Riiven Wild
How Bison Wallows Add Wet Pockets to Dry Prairie
On the tallgrass prairie of Kansas, bison roll in the dirt until the ground itself changes shape, and the hollows they leave become temporary ponds, odd soils and new plant neighborhoods.
The Scene
Late spring on Konza Prairie Biological Station, in the Flint Hills of northeastern Kansas. A bull bison walks to a bare patch in the grass, drops to his knees, and rolls onto his side. He kicks, twists, rubs his shoulder into the dirt, and rolls again. Dust lifts and drifts across the big bluestem. When he stands and shakes, he leaves behind a shallow bowl of exposed soil with no grass in it. This is a wallow, and he did not make it alone. Bison have been rolling in this same spot for years, each visit pressing the hollow a little deeper. After a heavy rain, the bowl fills with water.
The Creature
The American bison is the largest land mammal in North America, and on Konza Prairie it lives as part of a long-running research landscape managed by Kansas State University. K-State biologists study the bison and the prairie together, because the animals do more than graze. They trample, they roll, and they reshape the ground. Wallowing is the behavior at the center of this story. A bison lies down on bare soil and rolls, again and again, at the same favored sites. The rolling strips away plants, loosens the surface, and compacts what lies beneath. Over time a wallow becomes a disturbance, a patch where the usual plant cover has been knocked back and the soil has been altered. Ecologists call an animal like this an ecosystem engineer, a species that physically builds or changes habitat that other organisms then use.
How It Works
The physical result is surprisingly large. At Konza Prairie, bison wallows can reach 5 m in diameter and 30 cm deep, and some hold water that serves as drinking water and habitat for wetland plants and animals. The soil inside changes too. A study of Konza Prairie wallows, reported in K-State research on tallgrass prairie soils, found lower organic matter and nitrogen in wallows than in comparison grassland soils, along with higher clay and sodium. That combination matters. Clay holds water and seals the bottom of the bowl, which helps explain why some wallows pond after rain. Less nitrogen and organic matter means a leaner, harsher place for prairie grasses to grow back. Plants respond accordingly. In a Konza Prairie study, wallows had lower plant species richness, meaning fewer kinds of plants, than non-wallow plots. Yet fenced wallows, closed off from bison, had higher richness than wallows bison could still reach. The continued rolling, not just the original hollow, keeps these patches distinct.
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The Ripple
Once a wallow holds water, other animals arrive. In 2003, Gerlanc and Kaufman monitored filled bison wallows at Konza Prairie as potential amphibian breeding habitat. Anurans, the group that includes frogs and toads, lay eggs in water, and their tadpoles need standing water until they transform into land-ready adults. In a wallow that pond is temporary, so the study noted that anurans must complete metamorphosis before the wallows dry. The plant community shifts as well. McMillan and colleagues described vegetation responses to wallows in tallgrass prairie, treating them as a distinct animal-generated disturbance. Hocking and colleagues later examined how wallows affect soil properties and vegetation composition. Together these studies show a grassland stitched with small, different patches: wetter, saltier, clay-rich hollows set into drier, richer sod.
Why It Matters
Bison rolling in the same spots for years carve hollows on Konza that change the soil, thin the plants, and, after rain, hold water long enough for frogs and toads to breed. Fence the bison out and the wallows begin to fill back in with more plant species. The bumps and puddles across the prairie are a record of where the bison have rolled.
Sources
- Gerlanc and Kaufman, Use of Bison Wallows by Anurans on Konza Prairie, 2003 (2003) · study
- K-State biologists study American bison and prairie home at Konza (2026) · study
- Impacts on tallgrass prairie soil, plant communities ... - K-REx · study
- McMillan et al., Vegetation Responses to an Animal-generated Disturbance (Bison Wallows) in Tallgrass Prairie, 2011 (2011) · context
- Hocking et al., Bison wallows effect on soil properties, vegetation composition and ..., 2022 (2022) · context
