Weather cycles may drive toad decline

For the first time, scientists have linked climate change to a specific mechanism

Too much ultraviolet-B radiation renders western toad eggs (top) susceptible to a funguslike killer (bottom). Kiesecker / Nature

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

colleagues propose an explanation in the April 5 Nature.

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

points out.

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.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.