For the first time, scientists have linked climate change to a specific mechanism
of doom for one of North America’s amphibians.
The plump, lumpy western toad, Bufo boreas, has been declining even in relatively
pristine havens like the heights of the Cascade Mountains in Oregon, laments
Joseph M. Kiesecker of Pennsylvania State University in State College. He and his
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colleagues propose an explanation in the April 5 Nature.
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This toad tale starts with the weather during the 1990s. The researchers found
that summer weather foretold how much rain and snow would pelt the Cascades the
following winter. It also predicted how well the toad’s egg hatching would go. The
researchers found at a least 75 percent of eggs hatched if they were in pools of
water deeper than 45 centimeters. That rate was less than 50 percent in water 10
cm deep or shallower.
The drop probably came from shallow water’s inability to shield eggs from
ultraviolet-B (UV-B) radiation, suggest Kiesecker and his colleagues. Perhaps as a
result, a funguslike pathogen, Saprolegnia ferax, was able to rip through and kill
many clutches of exposed eggs. When researchers added UV-B filters, however, eggs
in shallow water stayed healthier.
Another chronicler of amphibian declines, J. Alan Pounds of the Monteverde Cloud
Forest Preserve in Costa Rica, points out that populations elsewhere have shown
“alarming shifts” during changes in climate. During the 1990s, populations of
amphibians in Central America plummeted during droughts and haven’t recovered, he
Pounds says there’s more to this sad story. He cautions that “climate only loads
the dice for disease outbreaks; it does not dictate when and where they will occur
and whether or not they will spread.”