Masthead: Kaweah Range

Sierra Nature Notes, Volume 6, January 2006

With Recovery Efforts Underway, Danger still exists for the frogs
by Peter Stekel

Yellow-legged frog dead of Chytrid fungus near Mt. Whitney, Sequoia National Park.

At one time, the mountain yellow-legged frog (Rana muscosa) was one of the most common vertebrates in the high elevation aquatic ecosystems of the Sierra Nevada and Transverse Ranges of southern California. Then came the well-documented decline in the species that occurred with the introduction of non-native fish. The fish, primarily trout, ate frog tadpoles in large numbers. And now, an emerging infectious fungal disease, chytridiomycosis, has created another threat to the frog’s survival.

Chytridiomycosis is caused by the pathogenic fungus, Batrachochytrium dendrobatidis. Found in the skin of frogs and in the mouthparts of tadpoles, the fungus has created severe impacts on amphibian populations around the world. While tadpoles can carry the fungus without any known health consequences, the disease can kill adults.

Vance Vredenburg, a research scientist at the Museum of Vertebrate Zoology at UC Berkeley says there is no known non-aquatic life stage of the Chytrid fungus - it’s only spread from water source to water source. “How it got to the Sierra Nevada is the million dollar question.”

Sierran frogs are severely affected. “There has been something like a 25-30% decline in populations over the last five years due to fungus,” says Vredenburg. It’s ironic that fish removal has allowed mountain yellow-legged frog populations to recover and now Chytrid fungus threatens to knock them down.

Maybe Chytrid fungus has always been present in Sierran waters and recently become pathogenic. If true, researchers hypothesize that sensitivity to airborne industrial and agricultural contaminants has compromised the frog’s immune system and made them susceptible to fungal infections.

Healthy tadpoles, juvenile and adult Mountain Yellow-legged frogs (with radio tracking band)
Photo © Vance Vredenburg

However it got here, Vrendenburg says the mechanism Chytrid fungus uses to kill frogs is unknown. Since it grows in the skin of the frog, one idea is that it causes asphyxiation. Frogs breathe through their skin. If their skin is covered, they suffocate. Another idea is that the fungus produces a toxin that then kills the frog.

The real danger of Chytrid fungus lies in how isolated frog populations have become. “Frog populations are on the edge already, says Vrendenburg. “Genetic variability is low and the species ability to fight this infection is low,” Had this disease hit a hundred years ago, when mountain yellow-legged frog populations were more numerous and contiguous, “Maybe the
weaker individuals would have died, leaving the stronger ones to carry on.” But not anymore. “These small populations are dropping like flies.” At the headwaters of the Kern, in Milestone Basin, researchers found hundreds of dead and dying frogs last summer in what was once a healthy and expanding population.

Vrendenburg looks positively to the future. “We’ve discovered populations that are surviving the Chytrid fungus and have bounced back. They appear to have some kind of resistance to the fungus.” He also stresses the success land managers have experienced in re-establishing frog populations. “This shows the value and importance of the fish removals and that they need to go on - creating more habitat for these frogs to help them fight this disease.”

As if the froggies weren't having enough problems: Chytrid outbreaks in Mountain Yellow-legged frog populations in California as of 2004.
Data MVZ, CAS Specimens C.Davidson
Data: Knapp, Briggs et al., CDFG, USFS, USGS recent surveys
Chytrid Data: Vance Vredenburg

This article first appeared in a different format in Sierra Heritage magazine.

Further Reading

Rana muscosa: Mountain Yellow-legged frog (Amphibia Web) includes photos, sound recording of call, life history and research papers.

The Mountain Yellow-legged Frog: Can They be Saved?
by Vance Vredenburg, Ph.D

A Summer Spent Saving Frogs: Applying Research to the Real World
by Ryan Peek
Biology Department, UC Davis

FOLLOWING THE FROGS The continuing effort to find and save the mountain yellow-legged frogs of the Sierra Nevada
by Casey Ray
Field Biologist, Sierra Nevada Aquatic Research Lab (SNARL)

The AmphibiaWeb Watch List (extinct, missing or endangered amphibians of the world, with map).

The Mountain Yellow-legged Frog is Recovering in Sequoia and Kings Canyon National Parks
by Peter Stekel


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Masthead Photo from:
Kaweahs From Trailcrest, Kings Canyon National Park
© 2009, Howard Weamer