Where Have Yellowstone Amphibians Gone?
New study suggests that global warming plays a role in decline of frogs, toads and salamanders
Roger Di Silvestro
“These ponds are changing, the environment is changing, the landscape is drying up, and the amphibians no longer have a place to breed.”
So said Sarah McMenamin, a Stanford University graduate student in biology, last fall during a BBC News interview after she and her colleagues at Stanford published a report on a steep decline in frogs, toads and salamanders in Yellowstone National Park.
Yellowstone is home to four amphibian species—the blotched tiger salamander, the boreal chorus frog, the Columbia spotted frog and the boreal toad—that must lay their eggs in water. They make use of “kettle ponds,” which are glacier-formed depressions that fill in spring with groundwater and snow melt. In 1992 and 1993, Stanford biologist Elizabeth Hadly—who is McMenamin's graduate adviser and coauthor of the new report—with other researchers examined 46 of these ponds in the lower Lamar Valley (a frequent haunt of wolves hunting bison and elk). When McMenamin reexamined the ponds in 2006 to 2008, she found that the number of permanently dry ponds had quadrupled and that even the remaining wet ponds were supporting smaller numbers of amphibians. All of the park’s amphibian species associated with the ponds, except the boreal toad, had declined greatly—although the apparent success of the toad is dubious, as it was already so rare in 1992 that population trends are difficult to determine.
When McMenamin and her colleagues analyzed Yellowstone’s monthly temperature and precipitation data and examined satellite images of the park taken from 1988 to 2008, they found that decreasing rainfall and increasing temperatures during the warmest months of the year have altered the landscape significantly. "There is a pretty substantial signal of climate change in this region," McMenamin told BBC News.
The decrease in Yellowstone lakes, ponds and wetlands has been “astounding," according to John Varley, the park’s former chief scientist. Ponds and even lakes that have been considered permanent since first reported as early as the 1850s have disappeared or become temporary bodies of water.
Global warming may not only dry out amphibian breeding areas but also degrade their land habitats and make them more susceptible to diseases, such as the chytrid fungus that some studies suggest is driving down amphibian populations at sites around the globe.
“Precipitous declines of purportedly unthreatened amphibians in the world’s oldest nature reserve indicate that the ecological effects of global warming are even more profound and are happening more rapidly than previously anticipated," the biologists wrote in their report, which appeared last fall in the Proceedings of the National Academy of Sciences.