Michael A. Sawaya, Ph.D. (Sinopah Wildlife Research Associates)
Jesse Whittington, M.Sc. (Parks Canada)
Capture-recapture studies are frequently used to monitor the status and trends of wildlife populations. Detection histories from individual animals are used to estimate probability of detection and abundance or density. The accuracy of abundance and density estimates depends on the ability to model factors affecting detection probability. Non-spatial capture-recapture models have recently evolved into spatial capture-recapture models that directly include the effect of distances between an animal’s home range centre and trap locations on detection probability. Most studies comparing non-spatial and spatial capture-recapture biases focussed on single year models and no studies have compared the accuracy of demographic parameter estimates from open population models.
1) to compare grizzly bear demographic parameters estimated from open population non-spatial and spatial capture-recapture models using noninvasive genetic sampling data.
2) to compare the performance of open population non-spatial and spatial capture-recapture models using simulated data.
Noninvasive genetic sampling locations (i.e. hair traps, bear rubs, and crossing structures) for grizzly and black bears in the Bow Valley Study Area of Banff National Park, Alberta, Canada. Figure from Sawaya et al. 2012
We applied open population non-spatial and spatial capture-recapture models to three years of grizzly bear DNA-based data from Banff National Park (Sawaya et al. 2012) and simulated data sets. The two models produced similar estimates of grizzly bear apparent survival, per capita recruitment, and population growth rates but the spatial capture-recapture models had better fit. Simulations showed that spatial capture-recapture models produced more accurate parameter estimates with better credible interval coverage than non-spatial capture-recapture models. Non-spatial capture-recapture models produced negatively biased estimates of apparent survival and positively biased estimates of per capita recruitment. The spatial capture-recapture grizzly bear population growth rates and 95% highest posterior density averaged across the three years were 0.925 (0.786–1.071) for females, 0.844 (0.703–0.975) for males, and 0.882 (0.779–0.981) for females and males combined. The non-spatial capture-recapture population growth rates were 0.894 (0.758–1.024) for females, 0.825 (0.700–0.948) for males, and 0.863 (0.771– 0.957) for both sexes. The combination of low densities, low reproductive rates, and predominantly negative population growth rates suggest that Banff's population of grizzly bears requires continued conservation-oriented management actions.
For more details, see our paper in the open access journal PLoS ONE (pdf).
Spatial Capture-Recapture Models