Until the late 1800s , large herds of bison grazed the area until they were extirpated by human hunting .
Cattle have -- to a degree -- taken herds ' place as grazers , maintaining the grassland ecosystem in a mix of grazed and ungrazed patches .
Until the late 1800s , large herds of bison grazed the area until they were extirpated by human hunting , and cattle have -- to a degree -- taken their place as grazers , maintaining the grassland ecosystem in a mix of grazed and ungrazed patches .