Yellowstone National Park (YNP) spans over 2.2 million acres across Wyoming, Montana, and Idaho, representing one of the largest nearly intact ecosystems in the temperate zone. The park’s diverse landscape, from high-altitude forests to geothermal basins, supports a stunning array of wildlife. Assigning a single, precise number to the total animal population is impossible due to the immense biological diversity and size of the area. Therefore, scientists monitor population estimates for specific groups to understand the health and dynamics of this complex ecosystem and track trends for the most visible species.
Why Calculating Population Totals Is Complex
Calculating a definitive census for Yellowstone’s animal populations is a challenge rooted in both geography and methodology. The park’s fixed boundary of 3,472 square miles is only a fraction of the Greater Yellowstone Ecosystem (GYE), which encompasses roughly 18 million acres. Many large migratory species, such as elk and bison, spend the summer inside the park but move outside its jurisdiction onto state or private land during winter. This movement complicates counting efforts, as animals may be subject to different management strategies or hunting pressure once they cross the park line.
Wildlife biologists rely on various indirect methods to arrive at population estimates rather than total counts. Techniques include aerial surveys, where animals are counted from aircraft, often during winter when snow cover makes them easier to spot. Radio telemetry and GPS collaring track the movements, survival rates, and territories of individuals within a small sample. These methods provide a strong indicator of population trends and size but inherently introduce a margin of error, meaning the reported figures are scientifically informed estimates. The difficulty is compounded because the vast majority of animals are small, secretive, or aquatic species that cannot be individually tracked.
Population Estimates for Major Mammals
Yellowstone’s management focuses on the large, charismatic mammals whose numbers reflect the overall health of the ecosystem. These population estimates are crucial for understanding predator-prey dynamics and the effects of natural regulation. The reported numbers fluctuate annually based on weather conditions, calving success, and management actions.
Bison
Yellowstone is home to the largest continuously free-ranging population of American bison on public land. Recent estimates place the park’s population around 5,400 animals following the spring calving season. The management goal aims to maintain a population range between 3,500 and 6,000 individuals. This range is determined through coordination with tribal and state partners to balance conservation with concerns about disease transmission to livestock outside the park boundary.
Elk
Elk are the most numerous large mammal in the ecosystem, with an estimated 10,000 to 20,000 individuals belonging to seven different herds spending time in the park during the summer. The Northern Yellowstone Elk Herd, the most closely monitored, was estimated at approximately 5,597 animals in March 2024. This northern herd’s population has declined significantly since the reintroduction of wolves, reflecting a shift in the ecosystem’s structure. Only about 4,000 elk typically remain within the park’s boundary throughout the winter.
Wolves
Gray wolves were successfully reintroduced to the park in 1995 and now play a significant role in regulating ungulate populations. The wolf population size within Yellowstone is relatively stable, fluctuating between roughly 83 and 123 individuals since 2009. At the end of December 2024, the estimate was about 108 wolves distributed among nine different packs primarily residing inside the park. This number changes constantly as wolves are subject to natural mortality, disease, and hunting when they venture outside the park boundaries.
Grizzly Bears
Grizzly bears are monitored across the much larger Greater Yellowstone Ecosystem (GYE), as the park only provides a portion of their expansive habitat. The GYE population, which includes bears inside and outside the park, is estimated to be around 1,030 individuals based on a recent integrated population model. This model uses various data points, including female bears with cubs, to generate a more accurate number. The population has rebounded substantially since federal protections were put in place, demonstrating conservation success.
The True Scale: Uncountable Life
While large mammals capture the most attention, they represent only a tiny fraction of the park’s total animal population. The vast majority of life in Yellowstone consists of smaller organisms that are impossible to count individually. This enormous diversity includes nearly 300 documented bird species, with approximately 150 species nesting within the park’s boundaries.
The park’s waterways are home to 19 fish species, including 13 native types. The Yellowstone Cutthroat Trout, a native species, is a foundational food source for many other animals, including grizzly bears and ospreys. Even these countable vertebrates are dwarfed by the sheer numbers of invertebrates.
The true scale of life is found among the invertebrates, such as insects, spiders, and worms, which number in the billions or trillions. Scientists have documented thousands of insect species, including over 120 species of native bees vital for pollination. Aquatic invertebrate studies in the park’s wetlands have identified at least 187 different taxa, with monthly abundance estimates reaching hundreds of thousands of individuals per square meter in some areas. Considering the uncountable populations of mites, nematodes, and other microscopic life forms, along with the bacteria and fungi that underpin the entire ecosystem, the total number of living organisms in Yellowstone is effectively infinite.