The Greater Yellowstone Ecosystem

Paul Schullery
National Park Service
Greater Yellowstone is described as the last large, nearly intact ecosystem in the northern temperate zone of the earth (Reese 1984; Keiter and Boyce 1991). Conflict over management has been controversial, and the area is a flagship site among conservation groups that aggressively promote ecosystem management (Greater Yellowstone Coalition 1992). The Greater Yellow Ecosystem (GYE) is one of the world's foremost natural laboratories in landscape ecology and geology and is a world-renowned recreational site (Knight 1994).


Yellowstone National Park (YNP) boundaries were arbitrarily drawn in 1872 in hopes of including all regional geothermal basins. No other landscape considerations were incorporated. By the 1970's, however, the grizzly bear's (Ursus arctos) range in and near YNP became the first informal minimum boundary of a theoretical Greater Yellowstone Ecosystem that included at least 1,600,000 ha (4,000,000 acres; Schullery 1992). Since then, definitions of the GYE have steadily grown larger (Fig. 1). A 1994 study listed the GYE size as 7,689,000 ha (19,000,000 acres; Clark and Minta 1994), while a 1994 speech by a Greater Yellowstone Coalition leader enlarged that to 8,000,000 ha (20,000,000 acres; Wilcox 1994).

Fig. 1. Progressively lighter shading is used around the edges of a recent map of the Greater Yellowstone Ecosystem to illustrate the uncertainty that still plagues definitions of the ecosystem. Courtesy Desktop Assistance and Greater Yellowstone Coalition.
In 1985 the House Subcommittees on Public Lands and National Parks and Recreation held a joint subcommittee hearing on Greater Yellowstone, resulting in a report by the Congressional Research Service (1986) outlining shortcomings in interagency coordination and concluding that the area's essential values were at risk.

Ecosystem Management by Species

The GYE concept has been most often advanced through concerns over individual species rather than over broader ecological principles. GYE managers must keep at least two types of "long-term" status in mind. One is the known, or at least probable, trend of a species based on historical and prehistorical information. The second type is that which has existed since the beginning of formal scientific study. Though 20 or 30 or even 50 years of information on a population may be considered long-term by some, one of the important lessons of GYE management is that even half a century is not long enough to give us a full idea of how a species may vary in its occupation of a wild ecosystem.
For example, anecdotal information on grizzly bear abundance dates to the mid-1800's (Schullery and Whittlesey 1992), and administrators have made informal population estimates for more than 70 years (Schullery 1992). From these sources, we know the species was common in the GYE when Europeans arrived, and we know that the population was not isolated before the 1930's, but is now. We do not know if bears were more or less common than now.
A 1959-70 bear study suggested a grizzly bear population size of about 175, later revised to about 229 (Craighead et al. 1974). Later estimates have ranged as low as 136 and as high as 540 (Schullery 1992); the most recent is a minimum estimate of 236 (Servheen 1993). Although the GYE population is relatively close to recovery goals, the plan's definition of recovery is controversial (Mattson and Reid 1991; Schullery 1992). Thus, even though the population may be stable or possibly increasing in the short term, in the longer term, continued habitat loss and increasing human activities may well reverse the trend.
Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) have suffered considerable declines since European settlement, but recently began flourishing (Varley and Schullery 1983) in some areas. Especially in Yellowstone Lake itself, long-term records indicate an almost remarkable restoration of robust populations from only three decades ago when the numbers of this fish were depleted because of excessive harvest (Gresswell and Varley 1988). Its current recovery, though a significant management achievement, does not begin to restore the species' historical abundance.
Early accounts of pronghorn (Antilocapra americana) in the GYE described herds of hundreds seen ranging through most major river valleys (Schullery and Whittlesey 1992). These populations were decimated by 1900, and declines continued among remaining herds. On the park's northern range, pronghorn declined from 500-700 in the 1930's to about 122 in 1968 (Houston 1982). By 1992 the herd had increased to 536 (J. Mack, National Park Service, personal communication).

Among plants, whitebark pine (Pinus albicaulis) is a species of special interest, in large part because of its seasonal importance to grizzly bears, but also because its distribution could be dramatically reduced by relatively minor global warming (Blanchard and Knight 1991; Romme and Turner 1991; Fig. 2). In this case, we do not have a good long-term data set on the species, but we understand its ecology well enough to project declining future status.

Fig. 2. Top: Current distribution of whitebark pine portrayed by a computerized geographic information system (GIS). Bottom: Distribution of whitebark pine projected by GIS analysis under a modest increase in warmth and dryness, showing a decrease of approximately 90%. (Derived from Romme and Turner [1991] by the Yellowstone GIS Laboratory, Yellowstone National Park.)
Estimates of the decline of quaking aspen (Populus tremuloides) on YNP's northern range since 1872 range from 50% to 95% (Houston 1982; Kay 1993), and perhaps no controversy underway in the GYE more clearly reveals the need for comprehensive interdisciplinary research. Several factors are suspected in the aspen's changing status, including Native American influences on numerous mammal species and on fire-return intervals before the creation of the park in 1872; European influences on fire frequency since 1886; regional climate warming; human harvests of beaver and ungulates in the first 15 years of the park's history and of wolves and other predators before 1930; human settlement of traditional ungulate migration routes north of the park since 1872; ungulate (especially elk) effects on all other parts of the ecosystem since 1900; and human influences on elk distribution in the park (Houston 1982; Schullery and Whittlesey 1992; Kay 1993).


Research is but one component of land-management decisions (Varley 1993). While in some respects the GYE has fulfilled the promise of early scientists who described it as one of the foremost natural laboratories on earth, both managers and researchers need more information to deal with the increasing demands on the region's resources, either in terms of raw information or in terms of an ecosystem-level understanding. In YNP, a landscape model is being developed based on a computerized geographic information system that will integrate, analyze, and display information from many disciplines (Shovic et al. 1993). Through this level of synthesis we may be able to better understand trends in the GYE.
For further information:
Paul Schullery
National Park Service
Yellowstone Center for Resources
PO Box 168
Yellowstone Park, WY 82190

Blanchard, B., and R. Knight. 1991. Movements of Yellowstone grizzly bears. Biological Conservation 58:41-67.

Clark, T.W., and S.C. Minta. 1994. Greater Yellowstone's future. Homestead Publishing, Moose, WY. 160 pp.

Congressional Research Service. 1986. Greater Yellowstone Ecosystem, an analysis of data submitted by federal and state agencies. U.S. Government Printing Office, Washington, DC. 210 pp.

Craighead, J., J. Varney, and F. Craighead. 1974. A population analysis of the Yellowstone grizzly bears. Montana Forest and Conservation Experiment Station Bull. 40. University of Montana, Missoula. 20 pp.

Greater Yellowstone Coalition. 1992. Inside greater Yellowstone. Greater Yellowstone Coalition, Bozeman, MT. 16 pp.

Gresswell, R., and J. Varley. 1988. Effects of a century of human influence on the cutthroat trout of Yellowstone Lake. American Fisheries Society Symposium 4:45-52.

Houston, D. 1982. The northern Yellowstone elk, ecology and management. Macmillan, New York. 474 pp.

Kay, C. 1993. Aspen seedlings in recently burned areas of Grand Teton and Yellowstone National parks. Northwest Science 67(2):94-103.

Keiter, R.B., and M.S. Boyce. 1991. The Greater Yellowstone Ecosystem, redefining America's wilderness heritage. Yale University Press, New Haven, CT. 428 pp.

Knight, D. 1994. Mountains and plains, the ecology of Wyoming landscapes. Yale University Press, New Haven, CT. 338 pp.

Mattson, D., and M. Reid. 1991. Conservation of the Yellowstone grizzly bear. Conservation Biology 5(3):364-372.

Reese, R. 1984. Greater Yellowstone, the national park and adjacent wildlands. Montana Geographic, Helena. 104 pp.

Romme, W., and M. Turner. 1991. Implications of global climate change for biogeographic patterns in the Greater Yellowstone Ecosystem. Conservation Biology 5(3):373-386.

Schullery, P. 1992. The bears of Yellowstone. High Plains Publishing, Worland, WY. 318 pp.

Schullery, P., and L. Whittlesey. 1992. The documentary record of wolves and related wildlife species in the Yellowstone National Park area prior to 1882. Pages 1-4 to 1-174 in J.D. Varley and W.G. Brewster, eds. Wolves for Yellowstone? Vol. 4. A report to the U.S. Congress. National Park Service, Yellowstone National Park, WY.

Servheen, C. 1993. Grizzly bear recovery plan. U.S. Fish and Wildlife Service, Missoula, MT.

Shovic, H., M. Johnson, and H. Porter. 1993. A new view of an old land. Yellowstone Science 1(2):2-6.

Varley, J. 1993. Research in Yellowstone. Bioscience 43(3):3-4.

Varley, J., and P. Schullery. 1983. Freshwater wilderness, Yellowstone fishes and their world. Yellowstone Library and Museum Association, Yellowstone National Park, WY. 132 pp.

Wilcox, L. 1994. Sustaining wilderness. Panelist presentation, Cinnabar Symposium, Museum of the Rockies, Bozeman, MT. March 25. Unpublished.