Prairies are level or gently rolling grasslands where dry soil and frequent drought or fires limit or exclude trees and shrubs, and across North America their soils and climate support distinct mixes of native grasses, flowers, and other plants. In the Puget Sound region, prairies and oak woodlands occur in areas of dry soil within a landscape better known for forests, rivers, lakes, and saltwater bays. Following the retreat of glacial ice, torrents of meltwater left deep deposits of gravelly, well drained soil, and although forests first grew as the climate slowly warmed, about 10,000 years ago a warm period allowed prairie plants to take hold on the dry soils of Mima Mounds and nearby prairies. During the past 4,000 years, a wetter, cooler climate prevailed, and Native American burning practices maintained the prairies. These grasslands and oak trees provide food for insects, birds, and animals that cannot live in other habitats and are important to Washington’s biodiversity, but they are threatened by invasive species and development. Prairies once covered about 180,000 acres in western Washington, and high quality native prairies now cover only about three percent of that area.