Toxic chemicals used to rid rodents from illicit marijuana gardens in the Sierra Nevada range and elsewhere in California may have inadvertently poisoned dozens of vulnerable weasel-like mammals called fishers, according to a new study released today.
Biologists from UC Davis, the nonprofit Integral Ecology Research Center, and state and federal land agencies found that nearly 80 percent of a sample size of fishers found dead in the wild were exposed directly or indirectly to anticoagulant rodenticides – rat poison. They point to illegal marijuana cultivation as a likely culprit for the introduction of the chemicals to remote areas where the animals live.
Fishers are members of the weasel family and formerly ranged across the northern forests of North America. But logging and fur trappers lured by their once-valuable pelts drove the fishers' numbers down, wiping them out in some parts of the United States. The sample group of the rare animals, which could be listed for protection under the Endangered Species Act, were found over a five-year period ending in 2011 in Northern California and in the southern Sierra Nevada range. Some of the dead animals were found in remote wilderness areas with no roads or campground access.
The study, which documented exposure to such poisons in fishers for the first time, raises questions about the threat to other rare forest predators, such as the Sierra Nevada red fox, wolverine, gray wolf and various owl species. It also raises questions about the long-term environmental impacts from marijuana gardens treated with the poisons, which have become increasingly toxic as rodents build resistance to the chemicals. Nearly all of the fishers that died after coming into contact with the anticoagulants were exposed to highly toxic versions of the chemical.
Mourad Gabriel, a doctoral candidate at UC Davis and the study’s lead author, said in an interview that researchers were surprised to see such high levels of the chemicals in carnivores, which, he said, "really opened a can of worms," including possible exposure to humans.
"To humans, the threat is (the poisons used in marijuana gardens) can go into other species ... such as game resources for humans," he said. "It does pose a risk."
Drug enforcement authorities worry that the toxic chemicals could harm humans in more insidious ways – by smoking or ingesting marijuana that has been contaminated by rat poison, insecticides and other chemicals that ward off pests. Tommy LaNier, who directs the White House-funded National Marijuana Initiative, said other studies may be conducted to look at the long-term effects of chemicals sprayed on or applied near marijuana plants.
"This study is a real breakthrough on the effects of anticoagulants on wildlife," he said. "The animals bleed to death, their internal organs turn to mush. ... It's a terrible death."
The authors also warn that the use of the common rat poisons may need to be regulated further so only pest control professionals can use them. Some scientific studies have found anticoagulants in mountain lions and bobcats, but those incidents occurred in places with human development, either in urban areas or rural or agricultural land.
The authors point out that the deaths occurred within a short time period – from mid-April to mid-May, which is when marijuana gardens are often planted and seedlings are particularly susceptible to rodent damage. The chemicals are often placed near irrigation lines and hoses to prevent rodents from chewing through them.
“Our findings demonstrate that anticoagulant rodenticides, which were not previously investigated in fishers or other remote forest carnivores, are a cause of mortality and may represent a conservation threat to these isolated California populations,” the authors write. “Because we do not know the long-term ecological ramifications of these toxicants left on site long after marijuana grows are dismantled, heightened efforts should be focused on the removal of these toxicants at these and adjacent areas at the time of dismantling.”
State and federal land managers report that marijuana gardens on public lands have become an increasing problem over the past decade or so, with growers pushing into seemingly inaccessible terrain.
Authorities in California and elsewhere have long aimed to link illicit marijuana gardens on public lands with environmental damage – from diverting water resources and producing copious waste to trampling native vegetation and killing bears and other wildlife that may threaten the gardens, or the gardeners.
As part of the nation's intelligence community funding authorization for 2012, federal lawmakers last year directed top intelligence officials to consult with public land managers on how to address the issue of illegal grows, including the use of spy satellites and other equipment if necessary.
Yet, as California struggles with the issue of marijuana cultivation on public lands amid its budget crisis, such damage may linger for years as the state’s park system doesn’t have designated funds to clean up grow sites.
“We do not have a Department budget at the State level that is set aside and identified as funding to go toward marijuana eradication and the cleanup of pot gardens,” California State Parks spokesman Roy Stearns wrote in an email. “If the Districts and Sector have a garden and wish to take action, they must find the money in their existing budget to do the work.”