Rat Poison on Marijuana Farms Poses Lethal Threat to Spotted Owls

owls rat poison

Story at-a-glance -

  • California is the largest marijuana producer in the U.S. and such farms are known for their widespread use of anticoagulant rodenticides to prevent rodent damage to the plants
  • Rodents, unfortunately in this case, are one of owls’ favorite foods, and the birds are now suffering from dining on these poison-laced snacks
  • Seven of 10 spotted owls found dead in the wilderness in northwestern California had been exposed to anticoagulant rodenticides, as had 34 of 84 barred owls
  • The poisons work by inhibiting animals’ ability to recycle vitamin K, which leads to coagulation problems such as internal bleeding

By Dr. Karen Shaw Becker

Northern spotted owls are a threatened species that live in old-growth forests in northern California, the Pacific Northwest and parts of British Columbia, Canada. Their preferred habitat are forests with unique characteristics that typically only appear after 150 to 200 years of growth — things like multi-layered canopies of different tree species, standing and fallen dead trees, and space beneath the canopy where they can fly.1

Loss of this habitat is a primary reason for their decline; in California, northern spotted owls are experiencing up to 3 percent declines in population annually.2 Barred owls, which have moved into the spotted owls’ range, have added another threat to this species. More aggressive and resilient, as well as larger, barred owls have a survival advantage over spotted owls, which have been forced out of their preferred habitat to less productive areas.3

A third threat is also emerging, however, particularly in California, where marijuana farms (both legal and illegal) have increased exponentially since the mid-1990s. California is the largest marijuana producer in the U.S. and such farms are known for their widespread use of anticoagulant rodenticides to prevent rodent damage to the plants. Rodents, unfortunately in this case, are one of owls’ favorite foods, and the birds are now suffering from dining on these poison-laced snacks.

Spotted Owls Exposed to Deadly Rat Poison From Marijuana Farms

From 2009 to 2013, researchers from the University of California Davis in cooperation with the California Academy of Sciences found that 7 of 10 spotted owls found dead in the wilderness had been exposed to anticoagulant rodenticides, as had 34 of 84 barred owls.4 Rodents in the area were not found to contain the poison, but this is likely because any that had been would have died in a matter of days.

The researchers believe that barred owls can serve as “adequate surrogates” for exposure to rat poison in the rarer northern spotted owl. In all, 70 percent of northern spotted owls and 40 percent of barred owls were exposed to one or more anticoagulant rodenticides. The poisons work by inhibiting animals’ ability to recycle vitamin K, which leads to coagulation problems such as internal bleeding. Lead study author Mourad Gabriel, Ph.D., told the Los Angeles Times:5

“We have discovered a new potentially lethal threat to this struggling species that many conservationists have spent decades trying to save from extinction … If no one is investigating the levels at which toxic substances are being placed on private marijuana grows out there, the fragmented forest landscapes created by these sites can serve as source points of exposure for owls and other wildlife.”

Another study by Gabriel and colleagues tested a “freshly dead” northern spotted owl for 12 rodenticides, revealing the presence of brodifacoum rodenticide in its blood and liver.6 While it was unclear what role the poison played in the bird’s death, the chemical has previously been found to be fatal when relatively low levels were detected in the liver. It’s likely that the rat poison is being transferred to northern spotted owls from marijuana farms via the ingestion of rat-poison-contaminated prey.

Other Mammals, Birds Also Being Killed by Rat Poison Near Marijuana Farms

Additional previous research by Gabriel and colleagues found that fishers, a rare forest mammal similar to weasels, were also being exposed to the rat poisons. In a 2012 study, 79 percent of the fishers tested were exposed to an anticoagulant rodenticide. At least four of the fisher deaths were directly attributed to toxicity from the rat poison, including in a lactating female, “document[ing] the first neonatal or milk transfer of an AR [anticoagulant rodenticide] to an altricial fisher kit.”7

Other mammals have also been affected. “Images of dead and dying bobcats, mountain lions, coyotes and owls shared among residents in the vicinity of the Santa Monica Mountains National Recreation Area in 2016 attributed the animals' condition to the ingestion of prey contaminated with rodenticides,” according to the Los Angeles Times.8

Further, when 41 marijuana growing operations (MGOs) were investigated in three California counties, dead wildlife due to anticoagulant rodenticides were detected at nearly 22 percent of them.

Animals affected included bears, foxes, fishers, squirrels, deer and passerine birds. Further, from 62 to 90 percent of carcasses from three owl species in western Canada were also found to contain residues of at least one anticoagulant rodenticide, but only 2 to 12 percent of the deaths were deemed to be due to the rat poison. This could be because researchers estimate it could take up to 15 days for owls to bioaccumulate lethal concentrations of rodenticide in their livers.

Likewise, the researchers wrote, “Although raptors found dead with signs of AR poisoning were not found at MGOs, they prey on rodents affected by AR at MGOs and possibly die elsewhere.”9 Further research is needed to determine the extent and effects of poisoning from anticoagulant rodenticides, but it’s hoped that increased awareness will allow for more forest monitoring and protections to be put in place before vulnerable species like the northern spotted owl are stressed to the point of extinction.10

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