Yellowstone's roaming bison herds enhance nutrient cycles and boost ecosystem health at landscape scales, according to a new study. The findings, which challenge conventional grazing wisdom, suggest that restoring large-scale migrations could unlock the species' full ecological power. Historically, North America supported tens of millions of bison whose seasonal migrations transformed the continent's vast grassland ecosystems. Today, these once massive herds of wild, free roaming bison are no more; only about 400,000 bison remain, and almost all exist in small managed herds on private land or within parks and reserves. Although research suggests that bison play a powerful role in shaping ecosystems by diversifying habitats, influencing plant communities, and driving processes like nutrient cycling and productivity, the broader ecological impacts of large, migrating herds remain poorly understood because modern bison are mostly confined to limited areas. The restoration of bison migrations in the northern Yellowstone ecosystem, however, provides a rare natural laboratory for understanding how large herbivores shape ecosystems at the landscape scale.
Between 2015 and 2022, Chris Geremia and colleagues tracked bison grazing dynamics across 16 sites representing the animal's three main habitats, and measured their impact on carbon and nitrogen dynamics, plant communities, and soil microbiology. Geremia et al. found that bison stabilized plant production while accelerating nitrogen cycling, boosting aboveground nitrogen and improving landscape nutritional quality, particularly in wet, nutrient-rich areas that hosted bison densities and grazing at levels higher than typically recommended. Soil microbe density and nitrogen content in soils and plants also increased in grazed areas. According to the authors, the findings show that the ecological power of large, migrating herbivores lies not only in their size but in their numbers, density, and freedom to migrate. "To move forward with conserving migratory herbivores and grassland ecosystems, we must embrace landscape-scale heterogeneity – not at the scale of individual ranches or pastures, but at sizes that allow for thousands of migrating large herbivores to move freely across the landscape," write Geremia et al. "Our findings emphasize that ecosystems with large native herbivores, such as bison, can function successfully in today's world."
