The North American bison, a foundational herbivore within its native prairie ecosystems, plays a unique role in shaping the landscapes they inhabit. Their grazing habits and physiological adaptations are deeply intertwined with the health and biodiversity of the prairies. Understanding their dietary needs and behaviors illustrates their profound influence on this dynamic environment.
Primary Food Sources
Bison are primarily grazers, meaning their diet largely consists of grasses and grass-like plants, rather than woody vegetation. Grasses and sedges comprise the vast majority of their food intake, often exceeding 90% by volume. Specific grass types consumed include cool-season grasses like bluegrass and rye grass, prevalent in cooler periods, and warm-season grasses such as big bluestem, sand dropseed, and little bluestem, dominant in warmer months.
While grasses form the bulk of their sustenance, forbs (broad-leaf flowering plants) also contribute to the bison’s diet. These include legumes like clover and lupin, and non-legumes such as asters. Though smaller in volume, forbs add dietary variety and nutrients. Bison are distinct from “browsers” like deer, which consume leaves, twigs, and bark from shrubs and trees.
Seasonal Dietary Adaptations
The bison’s diet undergoes notable shifts throughout the year, adapting to the seasonal availability and nutritional content of plants. During lush spring and summer, bison thrive on abundant green grasses and sedges, offering high protein. As fall progresses into winter, fresh forage decreases, leading bison to rely on drier, cured grasses.
When snow covers the ground, bison adapt to access buried forage. They use their massive heads and powerful neck and shoulder muscles to sweep snow aside, creating trenches to reach the underlying grasses. This “snowplowing” behavior allows them to forage in snow depths up to four feet. If grasses are scarce, bison supplement their diet with woody vegetation, including twigs and bark from shrubs and sometimes tree bark, such as willow and aspen.
The Ruminant Digestive System
Bison possess a specialized digestive system to efficiently process the tough, fibrous plant material they consume. Like other ruminants, they have a four-chambered stomach, which includes the rumen, reticulum, omasum, and abomasum. This complex system facilitates the breakdown of cellulose, a primary component of plant cell walls, which most animals cannot digest.
Digestion begins when bison chew and swallow their food, which then enters the rumen, the largest chamber. Here, a dense community of symbiotic microbes (bacteria, protozoa, and fungi) ferments the plant material. This microbial action breaks down cellulose into usable nutrients, such as volatile fatty acids, absorbed for energy. Partially digested food, known as “cud,” is regurgitated for further chewing, increasing surface area for microbial action before passing through the remaining stomach chambers for further digestion and nutrient absorption.
Impact on the Prairie Ecosystem
The grazing patterns of bison profoundly influence the structure and biodiversity of prairie ecosystems. Their selective consumption of dominant grasses creates a mosaic of varying grass heights, rather than uniform vegetation. This patchwork of grazed and ungrazed areas provides diverse habitats, benefiting many other species. For instance, short-grass areas created by bison grazing are preferred by species like the Horned Lark and Thick-billed Longspur, while taller grass patches support others.
Bison also contribute to nutrient cycling within the prairie. Their waste (urine and feces) returns essential nutrients like nitrogen, phosphorus, calcium, and magnesium to the soil, acting as a natural fertilizer. This process supports soil microorganisms and attracts insects like dung beetles, aiding nutrient decomposition and distribution. Additionally, bison facilitate seed dispersal through their fur and droppings, spreading various plant species and increasing floral diversity.