Tropical America is one of many regions that have seen significant losses of amphibian species over the past few decades. In 2004, the first global assessment of amphibians revealed that almost a third of the world's known species are threatened with extinction. Early on, experts linked the mysterious disappearance of frog species throughout Central and South America to a fungal disease that was supposedly exacerbated by global warming. Now, according to new discoveries, it looks like climate change isn't to blame after all.

At first, researchers blamed habitat destruction or climate change and changes in ultraviolet radiation for the die offs. Then in 1998, Australian scientists reported finding a strange fungal infection on dead frogs in the rainforest. The fungus was soon identified by Joyce Longcore of the University of Maine and others around the world. Of the 120 amphibian extinctions since 1980, about 90 are attributed at least in part to the fungus.

Researchers first mapped the timing of species disappearances against changes in sea-surface and air temperature over the past few decades, and found that the frogs are disappearing almost exactly in step with climate change. This supposed link to climate change was widely reported at the time. “Disease is the bullet killing frogs, but climate change is pulling the trigger,” said Alan Pounds, an ecologist at the Monteverde Cloud Forest Preserve and Tropical Science Center in Costa Rica, commenting in Nature news online back in 2006. Pounds was expressing the opinion of a number of eco-types by placing the blame squarely on anthropogenic global warming.

The fungus threatens many species in South and Central America.

Confusingly, the fungus is deadlier under cooler conditions. How could rising temperatures be involved? A team looking at data Costa Rica to Peru found that the frogs were doing worst in areas where night-time temperatures were getting warmer, but day-time temperatures were cooler—conditions that would favor the fungus. The most likely connection, said the researchers, was that large-scale warming accelerated the formation of clouds. The clouds, in turn, made local conditions kinder on the fungus, and helped kill the frogs. A bit of a stretch, and it appears an incorrect one. It seems that researchers working at at the Smithsonian discovered the deadly fungus was also afflicting captive frogs—frogs not subjected the supposed stress of global warming in the wild.

Now, according to the October 23, 2009, issue of Science, researchers have gone a long way toward solving the mystery of how the fungal infection has been killing amphibians worldwide. In a paper entitled “Pathogenesis of Chytridiomycosis, a Cause of Catastrophic Amphibian Declines,” Jamie Voyles et al. have identified the mechanism by which the fungus kills its victims. They found that the fungus, Batrachochytrium dendrobatidis, causes such severe electrolyte imbalances that the frogs' hearts stop. Quoting from the abstract:

The pathogen Batrachochytrium dendrobatidis (Bd), which causes the skin disease chytridiomycosis, is one of the few highly virulent fungi in vertebrates and has been implicated in worldwide amphibian declines. However, the mechanism by which Bd causes death has not been determined. We show that Bd infection is associated with pathophysiological changes that lead to mortality in green tree frogs (Litoria caerulea). In diseased individuals, electrolyte transport across the epidermis was inhibited by >50%, plasma sodium and potassium concentrations were respectively reduced by ~20% and ~50%, and asystolic cardiac arrest resulted in death. Because the skin is critical in maintaining amphibian homeostasis, disruption to cutaneous function may be the mechanism by which Bd produces morbidity and mortality across a wide range of phylogenetically distant amphibian taxa.

This insight follows other, potentially promising findings reported in March and late August that certain skin bacteria can protect against fungal infection—there may be hope for the world's frogs after all. “The study clearly demonstrated that violacein concentration on amphibian skin can be manipulated by adding symbiotic bacteria,” says Doug Woodhams, a disease ecologist at the University of Zurich in Switzerland. “By manipulating the microbial community on amphibian skin, conservationists may be able to enhance disease resistance and prevent catastrophic amphibian declines.”

The fungus and one of its victims. Credits: J. Voyles & A Hyatt; (inset) Wikipedia.

But what about the origins of this fungal outbreak, was it spurred on by the dreaded anthropogenic global warming? Voyles et al. cite an earlier paper by Peter Daszak, Andrew A. Cunningham and Alex D. Hyatt, which appeared in the Journal of Biogeography back in February of 2003. “We suggest that, in common with many emerging infectious diseases (EIDs) of humans, domestic animals and other wildlife species, emergence of chytridiomycosis may be driven by anthropogenic introduction (pathogen pollution),” they state.

So yes, humans are at least partly to blame, but it was through people spreading the fungus, not by people causing global warming. Oops! I guess the “blame global warming for everything” crowd jumped the gun on this one, with an assist by a large number of lazy herpetologists who uncritically accepted the proffered false smoking gun. Seems that science can save frogs from the fatal fungus but nothing can save climate science from the folly of global warming.

Be safe, enjoy the interglacial and stay skeptical.