Testing infrared camera surveys and distance analyses to estimate feral horse abundance in a known population

Abstract

We tested the use of high-resolution infrared (IR) camera technology and distance sampling analyses to estimate abundance of feral horses (Equus caballus) during 2015–2016 in the McCullough Peaks Herd Management Area, Wyoming, USA. Infrared technology is becoming more common in ungulate population monitoring. The quality of IR cameras now allows ungulate species to be differentiated. Imperfect detection is a common problem in aerial surveys, so we tested the use of distance sampling analyses to account for imperfect detection probability. We conducted 2 aerial surveys in a sagebrush ecosystem with a demographically closed horse population. True abundance was known to within ±4 animals as a result of intensive, ground-based monitoring of each animal, all of which are uniquely identifiable. After truncation of our data, the most supported detection function was a uniform function with a detection probability equal to 1.0 out to 255 m. Our analyses yielded results that were within 10% of true abundance, but the coefficient of variation (CV) was large (36–58%) assuming a small sampling fraction. However, our truncated surveys covered approximately 95% of the herd management area. By including a finite population correction factor in our calculations of variance estimates, CVs (8–13%) were dramatically reduced. We found the combination of IR surveys and distance sampling analysis to be a useful method to estimate feral horse abundance in sagebrush vegetation type, which had limited cover to obscure horses. Repeated testing in sagebrush ecosystems as well as further testing in other habitat types and under differing conditions will inform how general our approach can be. © 2018 The Wildlife Society.