SREL Reprint #3826

Comparison of removal and spatial mark-resight models for estimating wild pig density

Charles R. Taylor1,2, Kim M. Pepin3, Ryan S. Miller4, John R. Foster4,5, and James C. Beasley1,2

1Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602, USA
2Savannah River Ecology Laboratory, University of Georgia, P.O. Drawer E, Aiken, SC 29802, USA
3United States Department of Agriculture, Animal and Plant Health Inspection Service, Wildlife Services,
National Wildlife Research Center, 4101 Laporte Avenue, Fort Collins, CO 80521, USA
4United States Department of Agriculture, Animal and Plant Health Inspection Service, Veterinary Services,
Center for Epidemiology and Animal Health, 2150 Centre Avenue, Fort Collins, CO 80526, USA
5United States Department of Energy, Oak Ridge Institute for Science and Education, Oak Ridge, TN 37830, USA

Abstract: Density estimation is critical to effectively manage invasive species and elucidate areas of highest concern. For wild pigs (Sus scrofa), the ability to estimate density is complicated because of their variable home range sizes and social structure. Common methods for estimating density (e.g., mark-recapture) may be unsuitable in management applications because additional data needs to be collected before and after management. Removal models offer a suitable alternative to estimate density changes following management and can be applied broadly across areas where management of wild pigs is ongoing. We collected wild pig removal and camera trap data from 25 private properties ranging in size from approximately 0.5 km2 to 95 km2 across 3 ecoregions in South Carolina, USA, from 2020-2023. We compared factors affecting consistency and precision of property-level density estimates between removal and spatial mark-resight (SMR) models. In general, excluding 1 large outlier, density estimates from removal models were between 0.60 and 15.85 wild pigs/km2 (median = 5.34) with a median coefficient of variation (CV) of 0.76 and 95% confidence intervals for the CV between 0.70 and 0.94. Similarly, excluding 1 large outlier, density estimates from SMR were between 0.22 and 30.97 wild pigs/km2 (median = 5.48) with a median CV of 0.39 and 95% confidence intervals for the CV between 0.38 and 1.20. We found the precision of removal models was affected primarily by the number of wild pigs dispatched in the removal period (3 months) and the ecoregion in which they were removed. None of the covariates, including the number of recaptures (a corresponding measure of sample size), influenced precision of the SMR models, although recaptures did influence the density estimates. At the individual property level, density estimates from our 2 estimators were dissimilar from each other in approximately 80% of instances, although none of the covariates we examined influenced dissimilarity. Our results provide unique insight into how sample size affects density estimates using 2 common methods and into novel SMR models that incorporate both marked and unmarked detections. In addition, the density estimates in this study can be used as a reference for wild pig densities in common land cover types throughout the southeastern United States.

Keywords: camera surveys, density estimation, invasive species, population control, removal model, spatial mark-resight, Sus scrofa, wild pigs

SREL Reprint #3826

Taylor, C. R., K. M. Pepin, R. S. Miller, J. R. Foster, and J. C. Beasley. 2025. Comparison of removal and spatial mark-resight models for estimating wild pig density. Journal of Wildlife Management(e70054).

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).