Introduced species are plants and species which have been translocated by human agency into lands or waters where they have not lived previously, at least during historic times. Such translocation of species always involves an element of risk if not of serious danger. Newly arrived species, depending on their interspecific relationships and characteristics, may act as or carry parasites or diseases, prey upon native organisms, display toxic reactions, or be highly competitive with or otherwise adversely affect native species and communities. They may be scientifically or aesthetically undesirable, and this is of special importance in natural areas which have been reserved for study or recreation. Some may become a nuisance through sheer overabundance. They may become liable to rapid genetic changes in their new environment or take advantage of abilities which were not significant to them in their original habitat. New food habits, for example, may cause irreversible damage in areas into which the species have been introduced. Possibly the greatest danger lies in the uncertainty of prior appraisal of a species proposed for an introduction. Many harmful introductions have been made by persons unqualified to anticipate the often complex ecological interactions which may ensue.
The reintroduction of exterminated species is bound to fail if the chosen species became extinct in the area too long ago and if the environment itself has undergone too many changes. Reintroduction needs years of careful planning - the approval of local population, technical conditions of the release, feeding system, protection and breeding control - and even then some unexpected problems may arise.
"Exotic", "alien", "introduced", "nonindigenous" and "nonnative" are all synonyms for species that humans intentionally or unintentionally introduced into an area outside of a species' natural range.
Man has transported both wild and domesticated species, and micro-organisms, across the world. In their new environments, many species have become established, have multiplied and have become nuisances. There exist numerous examples of adverse consequences of introductions, ranging from minor economic, cultural or scientific losses, up to unmitigated disasters. Some examples are foot-and-mouth disease Epizootic aphthae, which has often been transmitted both to domestic animals and wild ungulates around the world by introduced livestock. Introductions of the water hyacinth Eichornia crassipes to Africa and Asia, the European rabbit Oryctolagus cuniculus to Australia, the 'possum' Trichosurus vulpecula to New Zealand, the starling Sturnus vulgaris and gypsy moth Porthetria dispar to America were each, in various ways, calamitous. This epithet certainly also applies to many accidental introductions, such the chestnut blight Endothia parasitica fungus which has exterminated the beautiful and valuable tree Castanea dentata from North American forests, and the beetle which carries a parasitic fungus Ceratostomella ulmi, now decimating the American elm Ulmus americana.
Some introductions that have been deliberate, and even well-intentioned, have still had serious consequences. Thus, the rabbit in Australia (which devastated the Australian grasslands), the sea lamprey in Canada (which resulted in the extinction of the most important commercial fish from the Great Lakes), the giant African snail in the Pacific and the edible snail in California were purposely introduced to serve as food but multiplied beyond expectations to become pests. Many other destructive species have been and still are being introduced unintentionally to new regions. The problem is acute with commensals (such as domestic pests), many of which have spread from their centres of evolution to become partially or totally coextensive with man. Rodents despoil both standing and stored food-crops. The connected series of introduced rat, its parasitic fleas and the plague-virus of those insects, emphasizes the human disease aspect of introduced organisms.
Over the last 25 years the riparian forests of coastal southern California have become infested with Arundo donax which has spread by flood-fragmentation and dispersal of vegetative propagules. Arundo donax dramatically alters the ecological/successional processes in riparian systems and ultimately moves most riparian habitats towards pure stands of this alien grass.