Ungulates that graze within YNP for much of the year often winter in the northern range in and adjacent to the park. Two-thirds of the northern winter range is within YNP; one-third is north of the park boundary on public and private lands (Figures 1–1 and 1–2 in Chapter 1). Elk and bison populations in the northern range have increased dramatically in recent years, particularly in years with mild winters, leading some scientists and members of the public to question the appropriateness of the park’s natural-regulation policy. These critics believe the northern winter range is overgrazed and that woody vegetation and riparian areas are being damaged, mainly by elk. Further, they see overgrazing by elk and bison as contributing to serious erosion and stream degradation. However, other scientists and resource managers note that ungulates have influenced vegetation on the northern range for thousands of years and believe that natural density-dependent factors such as forage availability, predation, and disease are regulating population dynamics so that current conditions fall within the natural range of variability.

In recent years, the controversy over natural regulation has heightened, especially in the northern range—wintering range of Yellowstone’s elk herds. In 1998, the U.S. Congress directed the NPS “to initiate a National Academy of Sciences review of all available science related to the management of ungulates and the ecological effects of ungulates on the range land of Yellowstone National Park and to provide recommendations for implementation by the Service.” In response to that mandate, the National Research Council convened the Committee on Ungulate Management in Yellowstone National Park. This committee of experts was charged to review the scientific literature and other information related to ungulate populations in the Yellowstone northern range and to attempt to clarify what is known and not known about natural regulation and the ecological effects of elk and bison populations on the landscape.² The committee’s geographic focus has been Yellowstone’s northern range. The committee’s evaluation addresses the issue from a scientific perspective, which deals with only part of a multifaceted problem that includes sociological, economic, aesthetic, and other important dimensions beyond the scope of this study.