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A mixedgrass community occupied nearly all of central and western North Dakota. This community was less than 3 feet tall, and was dominated by a mixture of mid- and shortgrasses such as western wheatgrass, prairie junegrass, needle-and-thread, and blue grama. Besides the bison, elk, and pronghorn, characteristic mammals of this community included the kit fox and prairie dog. Common birds were the lark bunting, Baird's sparrow, and chestnut-collared longspur. A unique combination of mixedgrass prairie, brushlands, woodlands, exposed highly eroded bedrock, and flat-topped buttes exists along the Little Missouri River. This is the Badlands area that was home to the Audubon's sheep, kangaroo rat, coyote, and other western animals.

Most ecologists now agree that the prairie grasslands probably existed as an ever-changing, patchy mosaic influenced primarily by grazing, fire, climatic fluctuations, and to a lesser extent by the activities of the native Americans. Grazing by large ungulates, especially the huge northern bison herd which almost always was in slow migration, was probably the most important biological factor, but irregular outbreaks of orthopteran insects and the presence of large colonies of fossorial mammals also kept the grasses short over large areas. Fire devastated huge areas with great regularity, especially during the late summer and fall when lightning storms are common. Some ecologists attribute the basic formation of grasslands to fire and see strong correlations between the occurrence of grasslands and continental patterns of lightning frequencies. Unpredictable drought and wet periods, often of long duration, also resulted in shifts in the dominant plant and animal communities over broad areas.