Cannabis crops grown illegally on public lands along the west coast of the US are infringing on the habitats of native species, putting predators at risk of poisoning by dangerous pesticides.
Greta Wengert at the Integral Ecology Research Center in Blue Lake, California, first got an inkling of the scale of this human-wildlife conflict when cat-sized, ferret-like mammals called fishers (Pekania pennanti) turned up fatally poisoned by rodenticides. This was a surprise, considering the animals’ usual habitats are far from human developments.
Wengert and her colleagues suspected that the rodenticide was coming from illegal cannabis farms, where the poison is often used to control pests, because this is one of the few major potential sources of the poison in the region’s forested wilds.
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To probe whether it was happening, the team used modelling to predict where illegal cannabis farms and three different threatened predators – fishers, Humboldt martens (Martes caurina humboldtensis), and northern spotted owls (Strix occidentalis caurina) – would be most likely to intersect.
The team gathered law enforcement data from 1469 illegal cannabis cultivation sites in northern California and southern Oregon from 2007 to 2014. Using the environmental characteristics of the sites, the team made maps of where people were most likely to illegally grow cannabis, then compared these maps with similar models of suitable habitat for the three predator species.
Areas with a moderate-to-high likelihood of cannabis cultivation overlapped with more than 44 per cent of fisher habitat, and about 48 per cent of spotted owl and 40 per cent of Humboldt marten habitats, suggesting that the animals’ risk of pesticide exposure is high.
When the team surveyed some of the areas predicted to have a higher likelihood of hosting cannabis farms, 16 previously unknown sites were uncovered, validating the model’s predictive potential.
Such farms, says Wengert, don’t just poison predators. They also divert water sources and reduce the populations of rodents that serve as the predators’ prey.
“It really seems like the problem is way more extensive than originally thought,” says Wengert. “It really is impacting places that we just wouldn’t think growers would get to and use.”
Phoebe Parker-Shames at the University of California, Berkeley, says the study shows how the efforts of small-scale, legal cannabis growers to engage in best practices – something Wengert describes as minimal pesticide use and complying with water use regulations – can have a very different, and often much smaller, environmental impact than illegal cultivation.
“It’s really interesting how the same plant being grown in the same geographic region can end up with extremely different ecological outcomes depending on the way it is grown,” she adds.
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