Yellowstone National Park is a unique and largely intact ecosystem, home to the American bison. Yellowstone harbors one of the few continuously wild bison populations in North America, tracing its lineage back to a small remnant group of about two dozen individuals that survived the 19th-century slaughter. This herd represents the largest bison population managed on public lands, symbolizing successful conservation efforts.
Current Population Estimates
Yellowstone’s bison population regularly fluctuates due to natural cycles and management activities. In 2024, the estimated pre-calving population was 4,550, with the overall population reaching about 5,400 animals. The population consists of two primary breeding herds: the northern herd (around 2,300 individuals) and the central herd (about 1,400 animals). A new 2024 management plan aims to maintain the population between 3,500 and 6,000 animals after calving, targeting an average of 5,000.
Methods for Counting Bison
Estimating bison numbers in Yellowstone National Park involves specialized techniques. Park biologists primarily conduct aerial surveys to count the animals across the vast landscape. These surveys are typically performed annually, often in winter when bison are more visible against the snow. Challenges arise from rugged terrain, varied habitats, and the tendency of bison to form large or scattered groups, which can affect detection rates.
To enhance the accuracy of population estimates, these aerial counts are supplemented with ground observations. Researchers also employ telemetry, such as radio collars, to track the movements of smaller groups and monitor survival rates of individual bison. This comprehensive approach allows park managers to integrate various data points, including male-to-female and female-to-calf ratios, to understand the herd’s status and trends.
Managing the Yellowstone Bison Population
Managing the Yellowstone bison population is a complex undertaking, balancing conservation goals with external concerns, particularly regarding disease and migration beyond park boundaries. Brucellosis, a bacterial disease causing abortions in cattle and bison, has been a significant challenge. While no documented cases exist of wild Yellowstone bison directly transmitting brucellosis to cattle, livestock industry concerns have historically influenced management strategies. Recent scientific understanding indicates that elk are more likely to transmit brucellosis to cattle than bison.
To address these issues, management efforts include testing and, at times, hazing or culling bison that leave the park. A key initiative is the Bison Conservation Transfer Program (BCTP), which captures, tests, and relocates brucellosis-free bison to tribal lands, helping them contribute to other wild herds. Bison migration beyond the park’s borders into private or state lands has historically led to conflicts.
In response to these challenges, the Interagency Bison Management Plan (IBMP) was adopted in 2000, bringing together federal, state, and tribal entities to guide bison management. This collaborative framework involves partners such as the National Park Service, USDA-APHIS, the U.S. Forest Service, Montana Department of Livestock, Montana Fish, Wildlife & Parks, and several American Indian tribes. A new management plan was announced in 2024, reflecting updated scientific information and aiming to maintain a wild, free-ranging population while continuing to work with partners to manage bison movements and disease risk.
Ecological Significance of Yellowstone Bison
Yellowstone bison are keystone grazers, playing a fundamental role in shaping the park’s diverse ecosystems. Their grazing patterns directly influence plant communities, promoting biodiversity by creating varied habitats and facilitating nutrient cycling through their waste. Their trampling hooves aerate the soil and aid in seed dispersal, contributing to grassland health and regeneration.
In winter, bison clear pathways through deep snow, uncovering vegetation accessible to other wildlife like elk and pronghorn. After death, bison carcasses provide food for scavengers, and their remains return nutrients to the soil. Beyond their ecological contributions, Yellowstone bison hold profound historical and cultural significance for numerous Native American tribes. For millennia, these animals provided sustenance, materials for shelter and clothing, and held a central place in spiritual traditions.