Areas that were mostly covered with grasses and subdominant forbs (broadleaved herbs) prior to human intervention are known as grasslands. Extensive tracts of these grassland ecosystems in the drier interiors of continents, particularly in North America, are known as prairies. Grasslands occur where there is not enough precipitation to support a forest but there is enough for herbaceous plants. The grasses and lack of precipitation are just two of the characteristics that set apart the prairies from the other ecoregions of North America.

The beginnings of the North American grasslands occurred during the Miocene-Pliocene transition, perhaps 5 million to 7 million years ago. This time period is associated with widespread drying. The increased aridity resulted from the cooling of the oceans as the Antarctic ice sheet ...

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