Factors influencing fisher occupancy over time in a landscape altered by tree mortality in the southern Sierra Nevada

Background/Question/Methods

The fisher (Pekania pennanti) is a forest-dwelling carnivore of conservation concern in the southern Sierra Nevada. In 2007, we began utilizing detection dog teams (handler & dog) to non-invasively study fisher occupancy in the Kings River watershed in the southernmost portion of the fisher distribution. Our surveys overlapped with two major disturbances in space and time: an extreme drought beginning in 2012 and extensive conifer mortality starting in 2015. Our primary objectives were to evaluate changes in fisher occupancy over time (2007 – 2018) within the Kings River study area, identify factors influencing occupancy to inform forest planning, and to better understand the ecological ramifications of climate related stressors at a broader scale. We overlayed a grid system across the study area to equally and randomly distribute effort, conducted surveys twice per year (i.e., late spring/early summer, fall), and visited each cell three times per season. During each survey, the detection dog teams collected scats estimated to be from fisher and documented their locations, along with recoding a track log of the areas surveyed. All scats were then sent to a lab for genetic verification to determine the depositor of the scat. We then used a dynamic occupancy model to evaluate trends in fisher occupancy over time while accounting for covariates that influenced the probability of detection, as well as habitat covariates that we hypothesized might influence initial occupancy, persistence, and colonization.

Results/Conclusions

Results show a gradual decline in occupancy from the start of the project through the drought years, followed by a small increase at the start of the tree mortality event, then a continued decline. Oak and live forest had a positive influence on initial occupancy, while live forest, tree mortality, and shrub had positive influences on persistence. For site colonization, oak had a positive influence while tree mortality and shrub had a negative influence. Factors influencing scat detection included distance to stream, season, and effort. Our findings are relevant for conservation and management because they highlight the importance of remaining live forest and the value of drought tolerant oaks for fisher. The downward trend in occupancy is concerning for fisher conservation, especially given the dramatic change to the landscape. Additionally, because tree mortality is interspersed with live forest in this region, it is more diffuse than other types of disturbance; thus, while fishers may not strongly avoid tree mortality, it may reduce habitat quality over time.