Yosemite National Park's Sierra Nevada yellow-legged frogs were so abundant in the mid-1800s that "it would be difficult to avoid stepping on them," according to researcher Roland Knapp of the Sierra Nevada Aquatic Research Laboratory, or SNARL, in Mammoth Lakes. But the introduction of invasive trout, combined with a lethal fungus known as Batrachochytrium dendrobatidis, or Bd, decimated their populations. Now, they're the rarest amphibian in the Sierra Nevada.
But there's hope now for the endangered frogs: Knapp's team not only reintroduced Bd-resistant frogs into their former habitats but also reestablished entire breeding populations, which may save the species from extinction.
Knapp has spent the past 17 years studying two frog species: the Sierra Nevada yellow-legged frog, which lives in water bodies above 6,000 feet or so, and the closely related southern mountain yellow-legged frog, which lives at lower elevations.
"They were clearly the species most likely to be wiped off the face of the earth if we didn't do something quickly," explains Knapp of why he zeroed in on studying the Sierra Nevada yellow-legged frogs, known as Rana sierrae. He estimates that by the mid-1950s, the frogs had been extirpated from about 80% of their local habitats by non-native trout introduced to create fisheries. About two decades later, the frogs took another hit: Bd was detected in the Sierra Nevada.
Knapp explained the seriousness of the threat in no uncertain terms: "To me, it's a transformation as dramatic as the loss of the dinosaurs," he said.
Fortunately for the frogs, Knapp and his team made an unexpected discovery while monitoring the species in Yosemite National Park in the early 2000s. They captured and tested a few frogs infected with Bd that were not only alive but relatively healthy. By analyzing skin swabs, the researchers found that a few frogs developed the ability to suppress infection levels, enabling them to live with the pathogen in their bodies. It's a resistance, not an immunity, but it was a breakthrough for Knapp's team.
SNARL fieldworkers immediately began capturing frogs from the few remaining populations in the national park. They waded into ponds, catching frogs by hand and scooping them into the scientific equivalent of a Tupperware container with air holes. Researchers inserted a small tracking chip into the backs of frogs with Bd resistance and then relocated them to one of 12 lakes elsewhere in the park where their populations had been wiped out. They moved them on foot, stacking the Tupperware containers into a bear bin in a backpack and then surrounding the bin with snow.
For the next decade, researchers returned to those 12 lakes every summer to recapture the tagged frogs. Their efforts paid off: In nine of the 12 lakes, they found both tagged frogs and dozens of healthy untagged frogs. It proved that the Bd-resistant frogs had successfully passed on the gene to their offspring, who had an entirely different genetic makeup.
"In a single generation, you can have evolution," Knapp told me with a clear passion during our conversation. Because Bd has such a high mortality rate -- about 99% in the Sierra yellow-legged frogs, he explained -- it takes just one generation for the Bd-resistant frogs to be the only ones left.
Throughout the study, researchers conducted 24 reintroductions across the 12 lakes, using frogs sourced from three healthy populations. While survival rates within each lake remained relatively consistent, they varied significantly from lake to lake. Knapp explains that this variation was partly due to researchers initially selecting less suitable lakes. However, a major variable beyond their control also influenced the frog's fate: the severity of the winter that followed their relocation. Attempting to account for that is part of the reason the project went on for nearly two decades.
In high-elevation lakes, a long, early winter could mean the frogs are trapped under ice for nine or 10 months. "They're reliant on fat reserves," Knapp explains, so if they don't have time to adjust to a new habitat before winter, they likely won't survive. He estimates the frogs' life spans can be up to 20 years, if environmental conditions permit.
As a scientist, Knapp doesn't discount threats such as the impact of climate change on frog habitats, or the additional challenges they'll face by working in areas with fewer environmental protections compared with Yosemite. Still, he describes the frogs' successes as "amazing to watch unfold," especially as the Bd-resistant frogs were found by accident during the early stages of researching the populations.
He's hopeful the project can serve as a model for saving other at-risk species susceptible to Bd, from the local Yosemite toad to Panamanian golden frogs in Central America. The fungus has led to what he considers the most dramatic loss of amphibian biodiversity in the historical record, he says, and he's already in conversation with the California Department of Fish and Wildlife and other national parks in the state.
While SNARL plans to continue to monitor frogs in the nine successful populations, his goal is to scale up the efforts and reintroduce the frogs to the former habitat, stretching from Sequoia National Forest to just south of Lassen Volcanic National Park.
"But instead of doing one or two introductions per summer," he says, "we need to be doing 20 or 30."