Singing tree frogs in California are facing a growing health threat: pesticides used on the state’s valley farms. The chemicals are collecting in tissues of tree frogs found in national parks such as Sequoia & Kings Canyon and Yosemite, according to a study published in the journal Environmental Toxicology and Chemistry Friday.

Kelly Smalling, the lead author of the study and a research hydrologist with the U.S. Geological Survey, or USGS, said the chemicals included two fungicides that had not previously been found in wild frogs, LiveScience reported. “Fungicides have been registered for use for many years, but, for some reason, they haven’t really been on anybody’s radar screen until recently,” said Smalling, who is based at the USGS California Water Sciences Center in Sacramento.

The California Central Valley is home to some of the most productive farms in the country. Crops grown in the region include kiwifruit, nuts, tomatoes and, of course, wine grapes. Pesticides are used to protect plants from invasive species of insects that could possibly damage crops, among other reasons. But the chemicals have been shown to travel as far as 100 miles to the east of the Central Valley, as they have been detected in the water, snow, air and amphibians of the Sierra Nevada mountain range.

Adding to the chemical runoff from the farms is similar runoff from illegal marijuana gardens set up on plots of public land. The fertilizers and rat poisons used to maintain these gardens can contaminate wildlife -- like mice -- as other researchers have shown. Those poisoned animals can then cause the deaths of members of rare predator species, such as spotted owls, which feed on the mice.

“The marijuana cultivators make trail systems to go in, and put toxicants at every clearing,” Mourad Gabriel, a University of California at Davis wildlife disease ecologist who studies the effects of rodenticides on rare species, told LiveScience. “A lot of predators will use any type of trail system, so you can imagine the potential risk to multiple different species.”