USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
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Effectiveness of reintroductions and probiotic treatment as tools to restore the endangered Sierra Nevada yellow-legged frog (Rana sierrae) to the Lake Tahoe Basin

Principal Investigators:
Roland Knapp, University of California-Santa Barbara
Vance Vredenburg, San Francisco State University

Proposal [pdf]

Final Report [pdf]

Project Summary

A century ago the Sierra Nevada yellow-legged frog (Rana sierrae) was common in lakes, ponds, and streams in the mid and upper elevations of the central and northern Sierra Nevada. Unfortunately, R. sierrae has declined precipitously in recent decades and is now absent from more than 90% of its historical range, including all of the Lake Tahoe Basin. As a consequence, R. sierrae was listed as "endangered" under the U.S. Endangered Species Act in 2014.

The decline of R. sierrae is being driven primarily by the extensive introduction of non-native trout into naturally fishless habitats and the emerging infectious disease, chytridiomycosis. Nearly all lakes and streams at higher elevations in the Sierra Nevada were naturally fishless, but starting in the late 1800s several species of trout were widely introduced into these habitats. Predation by trout on R. sierrae has caused widespread declines and extirpations of R. sierrae populations. Chytridiomycosis is an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis ("Bd"). The extraordinary virulence of Bd has caused the decline or extinction of hundreds of amphibian species around the world during the last several decades and hundreds more are considered at risk as Bd spreads into new areas. R. sierrae is particularly susceptible to Bd, and the spread of this pathogen across California during the past 30 years has caused the loss of hundreds of frog populations from remaining fishless habitats in the Sierra Nevada. Fortunately, some R. sierrae populations have persisted following Bd-caused declines, and recent experiments indicate that frogs in these "persistent" populations are more resistant to Bd than are Bd-naïve frogs.

Although numerous efforts are now underway to recover R. sierrae populations by removing non-native trout, ongoing chytridiomycosis in the remaining frog populations presents a major challenge for recovery efforts. Even in R. sierrae populations that are persisting with this disease, chytridiomycosis often causes recruitment failure that may decrease the chances of successful population reestablishment. This may be particularly the case when frogs from persistent populations are translocated to nearby habitats or are used in captive rearing efforts because of the typically small number of frogs involved in such efforts. Therefore, the success of translocations and reintroductions in reestablishing frog populations may be low unless actions are taken to maximize the number of translocated/reintroduced frogs, and/or reduce the impact of chytridiomycosis. Two possible methods to reduce frog susceptibility to chytridiomyocosis are (1) augmenting the frog skin microbiome of translocated frogs with probiotic bacteria that reduce Bd infection intensities, and (2) exposure of zoo-reared (Bd-naïve) frogs to Bd to prompt an immune response that could serve to protect frogs from future Bd infection.