Native prairie once covered 400,000 square miles of North America, one-third of the vast region stretching from Ohio to the Rockies and from Canada to Texas. North American grasslands include tallgrass, midgrass, and shortgrass prairies. Tallgrass prairies are dominated by tall grasses such as big bluestem, Indiangrass, switchgrass, cordgrass, and a variety of wildflowers, with grasses that can grow to heights of six feet and root systems extending downward to a depth of nine feet. Midgrass prairies are dominated by grasses such as western wheatgrass, green needlegrass, porcupinegrass, needle and thread, sideoats grama, and blue grama grass, which grow up to two or three feet tall and are often mixed with wildflowers. Today the tallgrass prairies are small islands in an ocean of development, and less than one percent of this ecosystem remains. Healthy prairie ecosystems supported vast populations of free roaming grazers such as the North American bison, and they benefited from periodic wildfire, which rejuvenates grasslands by removing old plant residue and destroying encroaching trees and shrubs. Prairies are among the most imperiled habitats nationwide, and although they can flourish with properly managed grazing, both over-utilization and under-utilization can weaken the delicate balance. A weakened plant community is easily overcome by invasive, non-native plants competing for valuable nutrients, and one of the greatest threats to native prairies is invasion by non-native species such as leafy spurge, smooth bromegrass, and Kentucky bluegrass.