page_title: Invasive species
section_title: Vectors
hierarchical_section_title: Invasive species / Vectors
caption_attribution_description: English: The Chinese mitten crab, Eriocheir sinensis. This male specimen was found ashore in 150 metres distance to the banks of the Elbe river in the German federal state of Brandenburg. It behaves defending against the viewer by erecting the body and showing its hairy claws. The carapax’s width is estimated at six centimetres. Deutsch: Chinesische Wollhandkrabbe (Eriocheir sinensis). Dieses kräftige Männchen wurde 150 Meter abseits des Ufers der Elbe (bei Mödlich in Brandenburg) an Land angetroffen. Es hat eine Abwehrhaltung gegenüber dem Betrachter eingenommen, wobei es sich nach oben aufrichtet und seine pelzig behaarten Scheren präsentiert. Die Breite des Rückenpanzers beträgt schätzungsweise sechs Zentimeter.
context_page_description: An invasive species is a nonindigenous species that spreads from the point of introduction and becomes abundant.
  The invasive species label attaches only to populations of species whose impact upon introduction has altered their new environment. Although this impact can be beneficial, the term as most often used applies to introduced species that affect the invaded habitats and bioregions adversely, causing ecological, environmental, or economic damage. This includes plant species labeled as "exotic pest plants" and "invasive exotics" growing in native plant communities. The term is also used by land managers, botanists, researchers, horticulturalists, conservationists, and the public for noxious weeds.
  The term "invasive" is poorly defined and often very subjective, and some broaden the term to include indigenous or "native" species that have colonized natural areas – for example deer considered by some to be overpopulating their native zones and adjacent suburban gardens in the Northeastern and Pacific Coast regions of the United States. The definition of "native" is also sometimes controversial.
context_section_description: Non-native species have many vectors, including biogenic vectors, but most invasions are associated with human activity. Natural range extensions are common in many species, but the rate and magnitude of human-mediated extensions in these species tend to be much larger than natural extensions, and humans typically carry specimens greater distances than natural forces.
  An early human vector occurred when prehistoric humans introduced the Pacific rat (Rattus exulans) to Polynesia.
  Vectors include plants or seeds imported for horticulture. The pet trade moves animals across borders, where they can escape and become invasive. Organisms stow away on transport vehicles. Diseases may also be vectored by invasive insects such as the Asian citrus psyllid and the bacterial disease citrus greening.
  The arrival of invasive propagules to a new site is a function of the site's invasibility.
  Species have also been introduced intentionally. For example, to feel more "at home," American colonists formed "Acclimation Societies" that repeatedly imported birds that were native to Europe to North America and other distant lands. In 2008, U.S. postal workers in Pennsylvania noticed noises coming from inside a box from Taiwan; the box contained more than two dozen live beetles. Agricultural Research Service entomologists identified them as the rhinoceros beetle, Hercules beetle, and king stag beetle. Because these species were not native to the U.S., they could have threatened native ecosystems. To prevent exotic species from becoming a problem in the U.S., special handling and permits are required when living materials are shipped from foreign countries. USDA programs such as Smuggling Interdiction and Trade Compliance (SITC) attempt to prevent exotic species outbreaks in America.
  Many invasive species, once they are dominant in the area, are essential to the ecosystem of that area. If they are removed from the location it could be harmful to that area.
  Economics plays a major role in exotic species introduction. High demand for the valuable Chinese mitten crab is one explanation for the possible intentional release of the species in foreign waters.
caption: Chinese mitten crab (Eriocheir sinensis)
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