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It's no wonder some view our time on Earth as a mass extinction. Biodiversity losses today have reached levels not seen since the end of the Cretaceous period 65 million years ago. Yet amphibians were able to hold on through past extinction spasms, surviving even when 95 percent of other animals died out, and later when the dinosaurs disappeared. If not then, why now?

"Today's amphibians have taken not just a one-two punch, but a one-two-three-four punch. It's death by a thousand cuts," says University of California, Berkeley, biol­ogist David Wake. Habitat destruc­tion, the introduction of exotic species, commercial exploitation, and water pollution are working in concert to decimate the world's amphibians. The role of climate change is still under debate, but in parts of the Andes, scientists have recorded a sharp increase in temperatures over the past 25 years along with unusual bouts of dryness.

But a form of fungal infection, chytridio­mycosis (chytrid for short), often administers the coup de grâce. It did for the mating pair in the Limón stream. Both animals tested positive for chytrid fungus, and the male died soon after the female.

Chytrid was wiping out amphibians in Costa Rica back in the 1980s, although no one knew it at the time. When frogs started dying in big numbers in Australia and Central America in the mid-1990s, scientists discovered the fungus was to blame. It attacks keratin, a key structural protein in an animal's skin and mouthparts, perhaps hampering oxygen exchange and control of water and salts in the body. African clawed frogs, exported widely for pregnancy tests beginning in the 1930s, may have been the initial carriers of the fungus. "It's amazing we haven't seen even more population crashes, the way we shuffle things all over the world, complete with pathogens," notes Ross Alford of Queensland's James Cook University.

Chytrid is now reported on all continents where frogs live—in 43 countries and 36 U.S. states. It survives at elevations from sea level to 20,000 feet and kills animals that are aquatic, land-loving, and those that jump the line. Locally it may be spread by anything from a frog's legs to a bird's feathers to a hiker's muddy boots, and it has afflicted at least 200 species. Gone from the wild are the Costa Rican golden toad, the Panamanian golden frog, the Wyoming toad, and the Australian gastric-brooding frog, to name a few. Some scientists play down the importance of any single factor in overall declines. But in a 2007 paper, Australian researcher Lee Berger and colleagues, who first laid blame on the fungus, put it this way: "The impact of chytridiomycosis on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history."

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