Some illegal marijuana growers in northern California are putting wildlife at risk. Growers establish grow sites in remote areas in the coniferous forests of the Coastal, Klamath, and Sierra Nevada Mountain ranges. They clear cut a site and use stream water to irrigate the plants. Rats are attracted to the sites and can damage the valuable plants. To control these rodents, growers use powerful rodenticides, a class of pesticide formulated to kill rats.

Once poisoned, the rats can survive many days during which they can be easily picked off by predators. In the secluded forested valleys where these grow sites are, black bears, mountain lions, bobcats, foxes, fishers, and spotted owls occupy the top rungs of the food web. The concentration of rodenticide in the rat is then often high enough to harm or kill the predator, through indirect exposure. Predators can also be poisoned directly when rodenticide-laced meat is placed around the grow sites.

Researchers are particularly concerned with the effects of these toxicants on fisher, a large member of the weasel family. Fishers are an at-risk species proposed to be listed under the California Endangered Species Act. Several fishers have been found dead or dying due to high blood levels of rodenticides associated with illegal marijuana production. In a study where fisher carcasses were tested, 46 out of 58 (79%) had been exposed to rodenticide.

Sources

Gabriel, M.W. et al. 2012. Anticoagulant Rodenticides on our Public and Community Lands: Spatial Distribution of Exposure and Poisoning of a Rare Forest Carnivore.

PLoS ONE DOI:10.1371/journal.pone.0040163

Thompson, C. et al. 2013. Impacts of Rodenticide and Insecticide Toxicants from Marijuana Cultivation Sites on Fisher Survival Rates in the Sierra National Forest, California. Conservation Letter DOI: 10.1111/conl.12038.