In the western U.S., large ungulates have played a major role in shaping sagebrush-dominated landscapes. A great deal of attention has focused on the cascading ecosystem effects of historic livestock grazing, mediated primarily via exotic species invasions. In contrast, the effects of wild ungulates, which also use these habitats, have been relatively under-emphasized. Better understanding wild ungulate effects (and their potential interactions with domestic livestock) is necessary for both current management of these landscapes and anticipating the ecosystem consequences of their loss and/or replacement by livestock. Here we use long-term exclosures to investigate how sagebrush plant communities respond to herbivory by both cattle and wild ungulates, including elk and mule deer. Our study areas were located in northern Utah and the Colorado Plateau, two areas that, respectively, represent the more mesic and drier ends of sagebrush distribution. We assessed responses of woody and herbaceous components of the plant community, as well as that of biological soil crusts.
In northern Utah big sagebrush sites we found that wild ungulates and (highly managed) cattle each elicited perceptible changes to the native vegetation, but did not precipitate major state changes such as those commonly observed elsewhere in the Great Basin. Slightly increased abundance of invasive species was associated with cattle presence, whereas reductions in biological soil crusts were associated with both wild and domestic herbivores. In Colorado Plateau sites, wild ungulate herbivory modified the size structure of sagebrush populations and shifted the plant community from shrub to grass dominance, though the extent of this modification appeared to be mediated by soil depth, an abiotic factor. Both cattle and wild ungulates also altered ground cover attributes, particularly biological soil crusts, a key functional component of this ecosystem. Together these results illustrate how cattle and wild ungulate effects on sagebrush-dominated landscapes vary according to presence of other herbivores, abiotic context, and likely total combined stocking rates of domestic and wild herbivores.