Amphibians around the world are in trouble.

Nearly one-third worldwide are threatened with extinction, according to the Global Amphibian Assessment. And in the face of massive die-offs in the wild, scientists are scrambling to preserve what they can in captivity. A network of zoos and laboratories operating under the moniker "Amphibian Ark" is laboring to save amphibians with the aim of one day reintroducing them to the wild.

Scientists often liken amphibians to the proverbial canary in the coal mine.

Human activity on earth is causing a mass extinction, and amphibians are among the first species to begin disappearing en masse. (The last mass extinction occurred when an asteroid slammed into earth 65 million years ago, ending the dinosaurs' reign.)

That's partially because amphibians, who interface with their surroundings through highly permeable skin, are extremely sensitive to pollutants and to changes in the environment. And so scientists aren't sure which of several possible culprits -- more ultraviolet light from a thinner ozone layer, pesticides, habitat loss, global warming -- is most responsible for the observed amphibian die-offs. Most likely, all of them contribute.

Nonetheless, a prime suspect is a fungus called Batrachochytrium dendrobatidis, or chytrid for short.

Scientists aren't sure where it came from exactly, but they suspect that it was accidentally spread around the world from Southern Africa in the early 20th Century. At that time, African clawed frogs were in demand for pharmaceutical uses. Many were captured in Southern Africa and exported.

Perhaps tellingly, South Africa is one of few regions where amphibians haven't recently declined. That suggests that native species tolerate the fungus with no problem. And that, in turn, indicates that amphibians and chytrid co-evolved there.

Elsewhere, especially in the New World, amphibians populations have crashed, experiencing dramatic and rapid reductions in both numbers and species diversity. (Here's a story on the golden toad disappearing in Costa Rica, and a story on efforts to save the Golden Frog in nearby Panama.

In Central America, chytrid has progressed some 28-100 kilometers per year since first appearing in the late 1990s. Scientists estimate that wherever chytrid appears, 50 percent of amphibian species disappear soon thereafter. On average, 80 percent of amphibian individuals disappear within one year of chytrid's first appearance.

Now, a new study in the journal Ecology Letters tells us what happens in the wake of these amphibian disappearances in Central America. As you might predict, the die-offs cause a massive loss of diversity. But the loss is not evenly spread.

The amphibian communities left in chytrid's wake tend to look the same more than before the fungus' arrival. The frogs that made each community unique have a higher chance of succumbing to chytrid.

The fungus thrives in moist and cool conditions. Amphibians that inhabit that niche tend to disappear more than those inhabiting other niches. Relatively speaking, frogs living on dry land have a better chance of making it through. Chytrid, the authors say, is a great ecosystem homogenizer.

In a press release, Kevin Smith, associate director of the Tyson Research Center at Washington University in St. Louis, and a co-author on the study, calls it "the McDonaldization of the frog communities."

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