Owls living in the remote forests of Northern California are being poisoned from an unlikely source at illegal marijuana grow sites, according to findings published in the journal Avian Conservation & Ecology. 

A group led by Mourad W Gabriel, PhD, from University of California Davis, found that owls are being exposed to highly toxic anticoagulant rodenticides (AR), colloquially referred to as rat poison. Among 84 barred owls collected, 40 percent tested positive for one or more type of AR. Among the extremely rare northern spotted owls, the rate was 70 percent.

Based on eight years of ongoing research by Gabriel and his colleagues, the unquestionable source of these deadly, man-made substances in the middle of uninhabited wilderness is trespassing pot farmers. 

“We have discovered a new potentially lethal threat to this struggling species that many conservationists have spent decades trying to save from extinction,” said Dr Gabriel to the LA Times.

Though the marijuana cultivation industry is currently in flux in California as Proposition 64 begins to take effect, regions of both public and private timberlands in Mendocino, Humboldt, Del Norte, and Trinity Counties have been used for clandestine farming for at least 30 years. When altering the landscape to accommodate their crops, the growers create a clearing that is bordered by heavy forest. This type of environment is irresistible to many animals.

To protect their plants from the hungry herbivores they’ve inadvertently attracted, the growers scatter copious amounts of flavored ARs and AR-laced food such as peanut butter. This group of compounds causes animals to bleed to death internally by interfering with the production of clotting factors. Sublethal doses of ARs induce anemia, fatigue, and behavioral changes, and the poison is stored in tissue rather than being excreted.

In 2015, Dr Gabriel’s team published a study linking rodenticides at grow operations in the same region to the deaths of fishers, a small carnivore related to the pine marten. Fishers eat the small rodents targeted by the poison traps and consume the laced food before they too are eaten by a larger predator, effectively spreading toxins up the food chain. 

The findings are especially disheartening because similar to fishers, California populations of the northern spotted owl have declined dramatically since the early 20th century following habitat destruction and fragmentation. Even after several decades of conservation efforts, the species is struggling to repopulate their historical range.

Due to their protected status, the authors only collected spotted owl carcasses that they happened upon while out in the field. The barred owls were collected for use as "surrogates" for spotted owls because they compete against spotted owls for territory and food. So, even though only 10 spotted owls were collected, it is likely they would be infected with ARs at the same rate as barred owls.

Somewhat hearteningly, necropsies of the northern spotted owl carcasses showed that none died directly from AR poisoning, and since tissue samples from barred owls were collected from living specimens, the tissue levels must have been sublethal in them as well. However, the authors speculate that the effects of even low-level poisoning are likely to make recovery of spotted owls even more difficult.

Mourad’s team also have evidence that ARs are leading to the deaths of bobcats, mountain lions, coyotes, and bears, suggesting that the careless actions of illegal growers are impacting not only a few select species, but much of the forest ecosystem.