Non-native plants are continually being introduced into natural ecosystems. These introductions occur by accident, when non-native species hitchhike to the U.S. on ships, planes, cars, or people. Quite frequently, however, non-native species are intentionally introduced. A good example of this is the use of non-native species as ornamentals for lawn and garden beautification. Of all the non-native species in the U.S., a few will demonstrate extreme vigor and competitive ability in their new habitat, and are referred to as invasive species. Famous examples of invasive species include: kudzu, garlic mustard, Japanese honeysuckle, and multiflora rose. The debate is ongoing concerning what characteristics allow particular non-native species to become invasive. Whatever the cause, invasives represent a major threat to native biodiversity in the U.S. due to their ability to outcompete native species. Once established, invasives often form dense, monotypic stands that prevent native re-establishment in the area.