Bison graze. But they selectively graze on dominant grasses, avoiding most forbs (broad-leaved herbs as opposed to grasses) and woody species. This results in a patchy distribution of vegetation, favoring increased plant species diversity. Avoiding forbs allows greater gas exchange, above ground biomass and density of plant cover (Fallon 2009). Their grazing style also encourages new growth. The bison tend to bite off the top parts of grass rather than tear away the near-ground growing shoots as do cattle and sheep, promoting new growth. Greater photosynthesis also results from this due to increased light availability (Wallace, 1990). But Bison do not graze in one location, they roam. While bison grazing is intense, roaming mitigates the adverse effects, rendering them temporary, encouraging the grasslands to recover. A nomadic herd of bison will move on from an area in which they grazed and not return for several years, preventing over-trampling of the grasses, and fouling by excessive urine and dung. Roaming transports seeds as well. Seeds are caught in the bison’s hair, dropping off later, or mixed in the bison’s excrement; also to be dropped off later (Bailey 2013. 45).
In addition, bison wallow and trample. Wallowing dusts their coats or, if the ground is wet, covers them in mud to hold the insects at bay. They trample, well, just because they have to stand on something. These actions stir up the soil and push seeds into ground, encouraging new seeds to take hold and germinate. In conjunction with the rain the depression and compaction resulting from the rolling back and forth of the bison, retains water and creates miniature wetlands. These depressions may seem rather small compared to the vastness of the Great Plains, but when bison numbered in the millions, the number of wallows created would more than make up for their size. The resulting network of miniature wetlands scattered across the Great Plains, in turn, provides habitat for frogs, toads, insects and birds, and watering-holes for other wildlife (Bailey 2103. 45). Additionally, the compaction also encourages a hardier type of vegetation that is more drought and fire resistant (Collins and Barber, 1986).
After an arduous day of grazing, bison do get thirsty and will visit the neighborhood stream, affecting the streamside vegetation. They do trample the streamside rather severely, but, again, because of their roaming habit, their streamside visitations are intermittent. The positive effects of this can be seen through a comparison with the predilections of cattle. Cattle will stay close to water, trampling the ground over and over. They eat steadily woody plants such as willow and dogwood that act to slow down flood waters and protect banks. Cattle eat the sedges that filter out sediment, muddying the water and preventing bank build up (Callenbach 1996. 30). Bison tend to water and then move on. Perhaps the greatest effect of bison grazing lies under ground. Bison affect the nutrient cycling of the soil through death and decay and through urine and dung; a feast to the soil. Though the nitrogen rich soil underneath a bison carcass is initially toxic, after a period of time the toxicity dissipates, rendering the soil nutrient friendly for early successional species of plants (Knapp, et. al. 1999). But bison provide nitrogen to the soil on a more on-going basis through the consumption of plant matter, digestion and the excretion of urine and feces. Urine is more effective in returning nitrogen to the soil than is the break-down of plant matter, encouraging greater plant growth (Ruess and McNaughton 1988). Feces encourages dung-dependent insects such as ants and dung beetles which interact with grasslands fungi (Bailey 2014. 45) and effect the absorption of nutrients from the dung into the soil to be used by plant-life (continue to part 3).