Midwest Fish & Wildlife Conference 2018

AUTHORS. Edward D. Davis, Western Illinois University; Tim C. Swearingen, Western Illinois University; Christopher N. Jacques, Western Illinois University; Robert W. Klaver, US Geological Survey; Chuck R. Anderson, Colorado Parks and Wildlife; Jonathan A. Jenks, South Dakota State University; Christopher S. DePerno, North Carolina State University; Robert D. Bluett, Illinois Department of Natural Resources

ABSTRACT. A variety of increasingly sophisticated methods are available for estimating population density from capture-recapture studies. Among these, spatial capture-recapture (SCR) models provide a rigorous analytical technique for inference that extends standard closed population models to include a spatially explicit model by accounting for the distribution of individuals in space. Spatial capture-recapture models rely on spatial information readily available with camera data and use distance between traps and animal activity centers to model spatially explicit (i.e., camera trap) encounter probabilities and have been used in population density estimation for a range of carnivores. We used Bayesian analyses to evaluate the utility of SCR models for estimating density of bobcats in an agriculturally dominated landscape of west-central Illinois. We defined the continuous state space by overlaying the trap array on a square region extending 5 to 20 km beyond camera traps in each cardinal direction. We deployed 50 camera stations over a 77-day period from 1 February–18 April 2017. We captured 23 uniquely identifiable bobcats 115 times and recaptured these same individuals 92 times. Our analysis revealed a slight effect on the posterior distribution of density for the 5-km continuous state-space model, though posterior summary statistics for the 10-km, 15-km, and 20-km continuous state-space models were similar. Densities ranged from 1.44–1.57 bobcats per 100 km2 with a 95% posterior interval of 1.07 to 1.90. Variation in the state-space extending beyond trap arrays affect bobcat density estimates and should be sufficiently large to minimize encountering individuals with activity centers (i.e., home ranges) beyond the state-space boundary. Increased size of home ranges of bobcats across Midwestern landscapes may necessitate the use of relatively coarser survey grids in SCR models to account for frequent movements to and from the state space or whose core areas are positioned beyond camera survey unit boundaries.