Earlier this year, 350 endangered mountain yellow-legged frogs hopped into their new home in Bluff Lake in Southern California’s San Bernardino Mountains. It’s the latest step in a long-running battle fought by authorities, including the U.S. Fish and Wildlife Service, the U.S. Geological Survey, the California Department of Fish and Wildlife and conservationists to stave off extinction of this highly threatened species.

The mountain yellow-legged frog (Rana muscosa), despite its name, can display a wide array of color variations, often a mix of brown and yellow, but also gray, red, or greenish-brown. There are two distinct populations of this medium-sized frog: one in Southern California and another up north in the Sierra Nevada.

Biologists say this frog was once so common you couldn’t walk through its range without tripping on one. That’s far from the case today in Southern California. The last comprehensive surveys carried out in 2023 counted fewer than 200 adults in the wild. Though exact numbers are unclear, conservationists say the northern population is much more robust.

The southern reintroductions began in 2010 in the San Jacinto Mountains. This year’s release is part of what Sean Bruce, assistant curator of fishes and invertebrates at Birch Aquarium, describes as a “numbers game” to push back against possible extinction. In 2023, 70 frogs were released into Bluff Lake, marking the first time they had swum in its waters since disappearing in the 1950s.

“There were hundreds of locations historically where these frogs were present, and now we’re down to only a few left in Southern California,” Bruce says. At one point, only 20 individuals remained in the San Bernardino Mountains. “We’re trying to produce as many frogs as we can … the numbers are so low that, you know, even one or two surviving is kind of a win.”

Mountain gnome ‘can’t catch a break’

Mountain yellow-legged frogs are affectionately known as mountain gnomes, due to their ability to survive harsh, cold winters at elevations of up to 3,660 meters (12,000 feet) through hibernation. But that unique ecological survivability has faced test after test as this frog confronts a panoply of threats.

The species’ decline began as early as the 1950s and ’60s. While the exact cause is unknown, a major factor was the introduction of nonnative rainbow trout (Oncorhynchus mykiss) into lakes and rivers, as these fish feast on frogs and tadpoles. Other threats, such as pollution and recreational activities, also played a role, according to experts.

More recently, climate change is compounding these threats with prolonged drought, wildfires and landslides, all combining to degrade habitat, says Jeff Miller, senior conservation advocate at the Center for Biological Diversity.

Then there’s the chytrid fungus, which causes chytridiomycosis, an infectious, deadly skin disease that’s caused by the fungus Batrachochytrium dendrobatidis. It thickens a frog’s skin, affecting oxygen intake and hydration, is often fatal, and has become the scourge of amphibians across the globe, infecting more than 500 species. It has also hit mountain yellow-legged frogs.

“Most recently, I would say chytrid fungus has been one of the big drivers of losses,” says Debra Shier, who is the Brown endowed associate director of recovery ecology at the San Diego Zoo Wildlife Alliance. “Really, over the last couple of decades, we’ve seen a pretty precipitous decline.”

With all these threats, as Miller says, “this species can’t catch a break.”

Reintroducing the species into this maelstrom poses a significant challenge. Conservationists are working to reduce the manageable threats, specifically by removing invasive trout.

“There’s been some pretty successful trout removal in some localized areas, in the San Gabriel Mountains and the San Jacinto Mountains,” Miller says, adding that these have set the scene for the ongoing reintroduction work.

Shier, whose organization has spearheaded these efforts in Southern California, says that without this intensive conservation triage releasing thousands of frogs it’s possible the species would be on the very cusp of extinction.

That’s also the conclusion of the latest IUCN Green List assessment, which Shier co-authored. The Green List, which tracks a species’ road to recovery, noted in its 2024 report that despite conservation efforts, it’s unlikely that Southern California’s mountain yellow-legged frogs will be able to rebound within the next decade.

“I think for most species, we’re really hoping for recovery, right?” Shier says. “But in this case, we’re trying to prevent extinction.”

Conservation triage

In this uphill battle to save the mountain yellow-legged frog, conservationists are testing out and adopting new strategies. One approach that has increased survival rates of reintroduced frogs is a “soft release.”

That involves keeping frogs in acclimation chambers — temporary enclosures built in the area that will become their new home — and feeding them, allowing them to slowly adapt to their new habitat.

But chytrid remains the major challenge as it can easily decimate a frog population. Shier says their work improves short-term survival, “but then chytrid takes them. We really have to somehow improve our outcomes in the face of chytrid,” she says.

This latest batch of released frogs is what Shier describes as a genetic “admixture,” taking frogs from roughly 10 sites, breeding them, and releasing them. San Diego Zoo pairs individuals and also uses assisted reproduction, such as gathering sperm from males in the wild, to produce eggs.

Admixing boosts genetic diversity in wild populations. Increasing traits that could positively impact survival rates, Shier says, may give frogs the ability to adapt to changes in the environment. Then they raise the frogs and release them as tadpoles or as mature frogs between 1 and 3 years old.

Researchers are also trying out a strategy adopted to save the northern population of Rana muscosa and the Sierra Nevada yellow-legged frog (Rana sierrae), a close relative of the mountain yellow-legged frog: translocating and breeding individuals that had recovered from chytrid outbreaks and  developed immunity.

This strategy has proved a success, with the Sierra Nevada frog rebounding. “The trajectory has completely changed from declining rapidly to extinction to now increasing rapidly, hopefully towards some approximation of their historical distribution,” says Roland Knapp, an ecologist at the University of California, Santa Barbara.

So far, there are no mountain yellow-legged frog populations in Southern California that have recovered from chytrid. Shier says one possible solution would be to breed northern frogs that have some chytrid immunity with Southern California frogs. While this might be key to their survival, it’s rife with challenges. Though the two species are genetically very similar, these distinct populations may have specific adaptations that could be lost through interbreeding.

Another strategy is “priming” the released frogs against chytrid. This process exposes captive frogs to chytrid, clears them of it using an antifungal treatment, and then releases them into the wild. It should work somewhat akin to a vaccine, Shier says. “That’s supposed to give them some resistance to chytrid,” she adds.

This work is ongoing, with some “primed” individuals already released. Like other reintroduced frogs, they’ll be monitored to determine how they fare in the wild.

“We’re not going to be removing chytrid from the system,” Shier says. “We have to figure out whether or not we can help the frogs adapt to its presence.”

Meanwhile, the question of how the frogs’ absence affects the wider ecosystem where they once lived is unclear. Since they consume large numbers of insects, such as dragonflies and beetles, and are prey for several native species, their role was likely large, Shier says.

In the coming months and years, more mountain yellow-legged frogs will be raised in captivity and released in Southern California, as researchers continue their studies, hopefully gaining a foothold toward saving this once-abundant but now enigmatic and rarely seen species.

It’s unclear whether these efforts will be enough to save these highly threatened populations.

“But we have to try,” Shier says. “We have to try everything in the conservation toolbox.”

Banner image: Mountain yellow-legged frogs have a wide array of color variations, ranging from yellow and brown to red and gray. Image courtesy of San Diego Zoo Wildlife Alliance.

Sean Mowbray is a freelance writer from Scotland. He often covers lesser-known species that fly under the conservation radar. His reporting also appears in New Scientist, Hakai, Earth Island Journal, Discover magazine, and others.

Citations:

Knapp, R. A., & Matthews, K. R. (2000). Non-native fish introductions and the decline of the mountain yellow-legged frog from within protected areas. Conservation Biology, 14(2), 428-438. doi:10.1046/j.1523-1739.2000.99099.x

Fisher, M. C., & Garner, T. W. (2020). Chytrid fungi and global amphibian declines. Nature Reviews Microbiology, 18(6), 332-343. doi:10.1038/s41579-020-0335-x

Knapp, R. A., Boiano, D. M., & Vredenburg, V. T. (2007). Removal of nonnative fish results in population expansion of a declining amphibian (mountain yellow-legged frog, Rana muscosa). Biological Conservation, 135(1), 11-20. doi:10.1016/j.biocon.2006.09.013

Knapp, R. A., Wilber, M. Q., Joseph, M. B., Smith, T. C., & Grasso, R. L. (2024). Reintroduction of resistant frogs facilitates landscape-scale recovery in the presence of a lethal fungal disease. Nature Communications, 15(1). doi:10.1038/s41467-024-53608-4

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