Illegal Cannabis Farms Threaten Endangered Wildlife On US West Coast


Ben Taub

Freelance Writer

clockSep 2 2021, 13:02 UTC

The environmental impact of cannabis cultivation is becoming clearer. Image: HubbardSteve/

Clandestine cannabis farms in California and Oregon infringe on the habitats of several threatened and endangered species, potentially exposing vulnerable animals to toxic pesticides, new research has revealed. According to the study authors, harmful rodenticides used by illegal weed growers have the potential to accumulate in local food chains and destabilize fragile ecosystems.

While cannabis has been legalized in both California and Oregon, unlicensed growers continue to produce the plant illegally, often selecting hidden spots in highly forested regions for their cultivation sites. Publishing their work in the journal PLOS ONE, the researchers show how these hidden away sites are affecting local forest predators, such as the endangered Pacific fisher and threatened Humboldt marten and northern spotted owl. 


The team collaborated with law enforcement agencies to obtain data relating to the location of 1,469 illegal weed farms that had been busted between 2007 and 2014. Based on this information, they were able to identify certain landscape characteristics favored by these illegal growers and map the likely distribution of unlicensed grow sites in the forested regions of California and southern Oregon. For example, they found that illegal cannabis growers tended to pick spots with low to mid-elevation and with moderate slopes, as such locations allow for gravity-fed irrigation while also benefiting from a reduced human presence compared to low-elevation sites.

Equipped with this insight, the authors were able to identify a 21,607-square-kilometer (8,343 square miles) region of forest with a high likelihood of illegal cannabis cultivation. To verify this, the team physically surveyed a number of streams within this region that they had identified as prime spots for illegal cannabis operations and discovered 16 previously undetected farms within a relatively small area.

They then superimposed their cannabis farm distribution map over the geographical habitats of the endangered Pacific fisher, the threatened Humboldt marten and the threatened northern spotted owl. Alarmingly, they noticed that “moderate to high-likelihood areas of trespass cultivation overlapped with 40 to 48 percent of modeled habitats of the three sensitive species.”

A Pacific fisher investigates the toxicant storage area of a trespass cannabis grow site on National Forest lands. Image credit: Greta Wengert

This overlap consisted of 37 percent of Pacific fisher denning habitats, 37 percent of marten habitats and 48 percent of spotted owl habitats. Within the southern Sierra Nevada, meanwhile, 100 percent of Pacific fisher female home ranges were found to lie within areas with a high likelihood of illegal cannabis cultivation.


The presence of these covert farms has already had a considerable impact on the health of these species. For instance, the researchers state that between 2007 and 2019, 82 percent of fishers in the Sierra Nevada were found to have pesticides in their tissues. Similarly, anticoagulant rodenticides were detected in the tissues of 70 percent of spotted owls within the study region.

Summing up their findings in a statement, the authors explained that “the problem of trespass cannabis cultivation is more extensive than previously thought”, and warned that “the environmental impacts of this illicit activity adversely affect the recovery of these three sensitive forest predators in ways that we haven’t yet begun to address.”


  • tag
  • conservation,

  • Cannabis