Toads Croak As Climate Warms
Warmer weather over the South Pacific during the past three decades has caused an environmental chain reaction that is killing most of the eggs of a type of toad in Oregon, according to a study.
It is believed to be the first study to directly link global climate change with declining amphibian populations. Other recent studies have linked large-scale climate changes to population fluctuations in American songbirds and European butterflies.
The warmer weather is causing less rain and snow in Oregon's Cascade Range, where western toads are found, resulting in shallower lakes and ponds.
Western toads, like all amphibians, lay their eggs in water. In shallow water, the eggs are exposed to more ultraviolet light, which makes them susceptible to a water mold that kills the embryos by the hundreds of thousands.
The study was published in Thursday's issue of the journal Nature and was led by Joseph Kiesecker, a professor of biology at Penn State University.
"It portends some rather dire things down the road for lots of species," said Roland Knapp, an aquatic ecologist at the University of California at Santa Barbara.
Amphibian populations have been declining worldwide since the 1980s, alarming scientists. Amphibians are considered a barometer of the Earth's health because they are so sensitive to environmental changes.
In the past decade, 20 amphibian species have become extinct, and many more are in severe decline. Scientists point to a variety of factors, including habitat destruction, use of fertilizers and pesticides and increased ultraviolet light from an ozone layer thinned by pollutants.
The study demonstrates the "growing recognition of the connection between climate and epidemics," J. Alan Pounds of the Monteverde Cloud Forest Preserve and Tropical Science Center in Costa Rica said in an accompanying commentary.
"Climate only loads the dice for disease outbreaks," Pounds said. "It does not dictate when and where they will occur, and whether or not they will spread."
Researchers say average sea-surface temperatures in the South Pacific have been elevated since the mid-1970s, causing drier winters in Oregon. Some scientists believe the ocean warming is caused by the greenhouse effect, the buildup of energy-trapping emissions in the atmosphere.
In this study, scientists collected the data from 1990 to 1999 at ponds and lakes in the Cascades, where the western toads lay their eggs in early spring. They typically breed by the edge of the pond, where the ice melts earliest.
The scientists were able to manipulate the depth at which the embryos developed, and thus their exposure to ultraviolet radiation.
About 80 percent of the embryos raised in less than about 8 inches of water developed water mold infections and died. In eggs developing in water deeper than about 22 inches, the death rate was about 12 percent.
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