How A Fungus Is Pushing Frogs Extinct

Costa Rican Red-eyed Treefrog (Agalychins callidryas)
Biologists in Australia have solved one of the great mysteries of modern ecology — how a simple fungus is pushing the world’s frog populations closer to extinction. The answer surprised them: It’s as if millions of humans were keeling over with heart attacks from athlete’s foot.
Frogs are an important part of the food chain and are crucial to a healthy ecosystem. “They are the food supply for snakes and birds and everything else that eats those animals,” Voyles says. “So losing them is quite devastating.”

The realization that frogs across the globe were dying came in 1989. It wasn’t until 1998 that the deaths were linked to a fungus named batrachochytrium dendrobatidis. Still, no one knew how this superficial skin infection was killing frogs by the thousands — until now.

The answer lies in the special nature of frogs’ skin, through which they can not only breathe but also absorb water. Researchers at the Amphibian Disease Ecology Group at Cook University in Australia discovered that when frogs contracted the skin disease chytridiomycosis from the fungus, it disrupted their ability to properly regulate electrolytes. Their paper is in today’s edition of the journal Science.

Electrolytes are crucial to the body’s ability to send the electrical signals that tell the heart to pump. When levels fall too low, cardiac arrest can follow. This sometimes happens with human athletes who exercise too long in hot weather and lose too much water and salt in their sweat.

The team took blood samples from sick and dying frogs and realized their electrolyte levels were well below normal. When they gave the frogs an electrolyte solution to drink, some of them recovered enough to try to jump to avoid being captured, says study author Jamie Voyles. In the end, though, all the treated frogs died from the infection.

Though the species of fungus has existed for half a billion years, it appears to have recently evolved into this more deadly form and begun to spread, says Erica Rosenblum, a professor of biology at the University of Idaho in Moscow, Idaho. She studies the genetics of the fungus. Where it first emerged is still unknown.

By some estimates, almost a third of amphibian species are threatened with extinction. A 2007 paper in the Journal of Herpetology estimated that the current extinction rate could be 211 times higher than it has been in the past.

A paper in the Proceedings of the National Academy of Sciences in 2008 found that more than 6,300 families of amphibians are at risk globally and reported the fungus had been implicated in serious declines and extinctions of more than 200 amphibian species, “posing the greatest threat to biodiversity of any known disease.”

While other trends contribute to frogs’ extinction, including habitat loss, new predators and water pollution, “the thing that is really imperiling them is this strange fungal disease,” Rosenblum says. “We know of many hundreds of species that are infected, but my guess is that the actual number is far higher.”

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