Wild Weed

Why illegal cannabis farms are a lethal threat to wildlife

Scientists reveal the hidden environmental repercussions of illegal cannabis farms.

Justin Sullivan/Getty Images

Greta Wengert, executive director of the Integral Ecology Research Center, found herself surprised by two specific findings linked to her team’s examination of illegal cannabis farms in California and Oregon.

  • The significant overlap between illegal cannabis grow sites and animal habitat range
  • How many unknown grow sites the scientists were able to detect

“I thought we might find one, two, maybe a handful,” Wengert tells Inverse. “The number that we did find in relation to the small amount of stream miles we covered during ground-truthing was alarming.”

This discovery holds true despite a seemingly confounding fact: Cannabis in Oregon and California is legal for medical and recreational use.

“A huge majority of sites in California are still illegal and non-permitted,” Wengert explains. “All trespass grows on public lands and large tracts of privately owned ranches and timberlands will always be illegal, and deleterious to our environment, and there is no immediate sign of slowing down.”

Wenger is a co-author of a study that illustrates the environmental cost: In a paper published Wednesday in the journal PLOS One, she and colleagues highlight the physical overlap between these illegal operations and the habitat range of endangered animals.

It’s not just sharing space that’s the problem: Farmers use pesticides, and animals consume these pesticides and die.

Imagine this — On a mountain slope within California’s Sierra Nevada Forest, a farmworker illegally cultivates a highly controversial cash crop prized by millions: cannabis.

A fisher is a small carnivorous member of the weasel family that eats rodents. The fisher is at risk of pesticide poisoning from illegal cannabis operations.


It’s a hot summer day, so the worker irrigates the precious leaves with water piped in from a nearby stream. A curious rodent, drawn to the water source, feasts on the cannabis. But what it doesn’t know is that the plant is laced with a highly toxic pesticide.

The rodent perishes, and a slender carnivorous mammal, known as the Pacific fisher (Pekania pennanti), eats its corpse. In a matter of days, the fisher — endangered in the southern Sierra Nevada region — meets its own demise from ingesting the poisoned rodent.

This deadly interaction — linking rodenticides, cannabis grow sites, and animals — is increasingly documented.

For example, a 2013 report by one of the same co-authors on this PLOS One study, Mourad Gabriel, found that 85 percent of fisher carcasses recovered tested positive for rodenticide.

Another 2018 study by Gabriel and colleagues found the rodenticide, Brodifacoum, in liver and blood samples of the northern spotted owl, which is listed as threatened under the Endangered Species Act.

But fewer studies have attempted to model the location of likely cannabis cultivation sites with the aim of understanding their influence on wildlife. That’s where this new study comes into play.

How the discovery was made — The U.S. Fish and Wildlife Service and the California Department of Fish and Wildlife, which have previously looked into the environmentally devastating effects of illegal cannabis grow operations, provided funding for the research.

Using data from local, state, and federal law enforcement agencies on nearly 1,500 cannabis trespasses in forested regions of California and southern Oregon, the researchers created a statistical model which analyzed the geographical overlap between cannabis trespasses — illegal grow operations on public lands — and the habitats of three “species of concern.”

These are the:

  • Northern spotted owl
  • Humboldt marten
  • Pacific fisher

Through the model’s findings, the study team created a map showing the overlap between cannabis trespasses and the habitats of these animals. They also used their statistical model to identify the environmental factors that correlated most strongly with likely cannabis cultivation sites

On a general level, this helps predict where growers illegally cultivating weed would likely set up camp. “We can predict the most likely areas, but not necessarily specific areas,” Wengert explains. “It can be used more as a tool to guide surveillance and reconnaissance, and for managers to understand the riskiest regions for wildlife associated with these impacts.”

The study team also conducted on-the-ground research — but restricted time in the field to the off-growing season to avoid encounters with illicit growers.

A map showing the overlap of moderate to high trespass cannabis cultivation site likelihood (red) with modeled habitat for Pacific fisher (left), northern spotted owl (center), and Humboldt marten (right).

Wengert et al

What they found — The researchers found “trespass cannabis cultivation is widely and predictably distributed in forested regions of California and southern Oregon.”

The model points to three factors that influence where illegal cannabis grow operations are most likely to occur:

  1. Mid elevation forests (2624-5249 feet above sea level)
  2. Moderate slopes (30-60 percent steep)
  3. Less than 700 feet away from a water source

Other correlating factors were more surprising. The found illegal grow operations often clustered in areas where the forest had burned roughly eight to 12 years before cannabis cultivation.

A Northern Spotted owl watches for prey in California.

Robert Alexander/Getty Images

Critically, they also pinpointed a high degree of overlap between these grow sites and the habitats of threatened and endangered species. “Moderate to high-likelihood areas of trespass cultivation overlapped with 40 to 48% of modeled habitats of the three sensitive species,” the researchers write.

Pacific fisher

  • Over 53 percent of the fisher habit in the northern Sierra Nevada region overlaps with areas with a moderate-to-high likelihood of cannabis cultivation. The percentage was lower in the southern Sierra Nevadas (22 percent) where the species is most endangered.
  • A shocking 100 percent of reproductive female fishers’ home ranges overlapped with areas likely to harbor cannabis cultivation sites.
  • The researchers write the denning period —when fisher young are most dependent on their mother — “coincides with the initiation of cultivation and heavy use of rodenticides.”

Northern spotted owl

  • Forty-eight percent of their core habitat overlaps with areas with a moderate-to-high likelihood of cannabis cultivation
  • Seventy percent of these owls are at risk of rodenticide exposure

Humboldt marten

  • Thirty-seven percent of their core habitat overlaps with areas with a moderate-to-high likelihood of cannabis cultivation
  • The marten is less at risk than the northern spotted owl or the fisher because it resides in old-growth forests, where cannabis trespasses are less likely to occur

A fisher investigates the toxicant storage area of a cannabis growth site on public lands.

Greta Wengert

Why it matters — While cannabis is currently legal in California and Oregon, illegally grown marijuana continues to flourish.

For example, the sheriff’s department of Humboldt County— an area in northern California infamous for illegal cannabis operations — estimates illicit grow operations number in the thousands. According to the Orange County Register, California seized $1.77 billion worth of black-market weed in 2020.

With such serious health and environmental effects resulting from the illicit weed business, law enforcement will need better science and predictive models — like the one in the PLOS ONE study — to identify cannabis trespasses using harmful pesticides.

The potential of this tool is significant: The researchers discovered 16 cannabis cultivation sites previously undetected by law enforcement.

The researchers conclude that their results “substantiate the concern that law enforcement detects only a small fraction of the trespass cultivation sites in California and Oregon.”

“I hope lawmakers will understand the legacy of impacts to our natural resources and sensitive species that our public land face, and the need for reclamation funds as a tangible, though partial, remedy to this issue,” Wengert says.

Abstract: Illegal cannabis cultivation on public lands has emerged as a major threat to wildlife in California and southern Oregon due to the rampant use of pesticides, habitat destruction, and water diversions associated with trespass grow sites. The spatial distribution of cultivation sites, and the factors influencing where they are placed, remain largely unknown due to covert siting practices and limited surveillance funding. We obtained cannabis grow-site locality data from law enforcement agencies and used them to model the potential distribution of cultivation sites in forested regions of California and southern Oregon using maxi-mum entropy (MaxEnt) methods. We mapped the likely distribution of trespass cannabis cultivation sites and identified environmental variables influencing where growers establish their plots to better understand the cumulative impacts of trespass cannabis cultivation on wildlife. We overlaid the resulting grow-site risk maps with habitat distribution maps for three forest species of conservation concern: Pacific fisher (Pekania pennanti), Humboldt marten(Martes caurina humboldtensis), and northern spotted owl (Strix occidentalis caurina). Results indicate that cannabis cultivation is fairly predictably distributed on public lands in low to mid-elevation (~800-1600m) forests and on moderate slopes (~30–60%). Somewhat paradoxically, results also suggest that growers either preferred sites inside of recently disturbed vegetation (especially those burned 8–12 years prior to cultivation) or well outside(>500m) of recent disturbance, perhaps indicating avoidance of open edges. We ground-truthed the model by surveying randomly selected stream courses for cultivation site presence in subsets of the modeling region and found previously undiscovered sites mostly within areas with predicted high likelihood of grow-site occurrence. Moderate to high-likelihood areas of trespass cultivation overlapped with 40 to 48% of modeled habitats of the three sensitive species. For the endangered southern Sierra Nevada fisher population, moderate-high likelihood growing areas overlapped with over 37% of modeled fisher denning habitat and with 100% of annual female fisher home ranges (mean overlap = 48.0% + 27. SD; n = 134) in two intensively studied populations on the Sierra National Forest. Locating and reclaiming contaminated cannabis grow sites by removing all environmental contaminants should be a high priority for resource managers.
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