Sierra Nevada yellow-legged frogs reintroduced in landmark event

Two weeks ago, 43 endangered Sierra Nevada yellow-legged frogs took to the skies in a helicopter from the Oakland Zoo. A team transported them to their new home in the high country of Sequoia and Kings Canyon National Parks. 

The zoo has now successfully translocated its 1,000th frog to its mountain home. This marks a milestone for the Sierra Nevada yellow-legged frogs and the zoo.

This high-profile release is the latest chapter in a long fight to save the species. 

How yellow-legged frogs disappeared 

The Sierra Nevada yellow-legged frogs (Rana sierrae), also called yellow-legged frogs, used to be common across California’s alpine lakes and streams. Keeping insect populations balanced and feeding predators like birds and snakes. 

Non-native trout introduced in the late 1800s were the first blow to this native species. The trout ate the tadpoles in large numbers.

Having always lived in fishless lakes, the tadpoles were defenseless. Then the frogs faced more threats because of habitat destruction.

The final blow was the devastating chytrid fungus (Batrachochytrium dendrobatidis, or Bd). Succumbing to it, their population crashed by more than 90% in a decade.

Young frogs fresh after their tadpole stage (called metamorphs) are most susceptible to this fungus.

“Yellow-legged frogs once were a keystone species in high-elevation lakes, but chytrid and other factors reduced their populations significantly,” the Oakland Zoo stated in a press release.

Conservationists feared the endangered species would vanish. But they rolled up their sleeves and went to work. Now the tide is turning through years of sheer persistence.

The taming of a lethal fungus

Researchers got together as part of the High Mountain Lakes Project. It is a collaboration between UC Santa Barbara (UCSB), the Oakland Zoo, and others.

Pooling their resources, they studied the chytrid fungus and its effects on frogs. They identified skin peptides and beneficial bacteria that have antifungal properties.

The UCSB researchers developed a trick to improve frog immunity. At the zoo, the frogs are exposed to a controlled dose of the chytrid fungus to train their immune systems.

Then, they undergo treatment with antifungal baths. This immune “training” gives the frogs a better chance of fighting the fungus. It’s the trick that prepares the frogs for the wild.

The zoo staff know each frog; they pit tag, weigh, measure, and swab every frog and keep a record.

First rays of hope in Yosemite

Scientists found that some frog populations were naturally resistant to the deadly fungus. These frogs were all Bd-positive, indicating that they had the fungal infection. But they continued to thrive. This finding was the beginning of a meticulous reintroduction plan. 

Researchers collected adult Sierra Nevada yellow-legged frogs from three large, naturally recovering populations. Then, they relocated the frogs to 12 suitable, empty lake habitats in Yosemite between 2006 to 2020.

Their long-term observations changed the outlook for the survival of these frogs in the wild. Of the 12 reintroduced populations, nine reproduced successfully – a 75 percent success rate.

The transplanted frogs kept low-to-moderate fungal loads even in new habitats. Just low enough to avoid severe fungal disease and death.

This showed that the resistance was built into the frogs’ genetic makeup. Therefore, it would pass on to their offspring.

Additionally, resilience could persist across different locations and did not hinge on environmental characteristics.

Yellow-legged frog survival rates

The full-scale study on the reintroduction of frogs in Yosemite reported that many factors influence frog survival. The study published in Nature Communications, led by Roland D. Knapp, Ph.D., from UCSB, found indicators that improve the survival of the frogs.

Surviving the first year was a powerful predictor of long-term persistence. If 50% of frogs survive their first year, the population often remains stable for decades, according to their model.

Notably, Bd load at the time of release did not predict survival. The frogs survived despite carrying a fungal load because of their inherent resistance. Therefore, choosing a resilient donor population was critical.

Where the scientists release frogs mattered as much as the frogs’ resistance. Sites with overwintering refuges, like submerged boulders, improved survival rates.

These protected the frogs from the cold. Chances of survival also improved if the winter was less severe a year after release. 

These suitable sites are not easy to reach, however. The conservationists hiked to remote areas or used helicopters to keep track of the reintroduced frogs.

Yellow-legged frogs have friends for life

Even after reintroduction, the conservationists continue to be the frogs’ friends. Field teams conduct capture-mark-recapture surveys to track individual frogs during the summer months. They also look for the juveniles and tadpoles to count them.

The pit tags identify the reintroduced frogs, acting like microchips on pets. Researchers capture the adult frogs in a hand-held net. They test the captured frogs for Bd, then measure their size and weight before releasing them. 

The rise in population can be tracked with “new recruits”. They are the progeny of the translocated frogs. Researchers tag them after their first capture.

Keeping track is critical to keep the frogs safe in their new home. If an outbreak occurs, a quick reaction is the only way to prevent the fungus from wiping them out.

This is where the Oakland Zoo’s Rapid Response team comes in. They can mobilize within days to treat the frogs in the wild.

Saving a keystone species

Yellow-legged frogs once anchored Sierra Nevada alpine ecosystems, controlling insects and supporting predators. Their disappearance disrupted those systems. Reintroducing them helps repair the ecosystem.

Challenges still stand in the way. Metamorphs, the young adults, remain highly vulnerable to Bd, slowing the growth of adult populations.

Genetic monitoring is critical to enhance diversity in reintroduced populations. Future efforts may use selective breeding to strengthen resistance even further. For now, the Sequoia and Kings Canyon release offers a clear sign of progress.

Samantha Sammons is the Oakland Zoo’s wildlife recovery program manager. “We need to keep this movement going to make sure that these frogs don’t disappear again,” said Sammons, calling the event “a major milestone.” 

Conservationists are giving the Sierra Nevada yellow-legged frogs a real chance at recovery. If these efforts succeed, the chorus of frogs may once again echo across California’s mountain lakes – a sign of restored ecosystems.

The full study was published in Nature Communications.

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