There have bee numerous well-documented extinctions of indigenous species caused by the introduction of non-indigenous predators and pathogens. However, surprisingly few extinctions of indigenous species can be attributed to competition from introduced species. For example, during the past 400 years, 4,000 plant species have been introduced into North America, and these non-indigenous plants urgently account for nearly 20 percent of North America's plant species. Yet no evidence exists that any indigenous North American plant species became extinct as a result of competition from introduced plant species. The scarcity of documented extinctions caused by competition from new species could Mean that such extinctions take longer to occur than scientists initially believed or, alternatively, that extinctions are rely caused by competition from non-indigenous species.