No escape: amphibians in undisturbed habitats are more susceptible to disease. Credit: Kelly Zamudio

Amphibian populations around the world are facing twin threats: habitat loss and the fungal disease chytridiomycosis. Unfortunately, solving one problem may exacerbate the other — it seems pristine habitats hold the greatest risk of the disease, according to research published this week in the Proceedings of the National Academy of Sciences1.

The study found that disturbed habitats, such as deforested lowlands, may provide some shelter from chytridiomycosis — but only for those few species that can tolerate habitat loss.

"This is a double threat; most tropical amphibians are specialists that can't tolerate habitat loss, so it's unlikely that they would find refuge in deforested areas," says lead author Guilherme Becker, a PhD student at Cornell University in Ithaca, New York. "It seems that amphibians can't win for losing."

Analysing the distribution of the pathogen, Batrachochytrium dendrobatidis (Bd), Becker and his supervisor, ecologist Kelly Zamudio, noticed that the disease appeared to be commonest in pristine forests. Data sets from Costa Rica and Australia confirmed that, when the effects of latitude, elevation, climate and species richness are controlled for, places where habitat has been lost show lower disease occurrence.

In a field study in Brazil comparing areas with similar climate, elevation and latitude, the researchers found that habitat loss was the only variable that predicted the spread of the disease.

The findings run counter to evidence that biodiversity loss increases the probability of diseases such as malaria and dengue fever2. But Bd has infected some 350 species of amphibians worldwide — and outbreaks often occur in pristine forests.

Missing the mark?

Becker and Zamudio suggest that the disease might struggle in disturbed habitats because such places contain fewer host species or a less favourable climate.

But others say those are the more important variables. "Everything in the paper is valid and it's an interesting counterintuitive finding — except there are alternative ways to explain these patterns," says Peter Daszak, president of the EcoHealth Alliance, a New York City-based conservation organization.

Vance Vredenburg, an amphibian ecologist at San Francisco State University in California, agrees. "Bd doesn't do well in warm temperatures, but we knew that — and I don't agree that this paper necessarily offers evidence counter to the mounting evidence that biodiversity loss increases pathogen circulation."

Another interpretation of the data, he says, is that habitat at lower elevations is often more trashed — overwhelmingly so in the case of Costa Rica, where the vast majority of conserved habitat is at high elevations. Including Costa Rica tilts the analysis towards finding a large-scale pattern of disease risk in pristine areas, he says.

Alternatively, the pathogen may have already hit modified landscapes, leaving a recovering population of resistant animals. "Pristine areas are the last places pathogens get to because there is less human activity — and when they get hit massive die-offs typically occur," Daszak says.

Becker counters that people automatically think habitat loss is going to increase Bd, which isn't what they found. He says outbreak predictions will be improved by incorporating metrics of disturbed habitats — an important step forward since multiple strains of Bd with varied virulence may cause recurrent problems.

Everyone agrees that heightened risk of disease in natural habitats poses a big problem to conservation efforts. "Simply protecting habitat, while needed, is not the answer to conservation," says Daszak.

Conservation conundrum

Daszak says disease is an under-appreciated threat to conservation. The most basic studies, often done when responding to livestock pathogens — infecting animals, clearing infection and re-infecting to test for resistance — still haven't been done for amphibians, he says. "We're back in the Stone Age when it comes to wildlife disease and conservation."

Daszak says that bold solutions such as breeding disease-free rescue populations in captivity will be needed to maintain amphibian numbers.

Most researchers have given up on halting the spread of Bd. "This is now a Bd world," says Karen Lips, an amphibian ecologist at the University of Maryland in College Park. As such, Lips and Vredenburg are turning their attention to understanding how some amphibians have survived. Vredenburg says any number of evolutionary pathways — from symbiotic bacteria to enhanced frog immune systems — could be responsible.

Vredenburg is increasingly optimistic, despite documenting the frog collapse in the King's Canyon basin of central California. "There used to be 10,000 frogs in the basin; now there are 175 — but those are breeding and not dying in the same way as before," says Vredenburg. "Evolution is happening, just maybe not on the timescale we want."