Livestock grazing on natural rangeland vegetation is one of the most extensive land uses on the earth, with important implications for livelihoods, food security and the environment. Factors such as population growth and urban development, a shift from resource-based to service-based economies, and intensification in the livestock industry change the extent and practice of grazing worldwide. We investigated how and why livestock grazing on public lands changed since 1940 in the High Divide region of the Northern Rocky Mountains through a detailed analysis of United States Forest Service (USFS) rangeland management records. Based on a 90-year land use history, we process-traced the proximate causes of changes in grazing, identified the decision-makers, and statistically tested which underlying factors were associated with changes in grazing. The forage annually consumed by livestock in our study area declined by 62% since 1940, the equivalent of about 33,000 fewer cows grazing on public lands for a three-month summer period. Livestock grazing was closed on 21% of the total allotment area. The reductions in grazing were mainly caused by land management and policy factors: evaluations of range condition (27%), carrying capacity estimates (21%) and legal and administrative requirements (14%) derived from the Endangered Species Act (ESA) and National Environmental Protection Act (NEPA). The socio-economic causes of ranch economics (14%) and amenity migration (8%) were comparatively small. Overlap with wilderness and proximity to amenity towns were significant spatial predictors of reductions in grazing. The fate of publicly-owned but privately-used rangelands largely depends on institutions that are able to reconcile the competing values and demands that influence how they are managed.

在天然牧场的草地上放牧的牲畜是地球上使用最广泛的土地之一，对生计，粮食安全和环境具有重要意义。人口增长和城市发展，从资源型经济向服务型经济的转变以及畜牧业集约化等因素改变了世界范围内放牧的程度和做法。我们通过对美国森林服务局（USFS）牧场管理记录的详细分析，调查了1940年以来北部落基山脉高分地区在公共土地上放牧的牲畜如何以及为何发生变化。基于90年的土地使用历史，我们追踪了放牧变化的近因，确定了决策者，并进行了统计检验，哪些根本因素与放牧变化相关。自1940年以来，我们研究区域的牲畜每年所消耗的草料下降了62％，相当于在三个月的夏季期间，在公共土地上放牧的母牛减少了约33,000头。牲畜放牧占总分配面积的21％。放牧量减少的主要原因是土地管理和政策因素：评估范围状况（27％），估计承载力（21％）和法律和行政要求（14％）源自《濒危物种法》（ESA）和国家环境保护法（NEPA）。牧场经济的社会经济原因（14％）和舒适性移民（8％）相对较小。与荒野的重叠和与宜人城镇的接近是放牧减少的重要空间预测指标。