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Understanding invasive species

Prohibited species Wisconsin Statute Section 23.22 (1) (c) officially defines invasive species as "nonindigenous species whose introduction causes or is likely to cause economic or environmental harm or harm to human health."

Humans move organisms around all the time. Sometimes when we bring a non-native species into a new area the species will take over and spread rapidly and widely throughout the area. When this happens, the spread can cause major harm to the native ecosystem or humans. When non-native plants, animals, or pathogens rapidly takes over a new location and alter the ecosystem, we consider them invasive species.

How they become a problem

One of the reasons that invasive species are able to succeed is that they often leave their predators and competitors behind in their native ecosystems. Without these natural checks and balances they are able to reproduce rapidly and out-compete native species.

Invasive species can alter ecological relationships among native species and can affect ecosystem function, economic value of ecosystems, and human health. You can learn more about the impacts of invasive species on our why should we care page.

Humans have created conditions where plants and animals can aggressively invade and dominate natural areas and water bodies in three ways:

  • introducing exotic species (from other regions or countries) who lack natural competitors and predators to keep them in check;
  • disrupting the delicate balance of native ecosystems by changing environmental conditions -- e.g., stream sedimentation, ditching, building roads) or by restricting or eliminating natural processes (fire for example); in such instances, even some native plants and animals can become invasive; and
  • spreading invasive species through various methods. Some examples:
    • moving watercrafts from waterbody to waterbody without removing invasive plants and animals;
    • carrying seeds of invasive plants on footwear or pet’s fur;
    • mowing along roadsides;
    • importing firewood and leaving in campgrounds;
    • driving and biking with invasive seeds in tire treads.

The net result is a loss of diversity of our native plants and animals as invasive species rapidly multiply and take over. About 42 percent of the species on the federal Threatened or Endangered species lists are at risk primarily because of invasive species.

Last revised: Monday February 27 2012