Exotic Species

Exotic species, also termed nonindigenous, nonnative, alien, naturalized, or introduced, are species that are not native to a particular geographic area and were introduced intentionally or unintentionally by human activities. These introductions allow the species to overcome natural barriers to dispersal. In practice, classifying a species as native or exotic can be difficult when it is unclear how or when a species was introduced or if hybridization occurred with native species. Exotic species can be any type of organism, including microorganisms, plants, or animals. It is important to recognize that a species exotic in one area has an area to which it is native and that the behavior of the species in the two habitats can be completely different. Key features of exotic species include their geographic distribution, threats to introduction areas, and amenability to control treatments.

The pervasiveness of human movement and activity means that exotic species are present throughout the world. Even the Antarctic Peninsula is home to an exotic spider crab (Hyas araneus) and unknown numbers of exotic microorganisms. Exotics typically become distributed by long-distance dispersal (e.g., moving from one continent to another by ships or planes), followed by local secondary dispersal (e.g., transportation by cars or animals from one area to another). Multiple introductions can be required for a species to become established, and a slow initial population growth may be followed by exponential growth as invasion proceeds. A classic example of an exotic species, the European rabbit (Oryctolagus cuniculus), invaded Australia in this way. Repeated introductions made over a 70-year period beginning in the late 1700s failed, but when establishment did occur in 1859, populations grew exponentially to devastate agricultural crops expansively.

Exotic species vary in their impacts on introduction areas. Some exotics become inert components of their new habitats or provide replacement functions for native species that have become extinct. Many exotics benefit humans and are often purposely introduced (e.g., medicinal plants, food species). Overall, however, exotic species are considered the second largest cause (behind direct habitat loss) of native species extinctions. Exotics also disturb human activities and cause billions of dollars of damage annually. Exotic mussels, for example, clog municipal water intake pipes, reduce the utility of beaches, and alter fisheries.

Competitiveness, invasiveness, and the ability to engineer ecosystems are characteristics that make exotic species threats to native ecosystems. Exotics often have competitive advantages in their new environment because they escape the agents that had kept their populations in check. For instance, the prevalence of the Asian bark fungus (Cryphonectria parasitica) remains low in its native habitat because host trees have attained resistance. When this fungus was inadvertently introduced to North America on imported trees near the turn of the 20th century, it eliminated the nonresistant American chestnut (Castanea dentata) as an overstory tree.

Many exotics are invasive, readily colonizing and dominating habitats, especially disturbed areas containing open niches available for colonization. Exotics also often invade and thrive in high-resource environments (e.g., nitrogenenriched soils due to pollution) more than native species do. The ability to invade and usurp [Page 1049]resources quickly provides exotics with further competitive advantages over native species.

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