SNAMP Pub #27: Using DNA to describe and quantify interspecific killing of fishers in California
Article Title: Using DNA to describe and quantify interspecific killing of fishers in California.
Authors: Wengert, G.M., M.W. Gabriel, S.M. Matthews, J.M. Higley, R.A. Sweitzer, C.M. Thompson, K.L. Purcell, R.H. Barrett, L.W. Woods, R.E. Green, S.M. Keller, P.M. Gaffney, M. Jones, and B.N. Sacks. 2014.
- This is the first study to use DNA to verify predators and differentiate species preying on fishers.
- Understanding the drivers of interspecific killing, and the species of predators responsible is essential for conservation efforts.
- Genetic analysis was used to identify predators of fishers; necropsies were conducted to identify predator species in the absence of genetic evidence.
- Predation frequencies were analyzed by species in relation to study area, sex, age class.
The Pacific fisher (Martes pennanti) is a mid-sized carnivore that inhabits coniferous and mixed hardwood-coniferous forests of the western and eastern United States, northern Rocky Mountains, and southern Canada. Interspecific killing, where individuals of different species compete for the same resource in an ecosystem, is common among carnivores and can have population-level effects on imperiled species. The Pacific fisher is a candidate for listing under the United States Endangered Species Act. Interspecific killing is poorly understood in fishers and a potential threat to existing western populations.
We studied the prevalence and patterns of interspecific killing of fishers in the southern Sierra Nevada and Coastal range of California. Until this study, the frequency of deaths of healthy adult fishers by other predators and their significance for mortality rates of resident fisher populations were unknown. Understanding the drivers of interspecific killing, the seasonal patterns in predation rates, and the species of predators responsible is essential for conservation efforts to mitigate population decline in a vulnerable carnivore population.
We recovered deceased, radiocollared fishers (equipped with a mortality sensor) from 1 to 18 days after death from three California research projects between 2007 and 2011.We collected forensic evidence and samples from the carcasses and predation sites, including evidence of predation versus scavenging. We conducted full necropsies when possible, and used molecular methods to determine species of predators responsible for killing the fishers. Our methods were able to detect felid (cat) and canid (dog) DNA, and to differentiate between lion and bobcat predation, butwere unable to detect black bear DNA.
We analyzed 101 radiocollared fisher carcasses and attributed to each a cause of mortality through evidence collected by necropsy or circumstantial forensic information. Of these 62 were killed by predators of another species, We found that bobcats, mountain lions and coyotes killed fishers in our study areas and that spring had a significantly higher frequency of interspecific killing.
Bobcats, not previously reported to prey on fishers, killed only female fishers; who were found to experience greater predation rates than males by smaller species of predators. Mountain lions more frequently killed males. This dichotomy in sex specific predation by different predator species probably stems from the pronounced sexual size dimorphism between male and female fishers. That both bobcats and mountain lions consumed fishers suggests they were killed as prey rather than competitors.
Coyotes rarely killed fishers even though they are the most frequently cited predator of fisher in the literature and when they did they did not consume the carcasses but tended to cache it.
- Interspecific killing was the cause of 61% of all fisher deaths we investigated.
- The proportion of fisher mortalities caused by predation was greater than reported previously in California.
- Most predation mortalities occurred several months up to several years after collaring, suggesting that predation risk was not related to the capture process.
- Fisher kits are completely dependent on their mothers for survival during March through July, when over 70% of female predation deaths by bobcats occurred. Therefore, bobcat predation is likely to affect the population dynamics of fishers.
- Understanding what factors drive bobcat predation on fishers is an important question with respect to our ability to manage this potentially influential mortality factor.
- Fluctuations in fisher mortality due to interspecific killing could constitute increases in mortality sufficient to limit population growth.
- Additional research is needed to explore habitat characteristics that may encourage or discourage bobcat predation on fishers, potentially providing managers with specific tools that can be used to manipulate habitat in favor of fisher survival, especially during the spring.
Using DNA to describe and quantify interspecific killing of fishers in California. The Journal of Wildlife Management 78(4):603–611; 2014; DOI: 10.1002/jwmg.698
The full paper is available at: http://onlinelibrary.wiley.com/doi/10.1002/jwmg.698/abstract
For more information about the SNAMP project and the fisher team, please see: