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Understanding factors driving resource selection and habitat use of different species is an important component of management and conservation. Feral horses (Equus caballus) are free ranging across various vegetation types in the western United States, yet few studies have quantified their resource selection and seasonal use. We conducted a study to determine effects of vegetation community, distance to water, and topographic variables on seasonal resource selection in 2 feral horse populations in Great Basin sagebrush (Artemisia spp.) ecosystems of west‐central Utah, USA: Conger Herd Management Area (HMA) and Frisco HMA. We deployed global positioning system (GPS) radio‐collars on 38 female horses and GPS‐transmitters braided and glued into the tail hair of 14 males, collecting locations every 2 hours for 1–4 years between 2016 and 2020. We calculated home range size and core use area of social groups (harems) and bachelor males using auto‐correlated kernel density estimators for each biologically defined season (breeding, fall, and winter) per study year. We examined seasonal home range size and overlap of harem groups and bachelor males and compared movement speed of bachelors and harems among seasons. We determined seasonal resource selection in a use‐availability framework using resource selection functions. We hypothesized that horses would select for areas of high herbaceous vegetation, that water would be a key variable in resource selection models like other equids, and home range size in winter would be largest because horses can eat snow for hydration and could therefore roam farther from surface water. Mean annual home range size was 103.12 ± 37.38 km2 (SD) for Conger harems and 117.47 ± 32.75 km2 for Frisco harems. At Conger there was no difference in home range size between harem groups and bachelor males, but home range size was smaller in winter than other seasons, whereas winter home range size at Frisco was larger than other seasons. Bachelor males moved at higher speeds than harems during all seasons, and harem groups from both populations had lower movement speeds in winter. Harem groups had distinct winter ranges with little overlap on breeding season ranges. In both populations, all horses selected for herbaceous vegetation types and avoided forest relative to shrubland throughout the year. Harems at Frisco were consistently located closer to water sources, whereas selection for water sources by Conger harems varied seasonally, with winter having the lowest selection. Harem groups at Conger had an average of 10.6% of their home ranges outside the HMA boundary and Frisco harems had up to 66.8% outside, likely because of the horseshoe shape of Frisco HMA in which shrub meadows (foraging areas) comprise the horseshoe center, which is outside the HMA. Our results highlight the importance of water sources, which were a key predictor of horse movement patterns in our study. We emphasize the utility of telemetry devices to understand resource selection of feral horses at a fine scale, enabling management to be more targeted and facilitate planning. [ABSTRACT FROM AUTHOR]

In recent decades, feral horse (Equus caballus; horse) populations increased in sagebrush (Artimesia spp.) ecosystems, especially within the Great Basin, to the point of exceeding maximum appropriate management levels (AMLmax), which were set by land administrators to balance resource use by feral horses, livestock, and wildlife. Concomitantly, greater sage‐grouse (Centrocercus urophasianus; sage‐grouse) are sagebrush obligates that have experienced population declines within these same arid environments as a result of steady and continued loss of seasonal habitats. Although a strong body of research indicates that overabundant populations of horses degrade sagebrush ecosystems, empirical evidence linking horse abundance to sage‐grouse population dynamics is missing. Within a Bayesian framework, we employed state‐space models to estimate population rate of change (λ) using 15 years (2005–2019) of count surveys of male sage‐grouse at traditional breeding grounds (i.e., leks) as a function of horse abundance relative to AMLmax and other environmental covariates (e.g., wildfire, precipitation, % sagebrush cover). Additionally, we employed a post hoc impact‐control design to validate existing AMLmax values as related to sage‐grouse population responses, and to help control for environmental stochasticity and broad‐scale oscillations in sage‐grouse abundance. On average, for every 50% increase in horse abundance over AMLmax, our model predicted an annual decline in sage‐grouse abundance by 2.6%. Horse abundance at or below AMLmax coincided with sage‐grouse λ estimates that were consistent with trends at non‐horse areas elsewhere in the study region. Thus, AMLmax, as a whole, appeared to be set adequately in preventing adverse effects to sage‐grouse populations. Results indicated 76%, 97%, and >99% probability of sage‐grouse population decline relative to controls when horse numbers are 2, 2.5, and ≥3 times over AMLmax, respectively. As of 2019, horse herds exceeded AMLmax in Nevada, USA, by >4 times on average across all horse management areas. If feral horse populations continue to grow at current rates unabated, model projections indicate sage‐grouse populations will be reduced within horse‐occupied areas by >70.0% by 2034 (15‐year projection), on average compared to 21.2% estimated for control sites. A monitoring framework that improves on estimating horse abundance and identifying responses of sage‐grouse and other key indicator species (plant and animal) would be beneficial to guide management decisions that promote co‐occurrence of horses with sensitive wildlife and livestock within landscapes subjected to multiple uses. Published 2021. This article is a U.S. Government work and is in the public domain in the USA. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society. : We evaluated the influences of feral horse abundance on the population dynamics of greater sage‐grouse. Sage‐grouse population abundance declined substantially when horse abundance increased above maximum appropriate management levels established by the Bureau of Land Management. Results can be used to identify management actions that preserve a balance between the presence of feral horses, other managed livestock, and native wildlife within sagebrush ecosystems. [ABSTRACT FROM AUTHOR]

Abstract: We compared soil-surface penetration resistance and abundance of ant mounds at 12 western Great Basin sites (composed of 19 plots) either grazed by feral horses (Equus caballus) or having had horses removed for the last 10–14 years. Across this broad spatial domain (3.03million ha), we minimized confounding due to abiotic factors by selecting horse-occupied and horse-removed sites with similar aspect, slope, fire history, grazing pressure by cattle (minimal to none), and dominant vegetation (Artemisia tridentata). During both 1997 and 1998, we found 2.2–8.4 times greater abundance of ant mounds and 3.0–15.4 times lower penetration resistance in soil surfaces at horse-removed sites. In 1998, thatched Formica ant mounds, which existed predominately at high elevations, were 3.3 times more abundant at horse-removed sites, although abundance varied widely among sites within treatments. Several types of analyses suggested that horses rather than environmental variability were the primary source of treatment differences we observed in ecosystem components. Tests of several predictions suggest that alterations occurred through not only direct effects, but also indirect effects and potentially feedback loops. Free-roaming horses as well as domestic grazers should be considered in conservation planning and land management in the Great Basin, an ecoregion that represents both an outstanding conservation opportunity and challenge. [Copyright &y& Elsevier]