Yellowstone National Park is home to roughly 3,500 to 6,000 bison at any given time, depending on the season, recent winter severity, and how many animals have been removed through management operations. The number is not fixed. It swings from year to year, sometimes dramatically, because Yellowstone’s bison population is actively managed through culling, hunting outside park boundaries, and quarantine transfers. What makes the count especially interesting is that it involves aerial surveys over some of the most rugged terrain in the lower 48 states, and that the “right” number of bison is itself a contested question involving genetics, disease, forage, tribal sovereignty, and ranching politics.
How Bison Are Counted
Getting an accurate headcount of thousands of large, dark-furred animals spread across 2.2 million acres of mountains, forests, and thermal basins is harder than it sounds. The National Park Service relies on aerial surveys, flying fixed-wing aircraft and helicopters over defined survey zones during specific seasonal windows. Research on these methods found that the best time for winter surveys is early in the season, around December and January, when bison are still relatively clustered. In summer, the optimal window is around early August, when bison gather into their largest groups on open grasslands.
Even under good conditions, not every animal gets spotted from the air. Winter surveys detect about 75% of bison groups and roughly 92% of individual animals, while summer detection is slightly better at 79% of groups and 97% of individuals. Large groups also tend to be undercounted because animals overlap and cluster tightly; that undercounting bias can reach 15% for the biggest aggregations. When all of these factors are accounted for, the overall precision of population estimates sits at a coefficient of variation of about 8%, which is considered strong for a free-ranging wildlife population of this size.1Montana State University. Aerial survey methodology for bison population estimation in Yellowstone National Park That means if the estimate comes back at 5,000, the true number is likely somewhere between about 4,600 and 5,400.
Two Herds That Aren’t Quite Separate
Yellowstone’s bison have traditionally been described as two herds: the northern herd, which winters in the Lamar Valley and along the lower-elevation areas near the park’s northern boundary, and the central herd, which occupies the Hayden Valley and Pelican Valley deeper inside the park. For decades, managers treated these as functionally separate populations. A 2012 genetic study supported this view, identifying two genetically distinct subpopulations whose differentiation was surprisingly close to that seen between bison herds that had been completely isolated from each other for over 40 years.2Journal of Heredity. Genetic Population Substructure in Bison at Yellowstone National Park
More recent work, however, has complicated that picture. A 2024 genetic reassessment using larger sample sizes and samples collected specifically during the breeding season found that the best statistical fit was a single population, not two. Admixture analyses using both newer and older genetic marker types failed to separate the geographically defined herds into genetically distinct groups.3Journal of Heredity. Genetic reassessment of population subdivision in Yellowstone National Park bison The likely explanation is that enough bulls move between wintering areas during the summer rut to keep gene flow high, even if the herds look separate for much of the year. For management purposes, the two-herd framework still matters because the herds occupy different parts of the park and face different pressures during winter, but genetically they appear to function as one large population.
Why Bison Leave the Park in Winter
Every winter, hundreds and sometimes thousands of bison migrate beyond the park’s boundaries. They are not wandering aimlessly. Bison move downhill along decreasing elevation gradients, accumulating in wintering areas and then progressing to even lower ground as conditions worsen. Research on these movements shows that snowpack is a major driver: deeper, heavier snow pushes bison toward the park’s edges where forage is more accessible. Herd size also matters, because more animals competing for the same snow-buried grass means each one needs to range farther. Dried plant biomass on the landscape plays a role too. When there’s more standing forage, fewer bison push past the boundaries.4PLoS ONE. Predicting Bison Migration out of Yellowstone National Park Using Bayesian Models
Bison also appear to learn migration routes over time. Movement models have found that migration pathways get used more heavily as the season progresses, suggesting that experience or social learning plays a part in how bison navigate the landscape.5PubMed. Integrating population- and individual-level information in a movement model of Yellowstone bison This matters because bison that leave the park enter a very different regulatory environment. Inside Yellowstone they are fully protected. Outside, they can be subject to tribal and state hunts, hazing operations that push them back into the park, or lethal removal by management agencies.
How Many Is Too Many, and How Many Is Not Enough
The question of how many bison should live in Yellowstone has no single right answer because the answer depends on what you’re optimizing for. A spatially explicit model of the park’s forage, climate, and bison dynamics estimated that the theoretical food-limited carrying capacity is around 6,200 animals. In other words, the park could probably feed that many bison in a typical year before forage limitation starts causing significant declines. But the same research suggested that a population of 2,500 to 4,500 would better balance forage availability, bison movement ecology, genetic diversity, brucellosis risk, and the social and political realities of bison wandering onto private land.6Biological Conservation. Carrying capacity, migration, and dispersal in Yellowstone bison
The genetics side sets a floor rather than a ceiling. Simulation studies have found that a herd of at least 2,500 has a high probability of retaining more than 90% of its initial genetic variation over 200 years. But to maintain the higher benchmark of 95% of genetic variation, which researchers consider necessary for long-term evolutionary potential, the herd needs to stay at 3,500 or more. Keeping removals below 40% of the population in any given action, maintaining a balanced sex ratio, and preferentially removing related individuals all help preserve genetic health.7The Journal of Wildlife Management. Predicted genetic consequences of alternative population control strategies for North American plains bison in Yellowstone National Park Separate simulations have also shown that removing only calves and yearlings rather than adults extends generation intervals and preserves more diversity over time.8Biological Conservation. Conserving genomic variability in large mammals: Effect of population fluctuations and variance in male reproductive success on variability in Yellowstone bison
So in practice, Yellowstone managers are balancing a genetic minimum around 3,500, a forage-politics compromise around 2,500 to 4,500, and a biological ceiling near 6,200. The population has at times pushed well above the management target range, which triggers more aggressive removals and hunts.
The Brucellosis Problem
Much of the controversy around bison numbers comes down to a single bacterium. Yellowstone bison carry brucellosis, a disease caused by Brucella abortus that can cause pregnant cattle to miscarry. Bison and elk in the Greater Yellowstone Area are the last significant wildlife reservoirs for the disease in the United States. During the 1990s, no known cases of transmission from wildlife to livestock occurred in the region, but from 2002 to 2012 at least 17 transmission events were investigated.9PubMed Central. Transmission of brucellosis from elk to cattle and bison, Greater Yellowstone area, U.S.A., 2002-2012
The fear of brucellosis spreading to livestock is a major reason bison are hazed back into the park or removed when they migrate beyond the boundaries. But the actual risk is uneven. Detailed risk assessments have shown that the probability of transmission is spatially and temporally patchy, with local hotspots of higher risk but an overall distribution that skews heavily toward low risk. Climate and bison abundance both influence how that risk map looks in any given year.10Journal of Applied Ecology. Wildlife-livestock conflict: the risk of pathogen transmission from bison to cattle outside Yellowstone National Park
One way to move bison out of the park without spreading brucellosis is through quarantine. Protocols established for this purpose require rigorous testing over 900 days, a roughly two-and-a-half-year process that certifies individual animals as disease-free. More than 200 bison have cleared quarantine and been transferred to Native American tribes for conservation and cultural purposes.11PubMed. Bayesian latent-class modelling of quarantine testing procedures for American Bison (Bison bison) in the Greater Yellowstone Area to determine Brucella abortus freedom The InterTribal Buffalo Council, which represents dozens of tribes, has been a leading voice in advocating for live transfers rather than slaughter as the default tool for population management.
What a Large Bison Herd Does to the Landscape
Bison are not just passively occupying Yellowstone’s grasslands. They are actively reshaping them. Research published in Science in 2025 found that Yellowstone’s free-moving bison herds stabilized aboveground plant production while speeding up nitrogen cycling. They increased the amount of nitrogen available in the system without reducing carbon stocks, which effectively improved the nutritional quality of the entire landscape. The effects were strongest in wet, nutrient-rich habitats that received more grazing pressure than standard rangeland management guidelines would recommend, and soil conditions suggested the landscape remained resilient even under that heavier use.12PubMed. Yellowstone’s free-moving large bison herds provide a glimpse of their past ecosystem function
This finding fits with earlier experimental work showing that grazing can actually stimulate plant growth. In Yellowstone’s mesic grasslands, simulated grazing through clipping more than doubled relative growth rates of plants, and real bison grazing had qualitatively similar effects. The mechanism does not rely on bison fertilizing the soil with their waste, as might be assumed. Instead, grazing exploits the dynamics of how plant populations grow: when a dense stand of grass gets thinned by grazing, the remaining plants grow faster because there is less competition for light and space.13PubMed. Density-dependent plant growth drives grazer stimulation of aboveground net primary production in Yellowstone grasslands These findings help explain why conservationists argue for maintaining a large, freely moving bison population. A small herd penned into a fraction of the landscape does not replicate these ecological effects.
Climate Change and Shifting Movement Patterns
Bison movements are tightly coupled to weather, which means that a changing climate will likely change where and when bison move. Research tracking plains bison in other large landscapes has found that weather and drought strongly influence movement patterns, and shifting climate conditions could cascade through bison behavior to affect grassland structure, function, and biodiversity across the ecosystem.14PubMed Central. Bison movements change with weather: Implications for their continued conservation in the Anthropocene For Yellowstone specifically, even early climate modeling warned that changes in the distribution and extent of grassland communities would affect large ungulate populations in the Greater Yellowstone Ecosystem.15Conservation Biology. Implications of Global Climate Change for Biogeographic Patterns in the Greater Yellowstone Ecosystem
What this means for bison numbers is uncertain but worth watching. Warmer winters with less snowpack could reduce the pressure that drives bison out of the park, potentially keeping more animals inside boundaries and reducing conflict with ranchers. But warmer summers could also shift grassland composition, alter forage quality, and change the timing of the green-up that bison depend on for calving season nutrition. The carrying capacity figure of 6,200 was modeled under historical climate conditions. If the park’s grasslands shrink or shift in character, that ceiling could drop.
Staying Safe Around Bison
Yellowstone’s bison are one of the park’s biggest draws, but they injure visitors every year and are statistically more dangerous to people in the park than any other animal, including bears. A study of bison-related injuries from 2000 to 2015 found patterns that are worth knowing. The average distance between a person and a bison at the time of injury was only about 3.4 meters, or roughly 11 feet. Eighty percent of the injured people had actively approached the bison before getting hurt. Nearly half were injured while photographing the animal. And 60% of injuries happened when groups of three or more people approached together, suggesting that being in a crowd makes people bolder or more distracted around a 900-kilogram wild animal.16One Health. Risk-enhancing behaviors associated with human injuries from bison encounters at Yellowstone National Park, 2000–2015
The park’s official guidance is to stay at least 23 meters (75 feet) from bison at all times. Bison can run at speeds that close that gap in seconds, and they give surprisingly few warning signs before charging. Unlike bears, which often bluff-charge, bison that charge typically make contact. Four of the 25 people studied were walking at night without a light source and stumbled into bison they could not see, a reminder that these animals graze in and around developed areas after dark. If a bison raises its tail, lowers its head, or starts pawing the ground, you are already too close.
Tribal Transfers and the Future of Bison Beyond the Park
Yellowstone bison are among the few remaining herds that descend continuously from wild, pre-colonial populations. Most bison alive today, including those on commercial ranches and many conservation herds, carry some cattle genes from 19th- and early 20th-century crossbreeding. Yellowstone’s herd has been confirmed as one of a handful that are free of detectable cattle ancestry, which makes it genetically irreplaceable.
This status is why the quarantine-and-transfer pipeline matters so much to tribal communities. The more than 200 bison that have passed through quarantine and been sent to tribal lands represent not just cultural restoration but a form of genetic insurance, placing animals with rare lineage onto multiple separate landscapes so that no single catastrophe can wipe out the line.11PubMed. Bayesian latent-class modelling of quarantine testing procedures for American Bison (Bison bison) in the Greater Yellowstone Area to determine Brucella abortus freedom The 900-day quarantine process is slow and expensive, and some advocates have argued that it is more cautious than the actual brucellosis risk warrants. But it remains the only pathway that satisfies both livestock regulators and conservation goals, and every bison that clears it adds to a growing network of genetically valuable herds across the northern plains and beyond.