Prairies exist in areas too wet for desert yet too dry to support healthy forests. Prairies respond to their environment, which includes soil type, water availability, and natural forces such as grazing and fire. These have resulted in three distinct prairie regions. In the West, in the dry Rocky Mountain rain shadow, there is the ankle high short-grass prairie with its buffalo grass and blue grama. The eastern prairies are wetter and support tallgrass prairies with Big bluestem, Indian grass, and Switch grass growing to heights of eight feet at times. Between lies the mid-grass prairie dominated by side-oats grama and wheatgrass, with a mixture of shortgrass prairies in dry sites and tallgrass in wetter sites. The prairie is well known for its fauna. Some authors have estimated that there were between 30-60 million bison roaming the prairies. Elk, deer, and antelope also grazed in astounding numbers. Large predators preying on the grazers included the grizzly bear and wolf. Hoards of smaller wildlife from birds to pocket gophers were inhabitants adapted to this unique ecosystem.