Yellowstone National Park is home to the only place in the United States where American bison have continuously lived since prehistoric times. This population represents a remarkable conservation success, having rebounded from a low of just a few dozen animals in the early 1900s. Today, the park’s wild bison herd holds immense ecological and cultural significance, functioning as the largest and oldest public herd in the country. The question of how many bison are in Yellowstone is complex, as the figure is not static but constantly managed, fluctuating based on seasons and specific management actions. This article will detail the current size of this iconic herd, explain the scientific methods used to determine that number, and outline the management strategies that govern its population dynamics.
The Current Population Estimate
The Yellowstone bison population is not a fixed number but rather a dynamic estimate that changes significantly throughout the year due to natural factors and human intervention. The National Park Service maintains an official target range of 3,500 to 6,000 animals after the annual calving season. This range is designed to sustain a genetically viable, wild population while also considering the constraints of the surrounding ecosystem and policy agreements. The population is typically at its highest immediately following the summer calving season, and estimates are often given as a post-calving figure. For example, the population was estimated to be 5,449 bison in August 2024, which includes the new calves born that spring. The number begins to decrease naturally through the winter months due to mortality and is further reduced by interagency management actions before the next spring’s calving begins.
Bison Counting Methodology
Park staff and partner agencies employ a sophisticated, multi-faceted approach to accurately estimate the bison population, which goes beyond a simple visual count. The primary method involves annual aerial surveys conducted from fixed-wing aircraft, often flown in the late summer when bison are highly visible and aggregated during the breeding season. These flights aim to census all areas of the park where bison are known to congregate, providing a raw count of the herd.
To increase accuracy, this raw aerial data is integrated with other scientific metrics. Ground surveys are conducted simultaneously to estimate the detection probability, accounting for animals missed by aerial observers due to group size or dense cover. Photographic analysis is also used to correct for undercounting, as larger groups of bison are sometimes difficult to count precisely from the air. Finally, the raw counts are incorporated into demographic models that use survival rates from radio-collared bison and ground-based calf-to-female ratios to create the final population estimate.
Population Control and Management
The population of Yellowstone bison is actively managed through a framework known as the Interagency Bison Management Plan (IBMP). This plan is a collaborative effort involving federal, state, and tribal entities. Its central goal is to maintain a healthy, wild bison population while minimizing the risk of disease transmission to domestic cattle outside the park. The primary concern driving this management is the bacterial disease brucellosis, which can cause domestic livestock to abort calves, although recent data suggests elk are a more frequent source of transmission to cattle than bison.
Management actions are primarily focused on controlling the population when bison migrate out of the park during the winter in search of forage. Methods used to control the numbers include hazing, which involves using horseback riders or snowmobiles to drive bison back inside the park boundary. When the population exceeds the desired threshold, the most significant control methods involve the use of the Stephens Creek capture facility near the northern boundary.
Bison captured at this facility are either removed from the population or channeled into programs designed to restore the species elsewhere. Removals occur through regulated tribal and state hunts outside the park boundary, as well as through the Tribal Food Transfer Program, which provides meat and hides to Native American tribes. A major priority in recent years has been the Bison Conservation and Transfer Program (BCTP), which quarantines and transfers brucellosis-free bison to tribal lands, providing a path for live restoration instead of removal.
Habitat and Herd Distribution
The Yellowstone bison herd is generally divided into two distinct groups based on their primary geographic ranges: the Northern Herd and the Central or Mesa Herd. These groups utilize different ecological zones within the park.
The Northern Herd primarily ranges across the northern portion of the park, including the Lamar Valley and the high-elevation Mirror Plateau. The Central Herd inhabits the core of the park, frequenting areas like the Hayden Valley and the Firehole Geyser Basins. While the two herds are not completely genetically isolated, their spatial separation influences their migration patterns and exposure to management actions. The Northern Herd is more likely to migrate outside the park’s boundary into Montana during harsh winters, making it the primary focus of the interagency management and population control efforts along the park’s edge.