Yosemite’s endangered frog is making a comeback

(Credit: Roland Knapp/UC Santa Barbara)

After decades of decline, the endangered Sierra Nevada yellow-legged frog (Rana sierrae) is making a dramatic recovery.

The frog’s abundance across Yosemite National Park has increased seven-fold, and at an annual rate of 11 percent, during a 20-year period, according to a new study.

“We now have a parkwide picture of what’s happening in Yosemite, and it shows convincingly that these frog populations are increasing dramatically.”

“We now have a parkwide picture of what’s happening in Yosemite, and it shows convincingly that these frog populations are increasing dramatically,” says Roland Knapp, a biologist based at UC Santa Barbara’s Sierra Nevada Aquatic Research Laboratory in Mammoth Lakes. “These new results show that, given sufficient time and the availability of intact habitat, the frogs can recover despite the human-caused challenges they face.”

Knapp joined a team of researchers who analyzed more than 7,000 frog surveys conducted at hundreds of sites over more than 20 years. The findings appear in the Proceedings of the National Academy of Sciences.

According to Fellers, who has been visiting Yosemite since the 1950s, the sight of yellow-legged frogs across the park were once common at many lakes, “more the norm than the exception.”

Since then, the species has disappeared from more than 93 percent of its historical locations.

Deadly fungus

Gary Fellers of the US Geological Survey and his field crews started surveying for amphibians throughout Yosemite in 1993; Knapp has been working in the region since 2000. Their combined data includes more than 2,000 sites.

“With this unprecedented, robust data set, we could look for patterns in frog population trends, and potential factors that might be influencing frogs in Yosemite,” Fellers says. “Fortunately, and unexpectedly, we found that in spite of a host of potential factors that could be working to depress or eliminate frog populations, the overall pattern has been for a slow, but widespread recovery of Sierra Nevada yellow-legged frogs.”

Chief among the culprits in Rana sierrae’s once-stark decline is deadly fungal disease called chytridiomycosis, which affects amphibians worldwide and has caused at least 200 species of frogs and salamanders to become extinct within the last 30 years.

To understand how frogs could have recovered in Yosemite despite ongoing chytridiomycosis, in addition to the extensive field surveys, the study also included a laboratory experiment that demonstrated that frogs in Yosemite that have been exposed to the disease for decades are less susceptible than are frogs from populations that are naïve to the disease.

“This suggests that frogs have evolved at least partial resistance to the disease,” says Vance Vredenburg of San Francisco State University. “That is a very hopeful finding.”

Some frogs are evolving to survive a killer fungus

Questions remain, however, on the exact mechanism leading to reduced susceptibility.

“We are working to identify those mechanisms,” says Cheryl Briggs of UC Santa Barbara. “That information will be critical to recovery efforts not just for yellow-legged frogs but also for other amphibians worldwide that are endangered by chytridiomycosis.”

Also a big factor in the original decline: fish introduced years ago into Yosemite waters in the name of recreational angling. According to Knapp, the new research provides clear evidence that efforts in Yosemite to reduce impacts caused by nonnative fish, such as the cessation of fish stocking 25 years ago to restore some lakes to their natural fishless condition, are succeeding.

Describing the group’s findings as “profoundly encouraging,” says Patrick Kleeman of the US Geological Survey, “The fact that this is occurring at a landscape scale in the face of multiple stressors lends hope that recovery of the species may indeed be possible.”

Still, he cautions: “Significant population declines are still occurring in other parts of this species’ range, and more work remains to be done to ensure the survival of this emblematic frog of the Sierra Nevada.”

Adds Knapp: “The observed recovery of frogs is particularly important because it is based on all of the yellow-legged frog populations in Yosemite. That provides a really strong foundation for the implementation of effective recovery measures.”

Source: UC Santa Barbara