Free-roaming equids (i.e., feral horses [Equus caballus] and burros [Equus asinus]) are widely distributed and locally abundant across the rangelands of the western United States. The 1971 Wild Free Roaming Horse and Burro Act (WFRHBA) gave the Bureau of Land Management (BLM) and United States Forest Service (USFS) the legal authority to manage these animals on designated public lands. To fulfill this responsibility, federal agencies established an Appropriate Management Level (AML), defined as the number of horses or burros that can be sustained on a given management unit under prevailing environmental conditions and land uses. Although the WFRHBA specifies that feral equids must be managed in ecological balance with other land uses, including conservation of native wildlife, population control measures such as gathers, contraception, and adoptions have failed to keep pace with intrinsic growth rates. Over 80% of federally managed herds currently exceed prescribed population levels, making the potential for competition between native ungulates and feral equids a growing concern among state wildlife agencies. Mule deer (Odocoileus hemionus), pronghorn (Antilocapra americana), elk (Cervus canadensis), and bighorn sheep (Ovis canadensis) are of ecological and economic value to the states where they occur, and all exhibit some degree of distributional, habitat, or dietary overlap with horses or burros. Notwithstanding the scale of the problem, to date there have been no range-wide assessments of competition potential among native and feral ungulates for space, forage, or water. To address this need, we compiled demographic, jurisdictional, and species occurrence data collected from 2010–2019 by federal and state agencies. We used these data to map the distributions of 4 native ungulate species across federal equid management units (FEMUS) in 10 western states (n = 174). We then made within-state rankings of the 50 units that were ≥2 times over AML and encompassed ≥3 native ungulates. Collectively, FEMUs covered approximately 225,000 km², representing 18% of all BLM and USFS lands in affected states. Each FEMU supported ≥1 native ungulate and 14% contained all 4. The degree of overlap between native and feral species varied by state, ranging from <1% for mule deer in Montana, to 40% for bighorn sheep in Nevada. Oregon had the largest proportion of units that supported all 4 native ungulates (58%), whereas Montana and New Mexico had the fewest equids, but all populations were over target densities. Despite the perception that the problem of equid abundance is limited to the Great Basin states, high intrinsic growth rates and social constraints on management practices suggest all affected states should monitor range conditions and native ungulate demography in areas where forage and water resources are limited and expanding equid populations are a concern. © 2021 The Wildlife Society.

中文翻译：

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西部牧场本土有蹄类动物与自由漫游马科动物之间竞争潜力的分布

自由漫游的马科动物（即野马 [ Equus caballus ] 和驴[ Equus asinus]) 在美国西部的牧场上广泛分布且当地丰富。1971 年的野生自由漫游马和驴法案 (WFRHBA) 赋予土地管理局 (BLM) 和美国林务局 (USFS) 在指定公共土地上管理这些动物的合法权力。为履行这一责任，联邦机构建立了适当的管理水平 (AML)，定义为在当前环境条件和土地使用情况下，给定管理单位可以维持的马或驴的数量。尽管 WFRHBA 规定必须在与其他土地用途（包括保护本地野生动物）的生态平衡中管理野生马科动物，但收集、避孕和收养等人口控制措施未能跟上内在增长率。目前，超过 80% 的联邦管理的畜群超过了规定的种群水平，这使得本地有蹄类动物和野生马科动物之间的竞争潜力成为各州野生动物机构​​日益关注的问题。骡鹿 （Odocoileus hemionus )、叉角羚 ( Antilocapra americana )、麋鹿 ( Cervus canadensis ) 和大角羊 ( Ovis canadensis ) 对它们出现的州具有生态和经济价值，并且都表现出一定程度的分布、栖息地或饮食重叠。马或驴。尽管问题很严重，但迄今为止，还没有对本地和野生有蹄类动物在空间、饲料或水方面的竞争潜力进行范围广泛的评估。为了满足这一需求，我们汇编了联邦和州机构从 2010 年到 2019 年收集的人口、辖区和物种发生数据。我们使用这些数据绘制了西部 10 个州联邦马科动物管理单位 (FEMUS) 中 4 种本地有蹄类动物的分布图（n  = 174）。然后，我们对 50 个单位进行了州内排名，这些单位是 AML 的 2 倍以上，并且包括 3 种以上的本地有蹄类动物。FEMU 总共覆盖了大约 225,000 公里²，占受影响州所有 BLM 和 USFS 土地的 18%。每个 FEMU 支持 ≥ 1 只本地有蹄类动物，14% 包含所有 4 种。本地和野生物种之间的重叠程度因州而异，从蒙大拿州长尾鹿的 <1% 到内华达州大角羊的 40%。俄勒冈州支持所有 4 种本地有蹄类动物的单位比例最大 (58%)，而蒙大拿州和新墨西哥州的马科动物最少，但所有种群都超过了目标密度。尽管人们认为马的丰度问题仅限于大盆地各州，但高内在增长率和管理实践的社会限制表明，所有受影响的州都应监测草料和水资源有限且不断扩大的地区的范围条件和本地有蹄类动物人口统计平等人口是一个问题。