Marijuana is proving to be a health hazard, although not for the reasons one would assume. Since the start of the year, the private cultivation of marijuana crops has been legal in California, making it the fifth US state to permit the practice. Business is booming; Humboldt County alone, in the north of the state, is home to at least 4,500 – and potentially as many as 15,000 – plantations, and there is an expectation that numbers will continue to rise. However, at present the majority of these remain unlicensed and unregulated, even despite the recent legalisation passing.

Recent studies on owls found deceased in remote locations across Humboldt and neighbouring Mendocino and Del Norte Counties is focusing attention on the ecological impact of these farms. For example, 70 per cent of northern spotted owls – which are in decline in both range and population, and are consequently listed as ‘threatened’ under the Endangered Species Act – tested positive for various anticoagulant rodenticides, utilised to control rat populations within these farms.

By preventing the recycling of vitamin K, rodenticides cause a series of clotting and coagulation problems in both birds and mammals, followed by uncontrollable internal bleeding. For owls feeding naturally at forest edges, which unfortunately often also happen to mark the edges of these plantations, their dead rat dinners could potentially be their own death sentence (the studied birds in question were judged to have died of causes ranging from vehicle strikes to predation, not poison).

‘Sublethal anticoagulant rodenticide impacts vary from species to species, and also within individuals,’ explains Mourad Gabriel, a research faculty member at the UC Davis School of Veterinary Medicine. ‘We detected levels in our study that have been reported as benign in other owls, but other studies have shown these levels to be lethal, or having a negative impact in other owls or raptors. So as you can see it becomes a case-by-case scenario.’

Gabriel believes that, eventually, the legal market for marijuana will drive out these unregulated plantations and their overuse of rodenticides, but raises concerns about how long this might take. ‘We have empirical evidence of landscape contamination, and others have demonstrated habitat fragmentation,’ he adds. ‘The northern spotted owl is not increasing in population numbers, and there is consideration of up-listing this species to “endangered”.’

This was published in the March 2018 edition of Geographical magazine

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