From this story on kqed.com:
_Scientists at UC Davis say illegal marijuana farms could be harming a rare California mammal. High doses of rat poison, commonly used by pot farmers, are showing up in Pacific fishers. Fishers are a weasel-like animal found in California's secluded forests. The species is currently being considered for protection under the Endangered Species Act.
According to the story, researchers discovered that 80 percent of the dead fishers discovered in California had been exposed to rat poison, which pot growers dump indiscriminately on the patches to keep away rodents. The fishers had been wearing radio collars so scientists know they hadn't gone into inhabited areas like farms, so illegal pot patches were the only possible means of contact.