Many researchers have believed that pristine forests might provide amphibians a refuge against their two main killers: habitat loss and fungal infections.

But a Cornell study shows for the first time that frogs and other amphibians have a higher risk of infection from the fungus Batrachochytrium dendrobatidis (Bd) in pristine forests. Bd is a generalist pathogen that infects as many as 350 amphibian species. The study, appearing online in the Proceedings of the National Academy of Sciences during the week of May 30, suggests that Bd thrives in the optimal conditions found in forests, while it may struggle in hotter microclimates found in human-altered habitats. Furthermore, pristine forests have higher amphibian numbers and species diversity, allowing the fungus more opportunities to spread than in disturbed habitats.

"Amphibian disease risk is paradoxically high in natural forests," said Guilherme Becker, a graduate student in ecology and evolutionary biology and the paper's lead author. "At the same time, habitat loss is contributing to amphibian declines. Most tropical amphibian species are habitat specialists, so it is unlikely they will find refuge in disturbed habitats."

"Tropical amphibians are threatened both by destruction of natural habitats and increased disease in pristine forests," said Kelly Zamudio, professor of ecology and evolutionary biology and a co-author. "We're not saying that altering habitat is a solution to anything, but our study shows that frogs can't get away from the threat of extinction by living in a pristine forest."

Bd spread through the Americas starting in the 1970s, possibly coming from Africa. It is spread aquatically, through streams or ponds, and also through frog to frog transmission. "When it hits a naïve population, it is pretty fatal and destructive," especially for species that breed in streams, said Becker. In such places as Panama and Costa Rica, Bd and habitat loss together have led to a 70 percent decline in amphibian species, with some species going extinct.

In an examination of larger spatial scales across Costa Rica and eastern Australia, the researchers found that areas with higher habitat loss predicted lower risk of disease from Bd in amphibians. Previously, other researchers have reported higher Bd infection rates at higher elevations, but Becker and Zamudio show that deforestation is likely the true driver behind varied Bd infection rates, as more land has been deforested in lowland areas as compared to inaccessible highlands, where forests are left intact and where Bd can thrive, leading to more infections.

To test their findings at smaller spatial scales, Becker also conducted fieldwork in two regions of Brazil's Atlantic Forest. Becker sampled frogs in ponds in forests with different levels of habitat disturbance and used molecular techniques to measure the number of Bd zoospores infecting each frog's skin at different sites. Holding climates, elevation and latitude constant, "it turns out the pattern holds true for the small scale as well," with lower Bd prevalence and infection rates for amphibians in disturbed habitats, Becker said.

The researchers plan to conduct lab experiments to better understand how Bd spreads and how species diversity contributes to Bd epidemiology.

The research was funded by the American Philosophical Society, Sigma Xi, Andrew Mellon Foundation, Cornell's Department of Ecology and Evolutionary Biology, the Mario Einaudi Center for International Studies, CAPES-Fulbright and the National Science Foundation.