One of the most significant stressors to the sagebrush biome is expansion and dominance of nonnative ecosystem-transforming species, particularly invasive annual and perennial plants. Presidential Executive Orders 13112 and 13751 define an invasive species as "a non-native organism whose introduction causes or is likely to cause economic or environmental harm, or harm to human, animal, or plant health." The use of the term "invasive species" requires two basic criteria to be met: (1) the species is alien, nonnative, or exotic to the ecosystem in question; and (2) the species has been documented as causing harm as noted in the definition. In addition, invasive annual and perennial plant species are categorized as either regulated species (nonnative species regulated under State or Federal noxious weed laws), or unregulated species (nonnative species which may pose a threat but have not been officially designated as regulated or restricted under State or Federal law). Cheatgrass (Bromus tectorum), for example, is not a Federally designated noxious weed, nor a State-designated noxious weed in many western States, but there are other State and local restrictions associated with this species in some areas.