Illegal marijuana farms in our nation’s forests are not only threatening the safety of humans in these recreational areas, but are also causing ecological damage to the land. And now, there’s proof that the animals that make the forests their homes are also being harmed.

Rat poison used on illegal marijuana grows in remote areas of California is killing fishers, a cat-sized, weasel-like critter, according to a recent study conducted by a team of scientists from the U.S. Forest Service’s Pacific Southwest Research Station, University of California Davis, University of California Berkeley, Integral Ecology Research Center, Wildlife Conservation Society, Hoopa Tribal Forestry, and California Department of Fish and Game.

(For more, see: California’s growing marijuana business impacting agriculture)

Researchers discovered commercial rodenticide in dead fishers in Humboldt County near Redwood National Park, and in the southern Sierra Nevada on the Sierra National Forest. The fishers became ill after eating the rodenticides directly, or by consuming prey that have ingested the poisons.

“The ecological consequences of this are frightening, but not well understood,” says co-author Craig Thompson, a Pacific Southwest Research Station research wildlife ecologist. “These poisons are entering the system not only through prey species, but also through the contamination of the soil and water.”

The researchers deduce that illegal marijuana farms are a likely source, because the fishers in this study were radio-tracked and many were not observed venturing into rural, urban or agricultural areas where rat poison is often found. Illegal marijuana cultivation on public lands is widespread, and some growers apply the poisons to deter a wide range of animals and insects from encroaching on their crops.

“Fishers face plenty of threats to their survival, including predators, disease, and vehicular collisions,” says co-author Kathryn Purcell, a Pacific Southwest Research Station research wildlife biologist. “Adding this new risk factor could mean the difference from a population that is stable or increasing, to a decreasing population. The fact that the problem is so widespread is particularly surprising.”

This new threat could have potential impact on other species already facing declining populations, including wolverines, martens, great gray and spotted owls, and Sierra Nevada red foxes, which may also be exposed to the poison, say the scientists.