The prairie grasslands of the mid-west of North America are a lot more complex than one would think. There are short grass, mixed and tall grass prairies. Each has their own suite of grass and flower species. Tall grass prairies typically have 40 to 60 different grasses and up to 200 species of forbs. Tall grass prairies are maintained by frequent fire, and the activities of mammals that are native to the plains. Grazing by large ungulates (bison, elk and deer) provides a source nitrogen and prairie dogs aerate the soils. Prairie dogs are considered a keystone species in the tall grass prairie.

Tall grass plant communities occur in soils that are predominately wind deposited (loess). These are rich and deep soils that are ideal for agriculture. This was quickly learned when the "sod-busters" moved into the midwest in large numbers in the early to mid 19th century.

It is not surprising, then, that one of the earliest destructive forces impacting these plant communities is intensive agriculture. Not only did agriculture replace the native species but it provided a conduit for the introduction of a number of invasive species throughout the plains.

Another major factor in the demise of tall-grass prairies is the control of fire. Like many grasslands, tall grass prairies need fire on a regular basis. Both grazing and fire discourage the encroachment of shrubs and trees in the prairies.

There are a number of efforts at rehabilitation of tall grass prairies. Stephen Packard has been an instrumental force in prairie restoration and protection, as has been The Nature Conservancy. You can read about TNCs restoration and protection efforts here. Finally, the National Park Service maintains a tallgrass prairie preserve in Kansas. See here

Packard, Steve (1988). "Chronicles of Restoration: Just a Few Oddball Species: Restoration and the Rediscovery of the Tallgrass Savanna". Restoration & Management Notes. 6 (1): 13–22.

Packard, Stephen; Mutel, Cornelia F., eds. (1997). The Tallgrass Restoration Handbook: For Prairies, Savannas, and Woodlands. Island Press.

Scott L. Collins,* Alan K. Knapp, John M. Briggs, John M. Blair, Ernest M. Steinauer Modulation of Diversity by Grazing and Mowing in Native Tallgrass Prairie. Science Vol 280. 1998

The University of Northern Iowa has abundant resources here.