Now, just a century and a half later, only one percent of the original 18 million acres of prairie remains. Urban sprawl, agricultural expansion, and gravel mining continue to threaten this rich resource and the mammal, bird, and insect species it supports. Protection is critical if this complex ecosystem, with its shifting array of color and form, is to survive into the future. Moisture levels vary throughout, determining the types of plant communities that develop. Since fire tends to eliminate trees and shrubs while enhancing the growth of prairie grasses, wildfire frequency also affects the types and persistence of prairie communities.
Biologists identify four subtypes of dry prairie.
The sparsely vegetated barrens subtype occurs on deep deposits of sand left primarily by glacial meltwater rivers. Winds have often reworked these deposits into dunes during subsequent periods of severe drought. The hill subtype, richest in species, climbs steep slopes of loamy till also deposited by glaciers. Along steep Mississippi River bluffs in southeastern Minnesota, the bedrock bluff subtype, often called goat prairie, sprouts from a thin layer of soil over bedrock. In all four cases, frequent fires as well as low moisture levels help to prevent the encroachment of trees.