The beaver (Castor canadensis) is a keystone species that affects habitat structure and dynamics through the damming and diverting of streams, and the felling of trees and other woody vegetation. The resulting ponds and flooding help create an environment favorable to willow and aspen, the beavers’ preferred winter foods and used in building their lodges. The territoriality of beavers probably deters two colonies from locating within 165 feet (50 m) of each other, and most streams in the park lack either suitable vegetation or a sufficiently low gradient to provide beavers with habitat, but information about the distribution and number of beaver colonies in the park over time adds to our understanding about the long-term effects of changes in vegetation and climate.
Number in Yellowstone
100 colonies estimated in 2015.
Where to See
Behavior and Size
Beavers live throughout Yellowstone National Park but are concentrated in the southeast (Yellowstone River delta area), southwest (Bechler area), and northwest portions (Madison and Gallatin rivers) of the park. These areas are likely important habitat because of their waterways, meadows, and the presence of preferred foods such as willow, aspen, and cottonwood.
However, beavers are not restricted to areas that have their preferred foods. Essentially no aspen exist in most areas where beavers’ sign is most abundant, such as the Bechler River and in other areas where beavers periodically live, such as Heart Lake, the lower Lamar River and Slough Creek area, Slide Lake, and the lower Gardner River. In these areas, beavers use willows for construction and for food. Where their preferred plants are few or absent, beavers may cut conifer trees and feed on submerged vegetation such as pond lilies.
Beavers are famous dam builders, and examples of their work can be seen from the roads in the park. Most dams are on small streams where the gradient is mild, and the current is relatively placid during much of the year. Colonies located on major rivers or in areas of frequent water level fluctuations, such as the Lamar River, den in holes in the riverbank. An old dam is visible at Beaver Lake between Norris and Mammoth.
When hunched over their food, beaver can resemble round rocks. Beavers are most active in the early morning and late evening, which seem to allow them to use areas near human use. Beavers do not appear to avoid areas of moderate to high levels of human use. Several occupied lodges in Yellowstone are close to popular backcountry trails and campsites.
The first survey of beavers in the park, conducted in 1921, reported 25 colonies, most of them cutting aspen trees. Although it was limited to parts of the northern range, comparing the locations of those beaver colonies with subsequent survey results demonstrates how beavers respond and contribute to changes in their habitat. A 1953 survey found eight colonies on the northern range, but none at the sites reported in 1921 and a lack of regrowth in cut aspen. Willow were also in decline during this period.
To help restore the population of beavers on Gallatin National Forest, 129 beavers were released into drainages north of the park from 1986 to 1999. Park-wide aerial surveys began in 1996 with a count of 49 colonies and increased to 127 by 2007; dropping to 118 in 2009 and 112 in 2011. While the long-term increase is partly attributable to the improving ability of aerial observers to locate colonies, the park’s population of beavers probably has grown in the last 15 years. Some of the increase likely came from beavers dispersing from the national forest, but they would not have survived without suitable habitat. The increase has occurred throughout the park and is likely related to the resurgence in willow since the late 1990s, at least on the northern range, and possibly in the park interior. Nearly all of the colonies documented in recent years were located in or near willow stands, none near aspen.
Willow, which is more common in the park than aspen, is a hardier shrub that quickly regenerates after being clipped by beavers. The reason for the prolonged decline and relatively sudden release of willow on the northern range, and whether aspen have begun a sustained surge in recruitment, are topics of intense debate. Possible factors include the relationship of these plant species to changes in the abundance of beavers and elk, fire suppression, the reintroduction of wolves, and climate change.
Consolo Murphy, S. and D.D. Hanson. 1993. Distribution of beaver in Yellowstone National Park, 1988–1989. In R. S. Cook, ed., Ecological issues on reintroducing wolves to Yellowstone National Park. Vol. Scientific Monograph NPS/NRYELL/NRSM-93/22. US Department of the Interior, National Park Service.
Consolo-Murphy, S. and R.B. Tatum. 1995. Distribution of beaver in Yellowstone National Park, 1994, Edited by National Park Service, Yellowstone National Park.
Jonas, R.J. 1955. A population and ecological study of the beaver (Castor canadensis) of Yellowstone National Park. Vol. MS. Moscow, ID: University of Idaho.
Murphy, S.C. and D.W. Smith. 2002. Documenting trends in Yellowstone’s beaver population: A comparison of aerial and ground surveys in the Yellowstone Lake Basin. In R.J. Anderson and D. Harmon, ed., Yellowstone Lake: Hotbed of chaos or reservoir of resilience?: Proceedings of the 6th Biennial Scientific Conference on the Greater Yellowstone Ecosystem, 172–178. Yellowstone National Park, WY: Yellowstone Center for Resources and The George Wright Society.
Rue, III, L.E. 1964. The world of the beaver. New York: J.B. Lippincott, Co.
Slough, B.G. 1978. Beaver food cache structure and utilization. Journal of Wildlife Management 42(3):644–646.
Smith, D.W. and D.B. Tyers. 2008. The beavers of Yellowstone. Yellowstone Science. 16(3): 4–15.
Smith. D.W., and D.B. Tyers. 2012. The history and current status and distribution of beavers in Yellowstone National Park. Northwest Science 86(4):276–288.
Last updated: July 12, 2017