Where the buffalo roam: Ungulate influences on quaking aspen and willow communities in the Greater Yellowstone Ecosystem

Quaking aspen (Populus tremuloides) and willows (Salix spp.) are keystone species of montane and shrub-steppe landscapes of the Western United States. Intact communities dominated by these species provide a wide range of ecosystem services, harboring an exceptional proportion of landscape biodiversity. Land use, especially overgrazing by large ungulates, is among the greatest threats to these ecosystems. To examine the effects of wild ungulates and levels of grazing at Yellowstone National Park (YNP) and the adjacent Gallatin National Forest, we sampled plant community composition and vegetation structure of aspen and willow communities both inside and outside of exclosures. Within the Park, we found that current grazing pressures from large ungulates, principally American Bison (Bison bison), have a dramatic effect on community composition and structure, resulting in a shift from a structurally diverse forest or tall shrub dominance to that of a grassland. On heavily grazed sites, shrubs common to semiarid uplands are now relatively abundant, as are exotic grass species, in contrast to an abundance of berry-producing shrubs within exclosures. Finally, large herbivores at unnaturally high densities are resulting in the simplification of landscape diversity outside of exclosures through a decline in the patch diversity of the site (i.e., homogenization of the landscape). Increases in exotic species and those adapted to semiarid environments suggest that current levels of bison use at YNP are amplifying the effects of climate change as well as resulting in a loss of biodiversity values. Inside exclosures, the inherent resilience of these keystone ecosystems was demonstrated by the recovery of plant diversity following decades of large ungulate utilization exceeding natural carrying capacity. This suggests that reductions in current grazing pressures within YNP would have positive feedbacks to beneficial ecosystem processes such as increased species and habitat diversity, increased carbon sequestration, and a greater adaptive capacity to climate change.