Northern spotted owls and other wildlife are being exposed to high levels of rat poison in northwest California, and illegal marijuana farms are the most likely source point, a new study suggests.
Researchers found that 7 of 10 northern spotted owls collected tested positive for rat poison—as did 40 percent of 84 barred owls.
The study 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.
The findings support previous accounts that rat poison is contaminating the food web in the region, as the primary food source for owls—rodents—is being contaminated.
Driving the issue is the increasing conversion of private timberland into private, illegal, and unpermitted marijuana cultivation sites. These sites often overlap with designated critical habitat for northern spotted owls.
“Spotted owls are inclined to feed along forest edges. Because grow sites break apart these forest landscapes, they are likely source points for exposure,” says lead author Mourad Gabriel, a research faculty member with the Karen C. Drayer Wildlife Health Center within the University of California, Davis School of Veterinary Medicine’s One Health Institute.
Gabriel’s earlier studies were the first to link rat poison and illegal marijuana farms to the deaths of fishers, weasel-like mammals living in remote forests of California and the Pacific Northwest.
Proposition 64, which legalizes recreational marijuana in the state, took effect this month. With its arrival, resource managers expect the number and size of unpermitted, private cultivation sites to grow, which could exacerbate the problem.
The study authors note that an estimated 4,500-15,000 private cultivation sites are in Humboldt County alone, 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 says.
“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,” he explains.
Listen to a northern spotted owl’s whistle:
Anticoagulant rodenticides inhibit the ability of mammals and birds to recycle vitamin K. This creates a series of clotting and coagulation problems, which can lead to uncontrollable internal bleeding.
Barred owls are a physically larger group of owls currently competing for resources and space in critical habitat designated for northern spotted owls. 40 percent, or 34 of 84, of the barred owl tissue samples collected for this study tested positive for anticoagulant rodenticide. The owls are being exposed through the prey they eat.
Listen to a barrel owl’s hoot:
Environmental contamination, when coupled with ongoing competition from barred owls, poses an additional stressor on northern spotted owls, the study says. The fact that barred owls are contaminated as well shows that the species may be used as potential surrogates for detecting these contaminants in northern spotted owls.
“Access to these owl specimens allows us to explore the health of the entire regional forest system,” says Jack Dumbacher, curator of Ornithology and Mammalogy at the California Academy of Sciences. “We’re using our collections to build a concrete scientific case for increased forest monitoring and species protection before it’s too late to intervene.”
Listen to a barrel owl and northern spotted owl interact:
No owls were killed for the study. Northern spotted owls were collected when found dead in the field, while barred owl tissue samples were provided by outside investigators conducting an unrelated barred-owl project.
The researchers report their findings in the journal Avian Conservation and Ecology.
The necropsies for the study were conducted at the California Academy of Sciences and the California Animal Health and Food Safety Laboratory System, which is part of the UC Davis School of Veterinary Medicine.
Coauthors are from the Green Diamond Resource Company, Hoopa Valley Tribe, and Humboldt State University. The United States Fish and Wildlife Service, Arcata, and Yreka California Field Offices funded the work.
Source: UC Davis