Each year, many thousands of species are being extinguished as a result of human activities, such as habitat destruction and exotic species introductions. The actual number of species in existence is unkown and can only be estimated because we really only know about the species that botanists, entomologists and other scientists have been able to collect, process and identify to date. . The term noxious is a legal designation used specifically for plant species that have been determined to be major pests of agricultural ecosystems and are subject, by law, to certain restrictions. The U.S. Department of Agriculture regulates noxious weeds. For a list of these weeds click here .
Many noxious weeds designated for their impacts to agriculture also threaten natural areas.
Additional listings of exotic pest plants affecting natural ecosystems are expected, as their ecological and economic impacts continue to grow. Back Impacts of Invasive Alien Plants. Invasive non-native organisms are one of the greatest threats to the natural ecosystems of the U.S. and are destroying America's natural history and identity. These unwelcome plants, insects and other organisms are disrupting the ecology of natural ecosystems, displacing native plant and animal species, and degrading our nation's unique and diverse biological resources. Still others contain toxins that may be lethal to certain animals.