Creative Conservation for Endangered Frogs

Jamie Hinrichs, Pacific Southwest Region

December 5th, 2024

A large group of people pose for picture with white coolers.

Conserving endangered species requires a bit of creativity. And when it comes to the Sierra Nevada yellow-legged frog in California, zoos, coolers and laundry baskets are part of an innovative toolkit.

The backstory to the 95% decline of this frog’s population is driven by multiple antagonists. Long-term drought and rising temperatures have caused pools and streams to dry out before eggs can hatch into tadpoles and tadpoles can morph into adults. Then there is the looming risk of becoming a snack of invasive trout, stocked by humans in these high-elevation habitats starting in the late 1800s.

But the unenticing icing on this danger cake is a fatal fungus. Batrachochytrium dendrobatidis — often shortened to “chytrid” or “Bd” — is a waterborne infectious pathogen that has been spreading globally since the 1970s. The fungus can cause a disease called chytridiomycosis that degrades the porous skin of amphibians, which disrupts fluid transport and air exchange.

A group of people hike into the wilderness lakes with special backpacks — How do you hike frogs into high-elevation lakes? You setup each backpack with about16-24 individually packaged frogs and 1-2 pounds of ice along with field gear. *(USDA Forest Service photo)*

These seem like insurmountable odds for the frogs. But the impossible has inspired innovation for the organizations and agencies that have leapt to their rescue. For a window into this kaleidoscopic conservation, we look to the ongoing efforts between the Plumas National Forest, Plumas Audubon Society, Red River Forests, Sierra Institute for Community and Environment, U.S. Fish and Wildlife Service, California Department of Fish and Wildlife, and San Francisco Zoo & Gardens.

Fostering Frogs

Back in 2013, a year before the listing of the Sierra Nevada yellow-legged frog under the Endangered Species Act, the forest’s wildlife biologist Colin Dillingham was noticing a decline in their numbers.

“I started monitoring more intensely. And after a couple years, I recognized how dire this situation was for the frog,” said Dillingham, who recently retired after 23 years on the Plumas National Forest.

A few more years of surveys revealed more dispiriting news — populations in three streams on the forest were now extinct. Learning of the statewide decline, Dillingham joined meetings with other conservation organizations. Previously, agencies had siloed their surveying. But the listing of the frogs as endangered catalyzed creative collaborations.

“We realized that bringing tadpoles or eggs into a zoo was a viable technique. So, we worked with the San Francisco Zoo to set up a program to rear the frogs.”

Hiker carrying frogs in a special backpack hikes across rugged mountain terrain — Some sites are off the beaten trail. Hiking with heavy packs full of endangered species requires extra care. *(USDA Forest Service photo)*

In 2016, tadpoles were rescued from a creek on the Plumas National Forest that was drying rapidly. This started the forest’s headstart program for the Sierra Nevada yellow-legged frog. San Francisco Zoo — who works with the Oakland Zoo, national forests, and national parks across the Sierra Nevada on these efforts — fosters tadpoles until they metamorphose into frogs. This gives them higher resilience against the threats they face in their natural habitat.

Fighting Fungus

Treatment for the fungal disease is a key component of the headstart program. Most tadpoles come in with some level of infection. Chytrid is widespread in waters throughout their natural habitat.

When they develop into a frog, the infection intensifies. Young frogs then have more skin with keratin — a skin protein that feeds the fungus. But the zoo has a remedy.

“We create an antifungal bath and soak the frogs for 10 minutes a day for a week and a half, during which the medication kills the fungal spores on the skin,” said Dr. Rochelle Stiles, director of field conservation at the San Francisco Zoo.

Frog in a glass jar is ready to release in the wild — Sierra Nevada yellow-legged frog ready for release back to its native habitat on the Plumas National Forest. *(Courtesy photo by Isaac Chellman)*

This process of clearing the frogs of chytrid is also part of an ongoing study on immunity. The zoo inoculates a subset of the frogs with chytrid, then clears them of the disease before infection reaches a concerning stage. Another subset of frogs is left untreated as a control group.

Before either group is reintroduced into their natural habitat, they get their photo taken and a tiny microchip placed under their skin with a unique identification number. When the national forest staff and partners conduct annual surveys, they temporarily capture each frog in a net and scan the frog with a chip reader. They can then tell if the surviving frog was immunized the previous year and record other observations about their health.

“Inoculations typically wear off within the first couple of years. And so the primary focus in those first few years is to see how much of a difference it makes in their survivorship,” said David Hamilton, district wildlife biologist for the Plumas National Forest.

A Ribbiting Return

Since 2017, the team has brought back an average of 84 frogs to the Plumas National Forest. But this year, they beat the average with 300 frogs reintroduced to the forest and split between three different sites.

How do the frogs get to the forests? At the zoo, they are packaged up into individual plastic containers with moist paper towels. The containers then go into coolers with ice packs for a consistent temperature on the car journey back to the forest.

But a car only gets them so far. Since this species lives in high-elevation habitats and wilderness areas not accessible by road, the plastic containers are carefully transferred into backpacks with ice. And human helpers carry them over the final rugged miles.

Some of these frogs are being included in a new conservation technique by being monitored for 10 days in temporary holding pens.

“They’re actually plastic laundry baskets that have a screening. The frogs can't escape and predators can't get in. We had about six per pond with approximately 10 frogs in each. And once a day, or every other day, we fed them crickets,” said Kelly Weintraub, assistant district wildlife biologist with the Plumas National Forest.

Two large white coolers with several small plastic containers, each with a frog inside — The frogs, in individual plastic containers with ventilation holes, are placed in coolers to keep them at a steady temperature. They’re ready for their drive back to the Plumas National Forest from their headstart facility at the San Francisco Zoo & Gardens. *(Courtesy photo by Isaac Chellman)*

Part of this monitoring helps determine whether the frogs get sick from chytrid in a short span of time at individual sites. The specialists swab the frogs’ skin to check for the presence of the fungus, and those samples are sent to a lab. Fortunately, none were showing signs of weakness at time of release this year.

“Another purpose is to get the frogs acclimatized to the site, to see if there's any difference in survival when compared with the frogs that we released immediately,” added Isaac Chellman, High Mountain Lakes environmental scientist with the California Department of Fish and Wildlife. “Even if a small number of these zoo-reared frogs can get recruited into the adult breeding population, it can help make a difference in whether the population persists over time.”

Ultimately, on the Plumas National Forest, two creek reintroduction sites have seen a five-to-tenfold increase in the wild frog populations after eight years of reintroduction efforts. So it seems the Sierra Nevada yellow-legged frog are an exemplary study in the power of partnerships.

“It's such a collaborative environment. We can do a lot for them with all the expertise that already exists and it's rewarding to release hundreds of healthy frogs back into the wild,” said Stiles.

Hamilton added, “With the decline in populations caused by the fish introductions and chytrid and human-caused threats, it's nice to be part of the solution.”