1 INTRODUCTION -- Background and Charge to the Committee -- Natural Regulation -- Historical Context -- Climate Versus Elk As Causality of Changes in Aspen and Willow -- Organization of This Report -- 2 HISTORICAL PERSPECTIVE: YELLOWSTONE'S CHANGING -- ENVIRONMENTS -- Scales of Change -- Paleobiology of the Greater Yellowstone Area -- Ungulate Use Over Time As Estimates of Population Densities -- Physical Changes from Prehistory to the Present -- 3 PRESENT CONDmONS: VEGETATION -- Upland Shrublands and Grasslands of the Northern Range -- Forest Types Associated with the Northern Range -- Aspen Communities of the Northern Range -- Riparian Communities of the Northern Range -- Wetland Vegetation of the Northern Range -- 4 PRESENT CONDITIONS: ANIMALS -- Population Dynamics of Northern Range Ungulates -- Major Predators in the Yellowstone Ecosystem -- 5 CONCLUSIONS AND RECOMMENDATIONS -- Nature Is Dynamic -- Management for Ecosystem State or Ecosystem Process? -- Large-Scale Interactions and Patterns -- Weather, Wolves, and Aspen in Yellowstone -- Indicators of Unacceptable Change -- Understanding the Consequences of Alternative Management -- Approaches -- Conclusions -- Recommendations -- Epilogue -- REFERENCES -- APPENDI A: UNDERSTANDING THE PAST -- APPENDIX B: BIOGRAPHICAL INFORMATION ON THE COMMITTEE -- ON UNGULATE MANAGEMENT IN YELLOWSTONE NATIONAL PARK.