Species of plants, insects, animals, or diseases which rapidly overtake and harm existing ecosystems can be classified as invasive species. These are usually (but not always) non-native species to the area. The most familiar example to people in the southeast is the kudzu plant, which prevents other plants from becoming established, and as a result takes away forage and habitat from indigenous species.
 
Groups concerned with invasive species include: 

At the University of Tennessee, several groups are working to provide resources for pest identification and integrated pest management, including:

 

Diseases:

Insects:

  

Plants:

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Snails:

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 Image of Kudzu plant Kudzu image from Kerry Britton, USDA Forest Service, Bugwood.org

Image of Giant Salvinia in the water (Photo from: www.bugwood.com Credit to: Scott Robinson)

White Garden Snail (Picture from www.bugwood.org credit to: Charles Olsen)