Prairie dogs and wildfires shape vegetation structure in a sagebrush grassland more than does rest from ungulate grazing

Understanding drivers of vegetation structure has direct implications for wildlife conservation and livestock management, but the relative importance of multiple disturbances interacting within the same system to shape vegetation structure remains unclear. We investigated the separate and interactive effects of multiple disturbance drivers on vegetation structure through a three-tiered, large-scale manipulative experiment in northeast Wyoming, USA. We used nested grazing exclosures to isolate the effects of herbivory from livestock, wild ungulates, or small mammals within areas affected by either historical wildfire, black-tailed prairie dog (Cynomys ludovicianus) colonies, or neither disturbance. We analyzed the interactive effects of disturbance history and contemporary herbivory by either small mammals alone, small mammals + native ungulates, or small mammals + native ungulates + livestock on vegetation structure by quantifying vegetation height, visual obstruction, shrub density, shrub canopy, and shrub leader growth. The exclusion of wild ungulates and lightly to moderately stocked livestock for two years did not significantly affect herbaceous vegetation structure, shrub density, or shrub canopy cover. Maximum vegetation height, visual obstruction, and heights of grasses and forbs were similar to 50% lower on black-tailed prairie dog colonies than in undisturbed areas. Prairie dog colonies contained 71% lower shrub densities than undisturbed sites. Sites with wildfire or black-tailed prairie dogs had 89% lower canopy cover of shrubs and Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), when compared to undisturbed sites. Shrub leaders experienced over 4.5 times more browsing on prairie dog colonies, when compared to undisturbed areas. For most metrics, disturbance history did not modify the effects of contemporary herbivory on vegetation structure. However, shrubs on prairie dog colonies experienced significantly more leader browsing in the combined presence of livestock, native ungulates, and small mammals than in treatments where livestock were excluded. Our research has direct implications for wildlife conservation and range-land management by demonstrating that short-term (1-2 yr) rest from large ungulate grazing may not substantially alter vegetation structure in this system. Instead, structural variation is strongly driven by black-tailed prairie dog colonization and historical wildfire. Understanding and managing multiple, potentially interacting disturbances is critical for maximizing wildlife conservation and livestock production in heterogeneous landscapes.