life. And unfortunately, it wasn’t the last time that I saw that,” he adds.

The killer turned out to be a fungus. Not the portabella mushroom kind of fungus, but a microscopic disease causing one called Batrachochytrium dendrobatidis. The name is a mouthful, but it’s worth knowing, because it is implicated in a huge global epidemic. You can also call the fungus Chytrid, or Bd. What you need to know is that it causes a massive skin infection that among other things, prevents frogs from breathing through their skin. Most frogs die of cardiac arrest due to the stress of the disease.

Vredenburg says that here in California, and throughout Central America and South America, the fungus has wiped out literally hundreds of species.

“But what’s really interesting is that even in those same habitats where these mass epidemics have occurred, there’s a few species that survive,” he says.

Researchers looked to discover what might be causing this resistance. A lot of us have heard of beneficial bacteria by now. There are actually skin bacteria that protect some of these frogs from infection.

“It turns out that [frogs] have these bacteria that produce antifungal compounds,” says Vredenberg. Further research confirmed that by introducing these bacteria -- a species called J. lividum -- you could prevent the frogs from getting the fungal infection. But while this works in controlled experiments, Bushell says this species isn’t abundant enough in the High Sierras.

“You can’t just dose the frog once with J. lividum and expect that it’s going to make it,” she says, “it has to continuously be exposing itself through the environment. If it’s not in the environment it’s not going to work.” She continues, “so instead what we’re doing is basically inoculating these frogs, like you would get a vaccination shot.”

Here at the zoo the frogs are getting these shots in a program organized by a coalition of researchers, including a combination of US Forest Service, CAL Fish and Wildlife, the Tahoe Basin Land Management Group, and other researchers.

These groups are trying to give the Mountain Yellow Legged Frog a boost, so that they can be released back into the wild and have a better chance against this epidemic.

“We had a really successful year this year, I have over 150 animals that will be released this year and next year,” said Bushell.

This is exciting news for Bushell and all the others involved. But put that in context. That’s still just 150 animals being released, of a species that used to be incredibly abundant.

“You [could] walk along the shores of these lakes, and there are so many frogs in the shorelines that as you’re walking, it sounds like rain hitting the water. And there [could] be schools of tadpoles that [could] be be black in the water,” says Vredenberg.

He says that this just goes to show how important the frogs have been to the ecosystem in the past.

”They’re also really important in their food webs, because, everything that can eat them in the High Sierra Nevada ate them. Everything from birds, black birds and ravens, small mammals, all the way up to bears, would actually eat these frogs, both the tadpoles and the adults,” he says.

But Vredenburg is hopeful for the future. “I really do think they are survivors, and they’ve been able to survive for 5 million years, probably a lot more, (so) they should be able to survive this,” he says.


Fifty years ago, it wouldn’t have even occurred to conservationists to raise these frogs from egg to adult because they were so common.

Maybe in another fifty years, this will be the case again.


Colin was a 2014-2015 Audio Academy intern and is currently training as an as-needed board operator / announcer. He is a producer for Philosophy Talk, #FSFSF, and My Mixtape here at KALW. When not making radio, Colin plays in bands, some of which are louder than others.