May 18, 2010

Captive-bred Frogs Released into the Wild for the First Time

The southern mountain yellow-legged frog (Rana muscosa) inhabits lakes, ponds, and streams in the southern Sierra Nevada and southern California. The southern California Distinct Population Segment (DPS) was listed as "endangered" under the federal Endangered Species Act in 2002. This DPS currently contains fewer than 200 adult frogs and these remaining populations are at extreme risk of extinction due to a myriad of threats.

In response to the perilous status of this southern California DPS, scientists from the U.S. Geological Survey and San Diego Zoo have been working to develop an indoor captive breeding population the could eventually produce sufficient offspring for release into the wild. In an effort to stimulate frog breeding, this spring captive adult frogs were refrigerated for several weeks to simulate winter conditions. Once temperatures were increased frogs quickly began breeding, resulting in the production of numerous egg masses. 

In mid-April several of these egg masses were moved to a creek within the James San Jacinto Mountains Reserve, a reserve managed by the University of California. This is the first time that captive-bred mountain yellow-legged frogs have been released into the wild, and represents an important milestone in the recovery of frogs in this DPS. Time will tell how well the released animals survive. 

In addition to aiding the conservation of mountain yellow-legged frogs in southern California, these efforts have also produced a wealth of information regarding the breeding of these frogs in captivity. Whether we'll ever need these techniques for restoring mountain yellow-legged frogs in the Sierra Nevada remains to be seen but it is nice to have these methods already worked out. I continue to believe that one of the highest conservation priorities for Sierra Nevada frogs is to develop several outdoor captive populations of frogs in artificial ponds at an accessible location. Such populations are much less expensive to maintain than indoor captive populations (frogs don't need to be fed, water doesn't need to be changed, etc.) and would provide lots of offspring for use in research and conservation. All we need is a few ponds. Efforts to identify suitable ponds are ongoing.

Back to The Mountain Yellow-legged Frog Site.

May 10, 2010

Two New Papers Detail Impact of Disease on Sierran Frogs

Two papers by our research group were published today in the journal, Proceedings of the National Academy of Sciences (PNAS). Both papers are open access and can be downloaded from the PNAS web site (Vredenburg et al. paper; Briggs et al. paper). These papers represent more than a decade of research by our group on the role of the amphibian chytrid fungus (Batrachochytrium dendrobatidis - Bd) in driving the decline of mountain yellow-legged frogs in the Sierra Nevada.

The first paper (by Vredenburg, Knapp, Tunstall, and Briggs) describes the frog-Bd dynamics in detail for the first time and provides a critical insight into how Bd outbreaks might be controlled. Bd was absent from the three study basins at the inception of the research but invaded the basins in 2004-2005. Following its initial arrival Bd spread through the basins at 0.6 km per year, eventually infecting all mountain yellow-legged frog populations. 

Within 1-2 years of its first detection in a frog population, the population began to show evidence of severe chytridiomycosis (the disease caused by Bd). Frog die-offs and population crashes occurred when infection intensities (amount of Bd on a frog's skin) reached a critical threshold. The fact that a disease threshold exists is important because it provides a target for intervention strategies, strategies designed to prevent infection intensities from surpassing this threshold. One strategy, clearing frogs of Bd using an anti-fungal drug, was tested in two of the study basins in 2009 and preliminary results are promising. We'll be conducting additional tests in 2010, one designed to investigate whether clearing frogs of Bd early in an epidemic can actually prevent the epidemic from occurring.

Although most frog populations are extirpated following Bd epidemics, a few persist despite ongoing chytridiomycosis. Scientists have suggested that this persistence could be a consequence of selection for reduced susceptibility of frogs or reduced virulence of Bd. In the Briggs et al. paper, we use a mathematical model to demonstrate that neither of these changes are necessary to explain the long-term persistence of infected frog populations in the presence of Bd. Instead, this outcome could be solely the result of density-dependent host-pathogen dynamics. Under this scenario, epidemics are the result of Bd invading naive frog populations existing at their naturally high densities. This allows the production of vast numbers of zoospores (the infective stage of Bd) that overwhelm any frog defenses and causes massive frog die-offs. Most populations are extirpated but frogs in a few populations survive by chance alone. In these populations, zoospore density is kept low by low frog numbers, and frogs are able to tolerate the resulting low Bd infection intensities. As a consequence frog populations persist at low densities with Bd over the long term. 

This model assumes no role for an adaptive immune response by frogs against Bd. Although this is consistent with our current knowledge of the frog-Bd interaction, a series of experiments we are currently conducting will provide clearer insights into the existence of any such response. 

For the first time in several years I feel like we are making substantial progress toward understanding and potentially controlling chytridiomycosis. Let's hope our advances haven't come too late.

Back to The Mountain Yellow-legged Frog Site.