The desert grasslands are widely distributed in North America, occupying much of southeastern Arizona, the southern half of New Mexico, and west Texas in the United States and extending southward through 13 states in Mexico, from Sonora to Puebla. They represent more than 500,000 km² of basin and valley lands that skirt the hills and mountain ranges of southwestern North America (McClaran 1995). The entire geographic range of the desert grasslands has seen a rapid decline in grass cover and an increase in cover by woody shrubs, especially mesquite (Prosopis spp.) and creosotebush (Larrea tridentata). Traditionally, the desert grasslands have supported extensive stands of perennial bunch grasses, known colloquially as the grama grasses, which reproduce primarily by stolons. Other, smaller bunch grasses occupied habitat patches such as infrequently inundated swales that support a cover of tabosa grass, Hilaria mutica.