Amphibians are vanishing at an alarming rate, even from areas we think of as pristine and protected. California’s Sierra Nevada is a prime example of this global problem—five out of seven amphibian species there are threatened. Researchers are still trying to pinpoint exactly why ponds that once held mountain yellow-legged frogs or California red-legged frogs are now devoid of amphibians.In a new study, a U.S. Geological Survey group focusing on how pesticides affect amphibians tested common Pacific chorus frogs and their habitats, including Yosemite National Park and Giant Sequoia National Monument, for around 100 agricultural chemicals. Even though researchers have looked at pesticides in Sierra Nevada amphibians for years, the new study’s most commonly detected chemicals, two fungicides and one herbicide, have never been found in amphibians until now.
“As pesticide use changes, our studies have to evolve as well,” says Kelly Smalling, a USGS hydrology and chemistry researcher, and the lead author on the study. As new pesticides are approved, it's difficult to keep pace with where they end up in the environment, so the USGS group tested for a large batch of them in seven remote locations. “That’s how we stumbled across the fungicides.” In 2005, the U.S. Environmental Protection Agency approved the two fungicides found in the new study, pyraclostrobin and tebuconazole, to combat a new soybean rust -- the spores of which may have landed in the U.S. from South America during the 2004 hurricane season.
Pesticides, and diseases like the chytrid fungus, plus habitat loss and climate change, are among the possible reasons amphibians are blinking out in pristine areas. Earlier studies established that pesticides get into Sierra Nevada snow, water and sediments by wafting from the Central Valley, one of the nation’s most intensive agricultural regions. Frogs downwind of the valley are declining more rapidly than coastal or northern frogs.
Researchers also found in previous studies that pesticides commonly applied in the Central Valley, chlorpyrifos, and DDT-like endosulfan (which is being phased out), showed up in declining populations of Sierra Nevada Pacific chorus frogs, and also in imperiled yellow-legged frogs. Smalling's study only looked at Pacific chorus frogs because they are not threatened, and so the population wouldn't be harmed by a few sampling causalities. Yet the work still may point the way to research that could help narrow down what’s harming more rapidly declining species like yellow-legged frogs.
The next step, according to Smalling, is figuring out how the fungicides could affect, or kill, amphibians. That means a lot of difficult laboratory work, partly because every frog species may respond to pesticides differently.
As for how pathogens like the chytrid fungus might be interacting with pesticides to kill frogs, that remains a mystery. “I think it’s quite likely that there is an interaction between pesticides and other stressors,” says Gary Fellers, a wildlife biologist on the study who has worked on amphibian declines since the ‘90s.
Fellers, who recently retired from the USGS, grew up backpacking in Yosemite, where he still does field work. “I know of frog populations that are entirely gone now,” he says. “I’m incredibly anxious to find what’s causing these declines before we lose entire species.”
Sarah Jane Keller is the editorial fellow at High Country News.