Habitat Use by Feral Horses in the Northern Sagebrush Steppe

Abstract

Distribution patterns of feral horses (Equus caballus) relative to plant communities, herbaceous production, and perennial water sources were studied from April 1979 to March 1981 in Oregon's Owyhee Breaks. Repeated observations of radio-collared and easily identified horses allowed estimation of home range sizes and documentation of the plant communities utilized. A map of plant communities was constructed, and composition and herbaceous production of key communities sampled. Time-lapse cameras monitored the daylight watering patterns of horses. One hundred thirty-three horses were initially censused and identified on the study area with the total population subsequently increasing at an annual rate of 13%. Home ranges averaged 12 km2 with the minimum convex polygon procedure and 27 km2 with the 90%0 confidence ellipse method. No seasonal shifts in home ranges occurred, and no correlations were detected between home range size and number of horses per band, densities of perennial water sources, or levels of forage production within home ranges. Six distinct were identified on the area. Only one band of horses moved from one herd to another during the 2-year study. Animals in each herd made greatest use of the most prevalent plant community, with no community being universally preferred to over another. Watering activities were most intense during the first and last periods of daylight. Horses rapidly vacated the watering areas after drinking. A seasonal trend was observed in which horses remained slightly closer to perennial water sources during warm, dry summer months than during spring periods when additional seasonal sources were available. Seasonal differences were not statistically significant, however. A knowledge of the distribution and movement patterns of large herbivores is especially valuable to managers charged with allocatAuthors are, respectively, former graduate student, Department of Rangeland Resources, Oregon State University, Corvallis 97331, and presently range scientist, USDA-ARS, Eastern Oregon Agricultural Research Center, Squaw Butte Station, Burns 97720, and superintendent, Eastern Oregon Agricultural Research Center, Burns 97720. This research was a cooperative effort jointly funded by the Bureau of Land Management, contract number YA-5 12-CT8-137, and the Eastern Oregon Agricultural Research Center. Technical paper number 7517. Oregon Agricultural Experiment Station. Manuscript accepted 29 August 1985. ing resources or assessing impacts of large herbivore use. With the passage of Public Law 92-195 in 1971, feral horses (Equus caballus) in the United States became a particularly vexing problem for public land managers. Since 1971 our knowledge of feral horses has increased, but our understanding of their habits is far from complete. Because of differences in habitat, results and management suggestions from specific studies are often inapplicable to other areas. This research was initiated because few data were available describing patterns of resource use by feral horses in the sagebrush (Artemisia spp.)/bunchgrass vegetation of the Pacific Northwest. The objectives of this study were to characterize the population and to document patterns of habitat use by feral horses relative to plant communities, herbaceous production, and permanent water sources in Oregon's Owyhee Breaks. Estimates of home range sizes of feral horses vary from 0.9 km2 in Nova Scotia to 303 km2 in Wyoming's Red Desert (Welsh 1975, Miller 1983b). Behavioral aspects influencing distribution patterns of feral horses include a highly developed social organization of harem groups or bands, multiple male and female bands, and bachelor groups (Pellegrini 1971, Feist 1971, Zarn et al. 1977, Berger 1977, Nelson 1978, Salter 1978, Miller 1980). We measured the home ranges of several horses during a 2-year study and tested the hypothesis that bands and bachelor males have home ranges of equivalent size. Miller (1983b) proposed that distribution and abundance of required resources may influence home range size. We examined his hypothesis by correlating variations in home range size with herbage production within home ranges, densities of water sources within home ranges, and the number of horses constituting a band. When various groups of horses have overlapping home ranges they may further combine to form larger groups or herds (Miller and Denniston 1979), which Miller (1983b) proposed as the logical management unit for feral horses. Researchers in several areas have noted movements of horses in response to plant phenology and forage availability (Nelson 1978, Salter and Hudson 1979, JOURNAL OF RANGE MANAGEMENT 39(3), May 1986 207 This content downloaded from 157.55.39.104 on Sun, 19 Jun 2016 05:54:55 UTC All use subject to http://about.jstor.org/terms Miller 1983b). With this in mind, we tested the hypothesis that individual of horses randomly used the plant communities within their respective herd areas. Numerous observers have also noted seasonal movements of horses in response to fluctuations in the availability of water (Welsh 1975, Hansen et al. 1977, Storrar et al. 1977, Miller 1983a). Where water supplies are restricted, the continuous occupation of a source by horses may hinder the use of water by other species (Miller 1980). We noted the distance of sightings of horses from water and tested the hypothesis that there were no seasonal fluctuations in the dispersal of horses around permanent water sources. We also monitored the daylight watering activities of horses with time lapse cameras and tested the hypothesis that horses watered at random throughout the day.