Prairie historically covered 400,000 square miles of North America. This sea of grass stretched from the Rocky Mountains to east of the Mississippi River, from Saskatchewan, south into Texas. It was the continent’s largest continuous ecosystem supporting an enormous quantity of plants and animals. Prairie began appearing in the mid-continent from 8,000 to 10,000 years ago and has developed into one of the most complicated ecosystems in the world, surpassed only by the rain forests of Brazil.

Prairies exist in areas to wet for desert and to dry for forests.  Today, the most fertile and well-watered region, the tallgrass prairie, has been reduced to less than 4% of it's original area. This makes it one of the rarest and most endangered ecosystems in the world.

Prairies respond to their environment, which includes soil type, water availability, and natural processes such as grazing and fire. Tallgrass Prairie National Preserve is an eastern prairie. Eastern prairies are wetter and support Indian grass, switch grass, little bluestem and big bluestem grasses growing to heights of  up to eight feet tall!
 
Eighty percent of the foliage is indeed made up of grasses, from 40 to 60 different species. The other twenty percent of the primary vegetation is made up of over three hundred species of forbs or flowers.