"Exotics", "Non-natives", "Aliens", "nonindigenous species", and "Invaders" all refer to a species that is not native to a particular area. There are several effects on native flora, fauna and landscape when they become established and start reproducing in time frames that are insignificant in respect to evolutionary (geological) time.
The great majority of invasive species today are a result (direct or indirect) of human-kind. The organisms are transferred as humans travel between villages, between countries, and now, across the globe. This new species can be either out competed and unsuccessful or become competitive with positive, neutral or negative effects.
There are four main questions to ask when determining the effect of an invader on an environment: (G.K. Meffe and C.R. Carroll, 1997)
1. Which species are most likely to invade communities?
2. Which native species are the communities most likely to loose?
3. Which invasive species will cause extensive extinctions of native species?
4. Which native species extinctions will lead to many further losses of species and change in community structure?
Ariel E. Lugo suggested that not all invasive species do harm to the new environment. He points out that some non native species " can be important tools for land rehabilitation and restoration of biological diversity in damaged sites where natural selection is arrested." He references a study where an exotic tree is planted on lava flows of Hawaii to help speed the process of succession by the nitrogen enrichment properties of Myrica faya. I think that these examples are in the minority when dealing with invasive species.
Some invasive species are known to be very harmful to the new environments such as domestic cats (Felis domesticus), predatory fish such as the mosquitofish (Gambusia spp.), rats, goats, zebra mussels, and fire ants just to name a few. The mosquitofish are intentionally introduced (sometimes in state funded programs) as a means to lower the mosquito populations near streams, rivers, and lakes. As for domestic cats, studies have shown feral cats to be more than twice as likely to cause damage to the new community than other introduced birds and mammals. Species such as these should not be introduced under any circumstances.
Links to sites with information on Invasive species around the world:
Fire ants (Solenopsis invicta) http://uts.cc.utexas.edu/~gilbert/research/fireants/faqans.html
Kudzu (Pueraria thunbergiana) http://www.ipass.net/~lineback/kud.htm
Australian invaders http://www.environment.gov.au/bg/plants/manageme/feral.htm
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