The Highs and Lows of Grazing: Effects of Ungulates and Elevation on Grassland and Sagebrush Steppe Vegetation Composition in Yellowstone National Park

Yellowstone National Park’s Northern Range is emblematic for the wolves, bison and elk that inhabit its grassland and sagebrush-steppe habitats.  Their interactions and populations have been the focus of considerable conservation debate. Recent changes in ungulate populations provide an opportunity to examine their influence on the vegetation communities. We conducted expansive vegetation surveys along an elevation gradient and at exclosure sites where ungulates were unable to graze. We collected a large database (n = 620 quadrats), which we analyzed using classic community ecology approaches. We found that non-native species have higher abundances at low elevations, and that grazing by ungulates reduces beta diversity and native cover, while promoting non-native cover. The magnitude of these changes are greater in bison-dominated than elk-dominated areas of Yellowstone, suggesting that bison may be overgrazing. These results provide valuable insights into major factors shaping vegetation communities, while also contextualizing management practices in a world-renowned ecosystem.

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