Where Bison Roam, Prairies Thrive
Ratajczak, Z., Collins, S. L., Blair, J. M., Koerner, S. E., Louthan, A. M., Smith, M. D., … & Nippert, J. B. (2022). Reintroducing bison results in long-running and resilient increases in grassland diversity. Proceedings of the National Academy of Sciences, 119(36), e2210433119.
The Konza Prairie Biological Station in eastern Kansas, operated by Kansas State University, is a world-class prairie research site. It’s part of the Flint Hills ecoregion, home to the largest intact tallgrass prairie on the North American Continent. Researchers are using the site to compare management strategies on prairie land.
The site is divided into three distinct sections for measurement. One is left ungrazed by large ungulates so the grasses and woody plants (forbs) grow uninterrupted. The second is grazed seasonally by cattle—a common land use in the Western United States. The third is grazed year-round by bison. Each of the three subdivisions have near identical soil composition, annual rainfall, and have never been plowed for agriculture. The three pastures of the Konza prairie make it the perfect comparative study site to observe different grazing practices.
A study published in the Proceedings of the National Academy of Science (PNAS) tracked how these three different land uses affected plant communities. The study took twenty-nine years and provides decades of data utilized by biologists to track the effects of bison reintroduction on the landscape. The study asked two questions: How do bison and cattle grazing impact the abundance and kinds of plants that grow in a tallgrass prairie? And, how does bison grazing affect plant community’s ability to cope with extreme drought? The results are striking.
The pastures grazed by the bison had a native plant species richness increase of 86 percent compared to the ungrazed portion of the prairie. Dozens of native plant species returned to the prairie. This suggests that the long-term presence of bison is inherently linked to these plants’ survival. Native plant diversity increased by only 30 percent under the cattle treatment—less than half the positive effect of bison (86 percent). As an added benefit, these gains in biodiversity were resilient to extreme drought. Bison-mediated restoration creates effective and durable conservation outcomes.
To explain these results, researchers hypothesize that bison exhibit a trait called keystone herbivory. This means that they control dominant grasses that would otherwise outcompete and take over the prairie ecosystem. Bison selectively graze four grass species that proliferate in the ungrazed system. It’s also hypothesized that bison boost biodiversity through a behavior called wallowing. Bison bathe in dust and dirt, creating shallow depressions in prairie soil that hold rainwater and runoff. Wallows create new and unique habitats where native plant species can gain a foothold. Selective grazing and landscape alteration through wallowing create a competitive advantage for dozens of new native plant species to return to the landscape, resulting in richer, more biodiverse grasslands.
A richer assortment of plant species gives rise to a greater variety of insects. Small mammals eat those plants, which in turn provides more options for their predators. Things come into balance. Life flourishes within these conditions of complexity, and bison are key to bringing about such biodiversity.
Going forward, land managers should consider the effects that bison have on the prairie. With climate change, drought will become longer, hotter, and more extreme on the Great Plains. Bison are a species that has great potential to maintain and enhance ecosystem biodiversity and resilience in the 21st century.