SNAMP Pub #23: Impacts of rodenticide and insecticide toxicants from marijuana cultivation sites on fisher survival rates in the Sierra National Forest, California
Article Title: Impacts of rodenticide and insecticide toxicants from marijuana cultivation sites on fisher survival rates in the Sierra National Forest, California
Authors: Thompson, C., R. Sweitzer, M. Gabriel, K. Purcell, R. Barrett, and R. Poppenga
- In April 2009, a male fisher that appeared to be in excellent health was found dead by members of the Sierra Nevada Adaptive Management Project (SNAMP) fisher research team.
- Necropsy by the California Animal Health and Food Safety Laboratory at UC Davis revealed that the animal had died of acute anticoagulant rodenticide (AR) poisoning.
- Consequently, archived liver samples from fishers previously submitted for necropsy from both the SNAMP and US Forest Service Kings River Fisher Project (KRFP) were tested for the presence of seven AR compounds. Over 83% of the samples submitted tested positive for the presence of at least one AR compound.
- Hundreds of illegal marijuana cultivation sites have been found and eradicated within the Sierra National Forest, and law enforcement agents report finding large quantities of rodenticides at these sites.
- We examined correlations between the number of known cultivation sites within an animal’s home range and the presence of AR compounds in that animal’s liver tissue.
- Female fisher survival was influenced by the number of cultivation sites within its home range.
- This is the first reported analysis of the potential impacts of illegal marijuana cultivation sites on the survival of free ranging carnivores.
This study was conducted on the west slope of the southern Sierra Nevada, in the High Sierra and Bass Lake Ranger Districts of the Sierra National Forest, California. Field work was carried out between 1,000 and 2,400 m in elevation, corresponding to the Pacific fisher occurrence in the region, and the study area included a mix of public and private land.
Fishers are a species of significant conservation concern in the western United States. Populations are small and highly fragmented and considered at high risk from events such as disease or wildfire. They are currently deemed a candidate species under the United States Federal Endangered Species Act, are a candidate for listing under California Endangered Species Acts, and are considered a sensitive species in the western United States by the U.S. Forest Service. Significant research efforts have been underway for the past five years, intended to document fisher ecological requirements and limiting factors.
Secondary exposure of wildlife to anticoagulant rodenticides has been well documented over the last 40 years, yet exposure is typically associated with agricultural or wildland-urban interface areas. Wildlife in undeveloped areas has generally been presumed to be free from risk. Testing of forest wildlife is difficult, as it requires the recovery of liver tissue from an intact, nonscavenged carcass.
Investigation indicated that the most likely sources for AR’s found in fisher are the numerous illegal marijuana cultivation sites currently found on public lands. At cultivation sites regulations are disregarded and multiple toxicants are used in large quantities with the intent of poisoning anything that might harm the crop.
Fishers are potentially directly exposed through the consumption of toxicants mixed with bait, and secondarily exposed through scavenging and predating upon contaminated small mammals and insects. If not killed directly by the poison the resulting compromised health may cause weakened reactions to other stressors such as parasites, pathogens, and predation which may allow the underlying role of toxicants to escape notice.
Sublethal doses of AR exposure have also been shown to cause: short-term hypothermia in mammals, impaired antipredator behavior, decreased ability to recover from physical injury, increased susceptibility to a variety of pathogens and parasites, and is transferable from a female fisher to dependent kits through lactation.
To determine whether the presence of cultivation sites predisposed fishers to mortality, we related survival rates to the presence and number of cultivation sites found within that animal’s home range over the past 10 years.
Female fisher survival was influenced by the number of cultivation sites within its home range. The number of AR compounds detected per individual fisher ranged from one to four. The most common toxicant detected was brodifacoum, an acutely toxic second generation AR. There is a statistically significant association between AR exposure and female fisher survival.
- The association between illegal marijuana cultivation sites, AR and other pesticide exposure, and fisher mortality is strong yet speculative. Determining a cause and effect relationship would require novel testing procedures, given the inherent dangers of visiting and monitoring these sites.
- The potential existence of an underlying, previously unrecognized factor increasing mortality rates for a USFWS candidate species previously thought to be free of such influences raises significant conservation concerns. Under current research protocols such a factor could easily go unnoticed.
- Given the variety of toxicants found at illegal cultivation sites and the fact that as many as four AR compounds were detected in an individual fisher, the risk of interactive effects of the poisons should be seriously considered
- Pesticides are cached near sites for future use; these caches may remain undetected and can continue to contaminate a site for several years. Therefore the potential for chronic exposure by second and third-order predators is plausible.
- Given the breadth of potential direct and indirect impacts described above, the possibility that widespread AR exposure is reducing fisher survival rates sufficiently enough to limit population expansion must be considered. The negative impacts are clear and priority must be given to the identification, documentation, and reclamation of these sites, and educating the public about these illegal actions on their communal lands.
Thompson, C., R. Sweitzer, M. Gabriel, K. Purcell, R. Barrett, and R. Poppenga. Impacts of rodenticide and insecticide toxicants from marijuana cultivation sites of fisher survival rates in the Sierra National Forest, California. Conservation Letters. Accepted May 2013.
The full paper is available here.
For more information about the SNAMP project and the Fisher team, please see the: Fisher Team Website.