Exotic species

	Sometimes an organism is found outside of its natural range. When organisms are found living in parts of the world where they are not native, these organisms are referred to as "exotic species" in that new location. Most organisms that reach new locations as exotics are transported to those locations by human activity. Many times, humans purposefully bring new species into an area. Other times, people move organisms about without really knowing it--for example, organisms may "hitch rides" on cars or ships or they may be found in fruits or wood or other cargo that is moved from one location to another.
It is estimated that at least 4,500 exotic species are now established in the United States. Many of these are very beneficial. Examples include most crop plants and domesticated animals that we raise, many types of sport fish, and many organisms that are natural enemies of important insect pests. However, exotic species also can be very harmful. They can become serious pests in the new areas where they have no natural enemies and plants have not developed resistance to the exotics. They may cause decline of native species with which they compete, alter structure and functioning of ecosystems, and lead to serious health concerns and economic loss for humans. The cumulative estimated loss due to selected harmful exotic insects in the United States from 1906-1991 was close to $93 billion.
	In American forests, exotic insects and disease organisms have been major pests and have caused much tree injury and death. Examples of important introduced pest species include chestnut blight, Dutch elm disease, white pine blister rust, common pine shoot beetle, pear thrips, and European gypsy moth. Exotic plant species such as honeysuckle, buckthorn, and purple loosestrife may compete with native plant species in American ecosystems. On the other hand, a variety of exotic species have been purposefully and successfully introduced to act as natural enemies of important forest pests and have greatly aided in control of these pests.