The History and Current Status and Distribution of Beavers in Yellowstone National Park
Level 4 - case-series / case-control
Level 4 by design analogy (historical ecological survey compilation and case series).
OpenAlex W2156708597 · doi:10.3955/046.086.0404
What was done
The authors compiled and analyzed historical beaver (*Castor canadensis*) population surveys in Yellowstone National Park spanning 1921 to 2009. Early counts (1921–1923, 1953, 1979–1980, 1988–1989, and 1994) consisted of localized or incomplete ground surveys, primarily in the northern range. From 1996 to 2009, researchers conducted standardized, park-wide aerial surveys across eight distinct years (1996, 1998, 1999, 2001, 2003, 2005, 2007, and 2009).
What was found
Initial 1921–1923 ground surveys recorded 25 active colonies in the northern range using aspen and willow. By 1953, northern range presence dropped to 6 sites with zero 1920s sites remaining active and no observed aspen use. Incomplete park surveys estimated 71 colonies in 1988–1989 and 44 colonies in 1994. Standardized aerial monitoring showed an upward trajectory from 44 active colonies in 1996 to a peak of 127 colonies in 2007, indicating a 90-year pattern of decline followed by recovery.
Why it matters
This synthesis provides a multi-decade baseline tracking the recovery of a keystone ecosystem engineer in Yellowstone, linking population rebounds to broader riparian vegetation dynamics such as willow recovery.
Limits
Survey methodology shifted substantially over time, moving from sporadic, localized ground counts to comprehensive aerial surveys, making long-term direct comparisons imprecise. Colony counts serve as an index rather than a precise individual census, and the causal link between willow dynamics and beaver population growth is observational.
Cited by
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