priceless scientific and cultural artifacts. In carrying out its responsibilities, the NPS must consider both natural and cultural resources, as well as the interactions between people and these resources.
The national parks are more than natural and cultural treasures—they are an important source of national self-esteem. They give Americans pride as well as access to places of significant aesthetic, recreational, and spiritual value. These interests, and a devotion to the concept of public stewardship of the nation's heritage, have been important forces behind maintaining and expanding the national park system.
Conditions in the parks today give cause for concern. Against a backdrop of significant human alterations to the Earth's landscape, the national parks have become "besieged treasures" (Forgey, 1990). Although the national parks were created for the enjoyment of the American people, increasing numbers of park visitors, and the facilities needed to accommodate them, are overwhelming some parks. Air pollution, often from distant and diffuse sources, already has compromised aesthetic values within several of the largest national parks, especially Grand Canyon, Yosemite, Sequoia-Kings Canyon, Shenandoah, and Great Smoky Mountains national parks. Actions outside park boundaries are producing critical changes in ground and surface water, accelerating pest introduction, increasing stream sedimentation, and threatening wildlife populations.
The parks are increasingly subject to diverse human influences that threaten further attrition in biological diversity and accelerated damage to aesthetic values, and imperil the integrity and stability of park ecosystems. In some instances, destruction of the very resources for which individual national parks were established is now increasingly probable and, in fact, is under way in some areas. For example, air pollution has degraded the renowned scenic vistas of Grand Canyon National Park (NRC, 1990), beach erosion has threatened the historic lighthouse at Cape Hatteras National Seashore (NRC, 1987a), and operation of the Glen Canyon Dam on the