Non-native species (a.k.a. alien, introduced, non-indigenous, or exotic species) are plant or animal species that were intentionally or unintentionally introduced into areas outside of the natural range of such species. The invasion of non-native species is one of the most serious problems of national parks. Non-natives invade and colonize parks by every means possible and can cause severe ecological damage by altogether displacing native species. If non-natives are not aggressively controlled, the National Park System is at risk of losing a significant portion of its native biological resources.