Data from: Grazing by non-native ungulates negatively impacts vegetation important to a native species of concern

Non-native grazers compete with native species across the globe. In the
northwestern Great Basin of the western United States competition among
livestock, feral horses, and Greater Sage-grouse has been the subject of
numerous legal actions and management policies, yet spatially explicit
temporal data documenting the details of this competition are lacking. We
present a novel approach to studying the composition of the herbaceous
understory across three study areas within the Great Basin with different
historic and contemporary grazing regimes.  We surveyed the
landscape using distance sampling for livestock and horse feces as an
index of use. In addition, we surveyed the herbaceous understory of random
sites as well as sites chosen by female Greater Sage-grouse to nest and
brood their chicks. We used a novel Bayesian hierarchical modeling
framework to link vegetation metrics with the spatial-temporal
distribution of horses and livestock while accounting for observation
error. When livestock and feral horses were not present, we found that
Greater Sage-grouse chose sites with higher percentages of perennial
grasses and forbs to build their nests and brood their chicks compared to
what was available to them. As livestock increased, we found evidence for
decreases in the percentage of perennial grasses, forbs, cheatgrass
(Bromus tectorum), and increases in the amount of bare ground. These
effects were consistent at available sites and brood sites, however, we
found less evidence for an impact of livestock at nest sites. As feral
horses increased, we observed similar results at available sites, but at
sites chosen by females to nest and brood their chicks, we observed
increases in the amount of invasive cheatgrass as feral horses increased,
which could reflect attempts by Greater Sage-grouse to compensate for
reductions in protective cover.  We present a noninvasive
approach to assess space use that can be applied to other species. More
importantly, we document that grazing by non-native ungulates impacts
components of the plant community important to Greater Sage-grouse
reproduction. We provide spatial-temporal maps of livestock and feral
horse use to aid managers attempting to balance the needs of livestock
producers, feral horses, Greater Sage-grouse, and ecosystem function.