California Pot Farmers’ Use of Rat Poison Harms Northern Spotted Owls

State’s legalization of marijuana likely to spur more unpermitted cultivation, further exacerbating the problem

Pot users in California may be rejoicing at finally being able to smoke marijuana for recreation without fear of being arrested. But the state’s new law legalizing weed for fun — which officially took effect January 1 — may be bad news for the already beleaguered northern spotted owl and other wildlife in the state’s northwest regions.

photo of spotted owlPhoto by J.Mark Higley: Hoopa Tribal Forestry / UC Davis A new study found that 70 percent of spotted owl carcasses and 40 percent of barred owl carcasses collected from California’s major pot-growing counties tested positive for rat poison.

The northern spotted owl (Strix occidentalis caurina) — a rather retiring raptor that tends to be very territorial and intolerant of habitat disturbance — is already being exposed to high levels of rat poison from illegal marijuana farms, says a new study led by the University of California, Davis, with the California Academy of Sciences. And experts fear that legalization of recreational marijuana will spur an increase in unpermitted cultivation, further exacerbating the problem.

The study, released today in the journal Avian Conservation and Ecology, showed that seven of the ten northern spotted owl carcasses collected from the state’s major pot-growing counties — Humboldt, Mendocino, and Del Norte — tested positive for rat poison, while 40 percent of 84 barred owls collected also tested positive for the poison. 

The research is the first published account of anticoagulant rodenticide in northern spotted owls, which are listed as a threatened species under federal and state endangered species acts, but it supports previous accounts that rat poison is contaminating the food web in this region.

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“Spotted owls are inclined to feed along forest edges. Because grow sites break apart these forest landscapes, they are likely source points for exposure,” Mourad Gabriel, lead author of the study and a researcher with the UC Davis Karen C. Drayer Wildlife Health Center, said in a statement. The owl and other raptors and carnivores that prey on mice and rats often die from eating animals that have consumed anti-coagulant rat poisons. These rodenticides inhibit the ability of mammals and birds to recycle vitamin K and thus creates a series of clotting and coagulation problems, which can lead to uncontrollable internal bleeding, and ultimately, death.

The study notes that Humboldt County alone has an estimated 4,500 to 15,000 private cultivation sites, yet the county has seen legal permits for only a small fraction of them. That means there are thousands of unpermitted private grow sites with no management oversight.

“When you have thousands of unpermitted grows and only a handful of biologists that regulate that for multiple counties, we’re deeply concerned that there aren’t sufficient conservation protective measures in place,” Gabriel said. “If no one is investigating the level at which private marijuana cultivators are placing chemicals out there, the fragmented forest landscapes created by these sites can serve as source points of exposure for owls and other wildlife.”

The impact on wildlife is one among many environmental damages caused by illegal weed farms on public lands in California. “Tresspass growers,” as they are called, often clear cut forest groves, trash wild areas with irrigation equipment and human waste, and use large amounts of chemical pesticides. Given that they operate in remote areas, these growers usually have to deal with a serious pest problem. Rats and other rodents, it seems, like to eat cannabis plants, especially when they’re young and tender. To protect their plants, growers use vast quantities of rodenticides and wildlife often ends up as collateral damage.

The spotted owl isn’t the only creature impacted. The list of animals and birds harmed by rat poison includes other wild rodents such as opossums, skunks, and raccoons, grey foxes, barn owls, barred owls, Cooper’s hawks, red tail and red shouldered hawks, great horned owls, and the endangered Pacific fisher, fishers, a weasel-like mammal living in remote forests of California and the Pacific Northwest. In fact, Gabriel’s earlier studies in 2012, 2013, and 2015 were the first to link rat poison and illegal marijuana farms to fisher deaths, bringing broad attention to the issue.

“This is what we were afraid was going to happen [when marijuana got legalized],” Lisa Owens Viani, cofounder of Raptors Are the Solution (RATS), said about the study’s findings. “This is unacceptable. The state needs to crackdown on these [farms]. But, of course, there will still be huge mortality of wildlife as long as these rodenticides are in use.”

RATS — an Earth Island project that works to educate the public about the dangers to children, pets, and wildlife from rodenticides — has long been campaigning to get California ban the use of anti-coagulant rodenticides.  “We filed an administrative challenge with the state before the holidays asking why they were continuing to register these products even though the state knows that these poisons are impacting endangered species,” she said. “We are now waiting for them to respond.”

“I hope people will become more concerned about where their pot is coming from and not support any grower that uses poison,” she added.

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