“Unfortunately, these early investigations of rodenticide exposure in fishers that are associated with illegal marijuana cultivation on public lands might only be the tip of the iceberg with regard to overall impact of pesticides on wildlife at these sites,” said co-author Robert Poppenga, a professor with the California Animal Health and Food Safety Laboratory System at UC Davis. Poppenga’s laboratory provided rodenticide analysis in support of the investigation.

While some fishers have died from either directly consuming flavored rodenticides or by consuming prey that had recently ingested the poisons, exposure may also predispose animals to dying from other causes. Exposure to lower doses or to combinations of the poisons results in slower reflexes, reduced ability to heal from injuries, and neurological impairment. This leads to death from other causes, such as predation or road traffic.

Fishers in the southern Sierra Nevada are highly susceptible to pesticide exposure because their diet consists of small mammals, birds, carrion, insects, fungi and other plant material. Numerous dead or dying insects and small mammals are often found in the vicinity of illegal marijuana sites.

The conservation implications of this study are far-reaching.

“By increasing the number of animals that die from supposedly natural causes, these pesticides may be tipping the balance of recovery for fishers,” said Craig Thompson, a wildlife ecologist with the U.S. Forest Service’s Pacific Southwest Research Station and the study’s lead author.

This new threat may also impact other species already facing declining populations, including the wolverine, marten, great gray owl, California spotted owl and Sierra Nevada red fox, which may also be exposed to the poisons, the researchers said.


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