Aquatic ambassadors

Sierra Nevada yellow-legged frogs can teach us about our planet

Hilary Clark

Pacific Southwest Research Station

January 22, 2024

A hand holds a frog with a yellow belly and black spots across its back. — A researcher holds and Sierra Nevada yellow-legged frog (Rana sierrae) found on the Tahoe National Forest, California. (USDA Forest Service photo by Jamie Hinrichs)

“Some think a frog face is one only a mother could love, but I think they’re cute. And in some ways, they’re both resilient and fragile,” said Pacific Southwest Research Station aquatic ecologist Karen Pope.

Sierra Nevada yellow-legged frogs can live up to 20 years and reach 1.5 to 3.75 inches long. Their coloring ranges from a combination of brown and yellow to a mix of gray, red, and greenish-brown, and their name comes from the yellow that tinges on their inner thighs.

A woman in a t-shirt sits and smiles outdoors on a mountainside in front of boulders. — Karen Pope is a Forest Service aquatic ecologist with the Pacific Southwest Research Station. Her research is helping Sierra Nevada yellow-legged frog populations recover. (USDA Forest Service photo)

“Like people who become pale when they’re not feeling well, frogs that are sick have a much more muted yellow than those that are healthy,” Pope explained. “They’re bell weathers for the health of our environment.”

They can teach us about the health of our planet, the interconnectedness of our food web, and how our actions have consequences.

In the past, there were so many frogs pocking the landscape that, in 1915, U.C. Berkeley biologist, Joseph Grinnell, complained he and fellow researchers couldn’t avoid stepping on them. Fast forward to 2023 and researchers, like Pope, have trouble finding Sierra Nevada yellow-legged frogs in their homes—mountain streams and lakes, 4,500 to 12,000 feet high.

Today, Sierra Nevada yellow-legged frogs are a federally endangered species that have suffered a 95% decline in their populations.

Deadly fungus

Three small frogs sit on a large rock in the middle of a rocky mountain stream. — In California, yellow-legged frogs inhabit rocky shaded perennial mountain streams with cool waters originating from springs and snowmelt. (USDA Forest Service photo)

Researchers noticed Sierra Nevada yellow-legged frog populations’ precipitous decline during the 1970s and 1980s. They knew something was very wrong. In the early 2000s, researchers discovered a deadly fungus called Chytrid had infected the mouthparts of tadpoles and the skin of Sierra Nevada yellow-legged frogs. The disease thrives in the cool and wet conditions where Sierra Nevada yellow-legged frogs live.

Frogs have permeable skin, which helps them osmoregulate or balance electrolytes and salt in their cells. The porous nature of their skin makes them susceptible to contaminants and diseases in the air and water. Hindering osmoregulation in frogs, Chytrid ignited a wave of die-offs.

Researchers returned to their study sites to find them littered with frog carcasses.

Invasive fish

A hand grasps a spotted tan fish with a hook in its mouth above a rocky stream. — Non-native trout stocked in high-elevation lakes where they do not naturally live, eat the frogs and tadpoles there, which devastated the Sierra Nevada yellow-legged frog populations. (USDA Forest Service photo)

Non-native trout, stocked in high-elevation lakes, had been preying on frogs since the mid-1800s, which had already battered the Sierra Nevada yellow-legged frog populations. In the late 1990s, researchers discovered that frogs and trout were not in the same lakes. They wondered if there was a connection.

Scientists performed experiments and found removing fish allowed frog populations to recover. Based on these findings, government agencies have worked together to remove non-native trout from some lakes in the Sierras.

“It’s helping some frog populations rebound,” Brown stated.

Small actions can make a big difference in the life of a frog that first appeared on the planet 370 million years ago.

“I encourage people to learn more about these amazing animals and how they can support their conservation.”

Hope for frogs and their ecosystem

Hands carefully scoop up mounds of jelly-like eggs from a puddle. — Biologists from the Eldorado National Forest and California Department of Fish and Wildlife carefully collect Sierra Nevada yellow-legged frog egg masses from a drying stream for relocation. (U.S. Fish and Wildlife Service photo by Ian Vogel)

Rebounding populations are a silver lining for these amphibians, which are a vital link in the food web.

Highly aquatic, they remain near water all year round. During frigid winter weather, frogs lay low, overwintering for up to nine months at the bottom of mountain lakes and streams. When spring enlivens the landscape, they breed in the same lakes they do every year where females lay masses of about 350 eggs each.

“All the eggs mean lots of potential frogs, but sadly most won’t make it through metamorphosis,” Pope said.

Three tadpoles swim in shallow water with a rocky bottom. — Frog tadpoles swim in a stream on the Tahoe National Forest, California. (USDA Forest Service photo by Jamie Hinrichs)

Tadpoles emerge from the eggs in 16-21 days and act like lake sweepers. Using their mouthparts, they scrape algae and other plant matter off muddy stream floors, helping prevent algae blooms. In two to four years, they enter the next and final stage of their life cycle as full-fledged frogs. Playing the role of both prey and predator, they chomp on insects, while also serving as a nutritious meal for other wildlife.

“Frogs provide a hefty packet of protein for birds, raccoons, snakes, coyotes, and even bears. They help fuel the food web,” Pope emphasized.

“It’s an important part of the ecosystem, but aside from that, the frog has a right to be here,” said Pope. “We should do whatever we can to protect it. We owe it that much.”