Amphibian Extinctions: Is Global Warming Off the Hook?

Brian Handwerk
for National Geographic News
December 1, 2008
The world's amphibians are in dire straits—but global warming may not be the problem, a new study suggests.

Previous research has pinned steep declines in amphibian species on rising global temperatures, which are said to be fueling the growth of a deadly fungus.

(Related: "Frog Extinctions Linked to Global Warming" [January 12, 2006].)

Most experts agree that the disease-causing chytrid fungus Batrachochytrium dendrobatidis is taking a terrible toll on frogs and toads. One in three species worldwide is threatened with extinction.

"There seems to be convincing evidence that chytrid fungus is the bullet killing amphibians," said University of South Florida biologist Jason Rohr, lead author of the study, published in a recent issue of the journal Proceedings of the National Academy of Sciences.

"But the evidence that climate change is pulling the trigger is weak at this point."

Beer to Blame?

Rohr and colleagues don't completely discount the role of global warming in amphibian declines.

But they say decades of data show only that some correlation exists between rising air temperatures and Latin American amphibian extinctions—and that data are well short of proving causation.

In fact, the researchers found that, in the Latin American countries they studied, beer and banana production were actually better predictors of amphibian extinctions than tropical air temperature.

While beer and bananas are certainly not to blame, the whimsical comparison makes a point.

"We can't jump to conclusions of causality based on a correlation—especially when we're talking about 60 or 70 species," Rohr said.

"The major message is that we need to be careful."

But J. Alan Pounds, resident scientist at Costa Rica's Monteverde Cloud Forest Preserve and an author of previous research on the topic, maintains climate change is a key factor in the extinction crisis.

"There is a clear link between global warming and amphibian declines," he said, citing a growing body of evidence, including a recent study in Yellowstone National Park.

"The analysis by Rohr et al. is seriously flawed, as we will demonstrate in due course."

Heated Debate

Some scientists say the fungus is an invasive species, entering an ecosystem and wreaking havoc on species with no natural defenses.

Pounds and others believe that rising daytime and nocturnal temperatures are narrowing the gap between daily highs and lows.

Fewer temperature extremes are advantageous for chytrid fungus, which grows and reproduces best at temperatures between 17 and 25 degrees Celsius (63 and 77 degrees Fahrenheit).

The new research suggests neither theory has it exactly right.

For example, Rohr explained that, although extinctions increased significantly in the 1980s, that period did not show smaller swings between average daily high and low temps that would theoretically produce more fungus growth.

In the 1990s, daily temperature swings were smaller, but amphibian extinctions declined during that period, the study concludes.

Rohr and colleagues conclude that an unknown mix of factors is likely endangering amphibians.

Sorting it all out remains a high priority—and for many amphibians, time could be running out.

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