Yellowstone National Park, established in 1872, is one of the largest nearly intact temperate zone ecosystems remaining on Earth. The park’s vast 2.2 million acres of mountains, forests, and high-elevation plateaus support immense biological diversity. This area, the core of the Greater Yellowstone Ecosystem, is home to nearly 300 species of birds, 16 species of fish, and 67 species of mammals. The park’s wildlife ranges from massive grazing herds to elusive top carnivores that collectively shape the environment.
The Iconic Hoofed Mammals (Ungulates)
Hoofed mammals (ungulates) form the herbivore base of the Yellowstone ecosystem, influencing plant life across the Northern Range and Hayden Valley. The park’s bison herd is genetically significant as one of the few wild populations never interbred with domestic cattle, fluctuating between 4,000 and 6,000 animals. Bison are a keystone species because their grazing and wallowing behaviors alter the soil and vegetation, enhancing forage for themselves and other species.
Elk are the most numerous ungulate, with 10,000 to 20,000 individuals in the Greater Yellowstone Ecosystem, making them a primary food source for large predators. They undertake seasonal migrations, moving from high-elevation summer ranges to lower-elevation winter ranges outside the park.
Moose, the largest member of the deer family, are less abundant than elk, with fewer than 200 individuals estimated in the park. They prefer dense willow thickets and aquatic plants, distinguishing their habitat from the grassy meadows favored by elk. Their specialized diet and long legs allow them to exploit deep snow habitats and move through wetlands.
Other ungulates include bighorn sheep and pronghorn. Bighorn sheep are found in rugged, steep, and rocky terrain like Mount Washburn, using cliffs for safety. Pronghorn, the fastest land animals in North America, are mainly confined to the open sagebrush steppe of the Lamar Valley.
Apex Predators and Large Carnivores
Yellowstone hosts all of its original large carnivore species, which manage ungulate populations and maintain ecosystem balance. The reintroduction of the gray wolf in 1995 and 1996, after a 70-year absence, initiated a trophic cascade.
By limiting elk density and behavior, wolves reduced browsing pressure on streamside vegetation like willows and aspens. This recovery of woody plants has stabilized riverbanks and created new habitat, indirectly benefiting species like beavers. Wolf kills also provide a steady supply of carrion throughout the winter, feeding scavengers such as coyotes, ravens, and grizzly bears.
The park is home to both grizzly bears and black bears, distinguished by physical traits. Grizzly bears have a prominent shoulder hump and long, less curved claws suited for digging. They assimilate more meat into their diet, including hunting elk calves and scavenging wolf kills, and prefer large, nonforested meadows.
Black bears lack the shoulder hump and possess shorter, curved claws that make them efficient tree climbers. Their diet relies more on plant material than a grizzly’s, and they are typically found in forested areas. The mountain lion (cougar) is the park’s most elusive predator, preferring to ambush prey in rugged terrain. While their primary prey is mule deer and elk, they often lose kills to dominant wolves and grizzly bears, sometimes shifting to smaller prey.
Avian Species and Wetland Habitats
Yellowstone’s rivers, lakes, and wetlands host approximately 300 avian species. Bald eagles and osprey are prominent raptors, often seen near water sources where they historically preyed upon the Yellowstone Cutthroat Trout (YCT).
The YCT is a foundational aquatic species, providing a seasonal food source for bears and raptors during its spring spawning run. The YCT population declined due to the introduction of non-native lake trout, forcing raptors to adapt. Bald eagles now prey on the flightless young of colonial nesting species like the American White Pelican.
Trumpeter swans, the largest waterfowl in North America, are a species of concern, relying heavily on wetlands for food and nesting. Their population faces challenges due to declining wetland health, often impacted by changes in precipitation and water levels.
The park’s unique thermal features influence localized avian habitats. Heat prevents water from freezing, creating warm microclimates that allow insects, such as the ephydrid fly, to remain active year-round. This localized food source attracts birds during the harsh winter months.
Small Mammals and Ecosystem Engineers
Small mammals play a significant role in maintaining the park’s hydrology and soil health. Beavers are recognized as keystone species and ecosystem engineers, physically altering the landscape. By constructing dams, they create extensive wetland habitats crucial for storing surface water and recharging groundwater, offering sanctuary for numerous species during drought.
River otters, the most aquatic member of the weasel family, are active year-round, hunting fish and crayfish in the park’s rivers and lakes. These crepuscular animals are often spotted in areas like the Lamar River.
The American pika is a small, rabbit-like lagomorph inhabiting high-elevation rocky slopes and talus fields. Pikas do not hibernate and survive the long winters by creating large “haypiles” of dried vegetation to consume.
Small rodents, such as the Uinta ground squirrel and yellow-bellied marmot, are a foundational prey base for carnivores and raptors. The Uinta ground squirrel, which hibernates as early as mid-July, is a substantial food item for mid-level predators like coyotes, foxes, weasels, and badgers.