Until 1968, Park Service officials contended that an unnaturally large elk population, which had built up in Yellowstone during the late 1800s and early 1900s, had severely damaged the park’s northern winter range, including aspen (Populus tremuloides) communities. However, agency biologists now hypothesize that elk and other ungulates in Yellowstone are “naturally regulated,” being resource (food) limited, and that the condition of the ecosystem today is much like it was at park formation (Houston 1982; Despain et al. 1986). Elk influences on Yellowstone’s vegetation are now thought to be “natural” and to represent the “pristine” condition of the park.[Kay, Charles E. 2001. Long-Term Aspen Exclosures in the Yellowstone Ecosystem. USDA Forest Service Proceedings RMRS-P-18.]
According to the Park Service, Yellowstone is not now nor has it ever been
overgrazed, and all previous studies to that effect are wrong (Houston 1982).
This conclusion was challenged by available evidence. For example, Kay showed the change in herbivore exclosures between 1946, 1963 and 1987 (images from Kay, 2001). The aspen clone is bisected by the exclosure's fence. While the two portions of the clone seem fairly similar in 1963, by 1987 the portion of the clone that was protected from herbivores has spread considerably.
William Ripple and Eric Larsen found* that the aspens had stopped regenerating in 1920, about the same time that wolves were eliminated from Yellowstone. However, there were questions on the role that wolves might play in regulating the elk population.
To this end, Ripple and Robert Beschta** looked at the level of browsing and the height of aspen saplings in areas with and without logs (which can provide cover for wolves). In both upland and riparian sites, aspen saplings were taller when downed logs were present.
The reintroduction of wolves has allowed allowed aspen to return to riparian areas (although not, so far, in upland areas). Willows and cottonwoods also returned - more quickly than the aspen, which are a preferred browse for elk. While populations reductions have played a role, so has the 'ecology of fear'.
*Ripple,W.J., Larsen, E.J., 2000. Historic aspen recruitment, elk, and wolves in northern Yellowstone National Park, USA. Biological Conservation 95, 361–370.
**Ripple, W.J., Beschta, R.L., Restoring Yellowstone’s aspen with wolves, Biological Conservation (2007), doi:10.1016/j.biocon.2007.05.006
Story at Science Daily.