Effects of Grazer Identity (Bison Versus Cattle) and Woody Encroachment in Mixed Grass Prairies in Adjacent National Parks and National Grasslands.

In the Great Plains of North America, habitat loss has reduced grasslands to 30% of their pre-colonization extent. Climate change, defaunation, invasive species, and other global pressures are exacerbating this loss. Some conservation efforts to maintain grasslands and their biodiversity have prioritized reintroducing the American bison (Bison bison), once the continent's largest and most widespread native grazer. Our study measures the effect of two National Parks with long-term bison reintroductions in mixed-grass prairies—Badlands National Park and Theodore Roosevelt National Park—compared to nearby areas with the more typical management of grazing by cattle—Buffalo Gap National Grassland and Little Missouri National Grassland. The motivation of this work was to expand the number of sites with quantified bison impacts, because the literature is currently heavily biased toward areas like tallgrass prairie and parts of the Rocky Mountains (e.g., Yellowstone National Park). We found that bison presence in mixed-grass prairies did not increase plant species richness or diversity at the sites we measured compared to areas grazed by cattle. Instead, at one of our sites, species diversity was lower in areas with bison compared to those grazed by cattle, potentially due to elevated invasive plant cover (Bromus spp.). In the remaining sites, bison–cattle comparisons were not significantly different. Woody encroachment is another conservation threat in many grasslands. We found that encroached plant communities were distinct from open grasslands. In two out of three sites, small amounts of encroachment (20%–50% landscape cover) increased species richness, but at all sites, more severe encroachment decreased Shannon's Diversity index. While critical for many restoration efforts, our results suggest that bison reintroduction exhibits variable outcomes across grassland types, emphasizing the need for site-specific conservation approaches. Our findings highlight the need for more expansive research on bison's effects throughout the Great Plains to inform future conservation and management decisions as bison reintroduction efforts continue. [ABSTRACT FROM AUTHOR]