A Complex Prairie Ecosystem. Prairies historically covered 170 million acres of North America. This sea of grass stretched from the Rocky Mountains to east of the Mississippi River and from Saskatchewan, south to Texas. It was the continent's largest continuous ecosystem supporting an enormous quantity of plants and animals. Prairies began appearing in the mid-continent from 8,000 to 10,000 years ago and have developed into one of the most complicated and diverse ecosystems in the world, surpassed only by the rainforest of Brazil. A Complex Prairie Ecosystem - Tallgrass Prairie National Preserve (U.S. National Park Service)