The American Bison is the largest land mammal in North America and once roamed the continent in the tens of millions. Its near-total extermination in the 19th century fundamentally altered the ecology of grassland systems. Today, conservation efforts focus on restoring this species to its native range, raising a central ecological question: Does the bison meet the strict criteria for classification as a keystone species? This designation is reserved for a select group of organisms whose influence on their environment is exceptionally powerful. Answering this requires a detailed examination of the bison’s unique ecological functions.
Defining Keystone Status
The concept of a keystone species refers to an organism that has a disproportionately large effect on its ecosystem relative to its abundance. Removing this species would trigger a cascade of negative effects, significantly reducing diversity and altering the community’s physical structure. The keystone species is not necessarily the most numerous or the largest by mass, but its specific activities maintain the ecosystem’s balance. This classification differs from a dominant species, defined by its sheer biomass, and a foundation species, which physically creates the habitat, such as corals building a reef. A true keystone species is a low-abundance player whose presence or absence determines the fate of many other species and the overall community structure.
Grazing Patterns and Nutrient Cycling
Bison herbivory creates a diverse landscape structure crucial for prairie health. They are selective grazers, preferring certain grasses and leaving other plant types, such as flowering forbs, untouched. This selective pressure prevents fast-growing grass species from dominating, allowing a greater variety of plants to thrive. This uneven grazing results in a mosaic of habitat patches, featuring short, intensely grazed grasses alongside taller vegetation. This heterogeneity is important for insects and small mammals that require different vegetation heights for shelter and foraging. Bison also play a direct role in nutrient cycling by depositing nutrient-rich dung and urine, which accelerates the nitrogen cycle and makes nutrients more quickly available to plants.
Ecosystem Engineering Through Wallowing
A behavior unique to the bison is wallowing, where the animals repeatedly roll in the dirt, creating shallow depressions called wallows. This physical disturbance is a powerful form of ecosystem engineering that directly shapes the landscape. These indentations expose bare soil by removing vegetation, which helps break up plant monocultures and allows seeds to germinate. When it rains, wallows collect water, temporarily forming small, muddy wetlands that support specialized plant life and function as breeding grounds for amphibians and aquatic insects. This cyclical process of creation and abandonment ensures the constant renewal of diverse micro-habitats across the prairie.
Documenting Biodiversity Changes After Removal and Reintroduction
The most compelling evidence for the bison’s classification comes from observing the changes that occur when they are removed or successfully restored to an ecosystem. The historical removal of bison led to a significant loss of prairie biodiversity, while long-term studies of reintroduction efforts have documented dramatic increases in species richness. For example, reintroducing bison to tallgrass prairies has been shown to double the native plant species richness compared to ungrazed control plots. This positive effect is greater than that achieved by traditional cattle grazing, highlighting the unique ecological value of the bison’s specific behaviors. Ultimately, the mosaic of grasses created by selective grazing and the foraging sites provided by wallows result in a more resilient and diverse ecosystem.