One of the most puzzling aspects about the impact of the amphibian chytrid fungus (Batrachochytrium dendrobatidis: Bd) on amphibian populations is the diversity of disease outcomes. In some landscapes, the arrival of Bd causes the complete extirpation of all populations, but in others it has little or no negative effect on amphibians. There are lots of reasons for these different disease outcomes, including differences between amphibian species in their susceptibility to Bd and different environmental conditions that change Bd virulence and amphibian susceptibility. The environmental condition that has received the most attention is temperature. Bd grows best at temperatures of 15-25° C and growth is greatly reduced at temperatures above and below this optimum range. Importantly, at temperatures above 30° C Bd is killed within a matter of hours, but no lethal effects are known at the low end of the temperature range (i.e., around 0° C).
This relationship between temperature and Bd growth likely explains several general patterns related to Bd infection intensities on frogs in hot climates (e.g., tropical and subtropical regions). These include lower Bd infection intensities at low versus high elevation, low versus high latitude, and summer versus winter. However, in temperate climates little is known as to whether similar patterns hold. That is, do colder temperatures limit the growth rates of Bd and impacts to amphibians? If so, then we would expect lower Bd infection intensities at higher elevations.
During the last several years, my colleagues and I have tested this idea using a series of studies on the Sierra Nevada yellow-legged frog (Rana sierrae) in Yosemite National Park. These studies included a park-wide (i.e., low to high elevation) survey of Bd infection intensities, detailed measurements of infection intensity over the entire ice-free period (from the cold temperatures immediately after ice-out to warm temperatures of mid-summer to cold temperatures of late-fall), and frog reintroductions in which we moved frogs from a single Bd-positive R. sierrae population to five nearby lakes that spanned a wide elevation range.
The results provided no support for the idea that the coldest habitats might provide frogs with a refuge from Bd. In the park-wide survey, Bd infection intensity was unrelated to elevation. In the seasonal study, despite temperatures that ranged from 4° C to 25° C, Bd infection intensities remained remarkably constant. And the reintroduction study indicated no changes in infection intensities related to elevation. As a consequence, in the paper that was recently published describing these results (see below) we concluded that in the temperate zone even the coldest habitats are unlikely to provide amphibians with a refuge from Bd.
Despite this discouraging finding, we did significantly advance our understanding of Bd-frog dynamics and learned a lot about frog reintroductions as a method of reestablishing R. sierrae populations in Bd-positive landscapes. Although three of the five reintroduced populations quickly declined and never showed evidence of reproduction, the remaining two populations did produce tadpoles in the years following reintroduction and at least one of these populations shows evidence of becoming a self-sustaining population despite ongoing Bd infections. Future reintroductions are planned to allow us to learn more about how best to conduct these reintroductions to maximize the chances of success.
The citation for the paper described above is as follows:
Knapp, R. A., C. J. Briggs, T. C. Smith, and J. R. Maurer. 2011. Nowhere to hide: impact of a temperature-sensitive amphibian pathogen along an elevation gradient in the temperate zone. Ecosphere 2:art93.The paper is available here: http://vesr.ucnrs.org/pages/knapp/publications/publications.html.
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