Sierra Nature Notes, Volume 4, June 2004

New Threat to Mountain Yellow-legged Frogs?
Lara Rachowicz PhD candidate
Department of Integrative Biology, University of California, Berkeley

Author with friend: Yosemite National Park Photo © Lara Rachowicz

Visitors to the high Sierra in the early and mid-1900s were struck by the incredibly high abundances of mountain yellow-legged frogs. Famous biologist Joseph Grinnell remarked that he and his crew could hardly avoid stepping on them as they surveyed the mountain lakes for wildlife. Unfortunately, I have not shared their experience; instead, I have more typically encountered dead frogs, frog skeletons, and frog-less lakes over the past three years while I have been doing research for my Ph.D. I am investigating the most recent threat to the mountain yellow-legged frog (Rana muscosa) – an emerging infectious fungal disease, chytridiomycosis. Why has this frog, who used to be one of the most common vertebrates in the high elevation aquatic ecosystems of the Sierra Nevada and the Transverse Range of southern California, become so rare during the past century?

Dramatic declines, such as those observed in the mountain yellow-legged frog, are not atypical. Amphibian species are declining worldwide, including 32 recent extinctions (AmphibiaWeb 2002). There is seldom an easy explanation for the decline or extinction of a species. Changes to the environment in the form of habitat loss, introduced species, over-exploitation, chemical contaminants, increased UV-B radiation, global climate change, and infectious diseases have been implicated in the declines of amphibians worldwide, and it is most likely that multiple factors are involved and no one factor can explain all declines.

Dead frogs: an increasingly common sight in the Sierra.

In the late 1990s, a new infectious disease appeared in amphibians almost simultaneously on several continents and was implicated in major die-offs in numerous species, including one probable species extinction (Berger et al. 1998, Longcore et al. 1999, Nichols et al. 2001). The pathogenic fungus Batrachochytrium dendrobatidis, often called the amphibian chytrid fungus, has been identified as having severe impacts upon amphibian populations in Australia, North America, Central America, South America, Europe, and Africa. The fungus is found on the skin of frogs and in the mouthparts of tadpoles, in keratinized cells. Tadpoles carry the fungus without any known negative health effects, whereas it can kill adults and massive die-offs have been seen in some species. The only known mode of transmission is an aquatic zoospore; no non-aquatic life stage of the fungus has been found.

A gathering of healthy Mt. Yellow-legged frog tadpoles.

The best-documented cause of the decline of the mountain yellow-legged frog is the introduction of non-native fish (Bradford 1989, Knapp and Matthews 2000, Vredenburg 2004). Introduced fishes are predators of larval mountain yellow-legged frogs and have fragmented frog populations; a fragmented species is often genetically vulnerable and cannot easily repopulate extinct sites. Additionally, some studies indicate that airborne agricultural chemicals from the Central Valley reach high elevation lakes and may be affecting amphibian physiology. These existing threats and recent decline have made this species extremely vulnerable to future threats, especially a catastrophic event such as a virulent infectious disease. In fact, the southern population of the species has declined so steeply that it has been listed as endangered by the U.S. Fish and Wildlife Service.

The mountain yellow-legged frog is a unique species in that it lives at very high elevations (up to 3660 m) and over-winters under frozen lakes for nearly 9 months a year. Because they have such a short growing season each year, they typically spend 2-4 years as tadpoles before metamorphosing into frogs. A “typical” frog only lives in the tadpole stage for a few months. Part of my research has been to identify the presence of the disease by a characteristic loss of black pigmentation seen in the mouthparts of tadpoles, where the fungus grows.

In the Sierra Nevada mountains, my co-researchers and I have been conducting experiments and surveys to understand how to identify the disease, how the fungus is transmitted, and disease prevalence. We are seeking to understand what effects it is having on mountain yellow-legged frogs and why it seems to be impacting some populations more than others. We do not know the extent of this disease in the Sierra, but initial data suggest that it is widespread and causing severe mortality in the mountain yellow-legged frog. Many questions remain, including if this fungus is a species that was always present in the environment, but has recently turned pathogenic, or if it has recently been introduced. Answering these questions is important for determining research and conservation priorities for the remaining populations, which are primarily in Sequoia-Kings Canyon National Parks.

Many prominent scientists believe that amphibians are a very sensitive group of animals and an indication of the overall health of the environment, a “canary in the coalmine.” Scientists have estimated that up to 90% of the mountain yellow-legged frog populations have disappeared in the past few decades. Some days I fear that the odds are great that this disease will finish off the mountain yellow-legged frog; I fear our field team has spent the past few years observing and documenting a species extinction. Other days I am less pessimistic and hope that these frogs who have withstood long, cold Sierran winters and extreme high-altitudes for millions of years surely can survive their current threats. We hope that our research on this disease will help us preserve this unique species and other amphibians suffering from chytridiomycosis.

Close-up of the effects of the chytrid fungus on tadpole mouth parts.

 

Further Reading
Dr. Roland Knapp's Mountain Yellow-Legged Frog site.

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).