Invasive exotic plants crowd out native plants that provide important food and habitat for wildlife. Across the country, ducks, geese, and muskrats cannot utilize wetlands taken over by purple loosestrife. In Montana, elk cannot find winter forage on grasslands replaced by spotted knapweed. Woodpeckers and bluebirds cannot nest in Russian olive trees lining rivers of the Great Plains. In Idaho, fires fueled by invading cheat grass eliminate the sagebrush depended on by sage grouse as well as the rabbits and rodents preyed on by the abundant hawks.
Invasive plants can change the entire structure of an ecosystem.
In the forests of the east, exotic vines and shrubs prevent tree seedlings from growing up to replace mature trees as they die. Buckthorn does the same in northern marshes. Preventing the introduction and spread of invasive plants provides the best protection for the vast areas of wildlands that are not yet infested and minimizes all environmental costs. People, vehicles, pack animals, and livestock entering an area are also likely to transport weed seeds to the now-hospitable conditions. Nearly three-quarters of the plant species invading our wetlands and forests, and a smaller but still substantial proportion of plants invading western grasslands are sold by commercial nurseries.