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Grasslands are nonequilibrium ecosystems, maintained by disturbances, notably drought, fire, and herbivory (Knapp and Seastedt 1998). Without such treatments, ecological succession will transform grassland into shrubland or woodland, depending on the precipitation regime. Attacks on above-ground growth—whether by fire, herbivore, or mowing machine—are well-tolerated (Manning 1995) because of the plants' deep root systems, and because the growth points of grasses are near the soil surface. Droughts, while termed catastrophes by humans, are a normal phenomenon, without which prairies would not exist (Manning 1995). Lack of moisture by itself is not sufficient to support prairie, however; other disturbances are necessary. The major ones are fire and herbivory.

Fire maintains grassland, especially in the eastern tallgrass prairie, where precipitation is adequate to support forest vegetation. The build-up of highly combustible dead grasses and forbs, open terrain that allowed winds to carry a fire long distances, and frequent summer thunderstorms that brought the match of lightning, all contributed to recurrent prairie fires. In the more xeric shortgrass prairie of the west and mixed-grass prairie between the shortgrass and tallgrass prairies, grazing by huge herds of bison (Bos bison), elk (Cervus elaphus), and pronghorn (Antilocapra americana), often in combination with fire, helped maintain prairie.

Americans have been ambivalent in their attitudes toward grasslands. To European pioneers, the prairies were a painful obstacle hindering them from getting from the eastern states to the West Coast, where gold was to be found. Or, for those who settled in the vast midsection of the nation, prairie was something to be turned upside down in order to farm. Yet, for some settlers, after trekking through hundreds of miles of thick forest, "seeing prairie was like seeing sun for the first time" (Madson 1995, 15). Humans had evolved in savannas, grasslands interspersed with scattered trees, so the "rush of freedom felt on encountering an open vista of grassland is racial memory" (Manning 1995, 51).

One reason that prairie has been underappreciated may be that much of it is hidden from view. But, as noted by William Least Heat-Moon, "The prairies are nothing but grass as the sea is nothing but water" (Knapp et al. 1998). Prairies harbor vitally important biological resources, including many endangered and threatened species, such as the black-footed ferret (Mustela nigripes), western prairie fringed orchid (Platanthera praeclara), and Dakota skipper (Hesperia dacotae). In Minnesota, 102 of 287 endangered species occur in prairie (Tester 1995). Some species are ghosts of prairies past; the plains grizzly (Ursus arctos) and gray wolf (Canis lupus) were dominant predators before settlement by Europeans. The elimination of these and other keystone species severely disrupted biotic communities.