Diet overlap estimation among species is important to understand interspecific interactions. Through these interactions, one species may extinguish others through competition or adapt through mechanisms such as resource partitioning. The introduction of domestic herbivores in a native assemblage may introduce competition for resources with wild herbivores. In southern Patagonian steppes, guanaco (Lama guanicoe) populations are increasing after a drastic demise during the past century. This is occurring in protected areas and extensive ranching areas for sheep, and in the latter ranchers have rising concerns regarding competition. In this observational study, we measured diet overlap, food selection and preferences, and niche width for guanaco and sheep to compare use of food resources during the summer season. Feces were analyzed through microhistological technique to determine sheep and guanaco diets. Diets of guanaco and sheep were dominated by tussock grass (Festuca gracillima, sheep > 60%, guanaco > 50%), the most abundant plant species. Although food niche widths, measured through Levin’s Index, were narrow, guanaco presented the broadest niche (P < 0.001). Of the 17 identified plant species present in guanaco and sheep diets, 5 were always selected by both ungulates and 1 was always avoided at the four study sites. Overlap between diets was high (> 0.90), evidencing potential competition. In addition, diet overlap coupled with our results of small variation in use and selectivity of food resources suggest the absence of resource partitioning. Therefore, both species might share resources or coexist, possibly due to high availability or spatial distribution of food resources and guanaco resilience to cope with numerically dominant competition from sheep. These findings offer new insight into the understanding of these species’ interactions. It is also a management challenge to emerging wild and domestic herbivores interactions, as well as livestock production concerns at large.

物种之间的饮食重叠估计对于理解种间相互作用很重要。通过这些相互作用，一个物种可能会通过竞争而灭绝其他物种，或者通过资源分配等机制进行适应。在本地种群中引入家庭食草动物可能会导致与野生食草动物争夺资源。在南部的巴塔哥尼亚草原上，骆马角（Lama guanicoe）在过去一个世纪急剧消亡之后，人口在增加。这种情况发生在保护区和广泛的绵羊牧场中，在后者中，牧场主对竞争的关注日益增加。在这项观察性研究中，我们测量了饮食重叠，食物的选择和偏好以及骆驼和绵羊的利基宽度，以比较夏季食物资源的使用。通过微观组织学技术分析了粪便，以确定绵羊和瓜纳科的饮食。骆驼科和绵羊的饮食以丛生的草为主（Festuca gracillima，绵羊> 60％，骆驼科> 50％），是最丰富的植物种类。尽管通过莱文指数测得的食物生态位宽度较窄，但瓜纳科地区的生态位宽度最宽（P<0.001）。在番石榴和绵羊日粮中发现的17种植物中，有蹄类动物总是选择5种，在四个研究地点总是避免1种。两种饮食之间的重叠率很高（> 0.90），表明存在潜在的竞争。此外，饮食重叠以及我们对食物资源的使用和选择性的微小差异导致的结果表明，缺乏资源分配。因此，这两个物种可能共享资源或共存，这可能是由于粮食资源的高度可利用性或空间分布以及羊驼的适应能力来应对绵羊在数量上占优势的竞争。这些发现为了解这些物种之间的相互作用提供了新的见识。这对于新出现的野生和家畜食草动物相互作用以及整个牲畜生产问题也是管理上的挑战。