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The arid and semiarid Great Basin of the western United States comprises parts of California, Nevada, Utah, Idaho, and Oregon and can variously be described by its hydrology, topography, or biology. Biologically, the area has been defined historically by the native sagebrush and shrubs that thrive in the dry valleys of the Sierra Nevada and Cascade Mountains. But, as a recent study undertaken by researchers at the University of Montana and the Department of Agriculture notes, these native plant communities are rapidly being colonized by nonnative annual grasses like cheatgrass (Bromus tectorum), red brome (B. rubens), and medusahead (Taeniatherum caput-medusae) to the detriment of wildlife and humans.