Stages of Invasion
Basic Landscape Ecology
By Robert Coulson
An invasive species is defined as a non-native species to an ecosystem whose introduction causes economical, medical, and/or environmental harm. Invasive species can include plants, animals, and microbes. The first of stage of invasion, introduction involves the movement of a species into a non-native area; human transport is the most dominate form of invasive species introduction. Humans have the capability to move species to places that would be otherwise inaccessible for species. People may introduce a species intentionally or accidentally. An intentional pathway can only be judged by the impacts of the organism. Some accidental pathways include ballast water discharge, importation of resources, and the immigration of people. However, species may invade through natural pathways like wind and water currents.The second stage of invasion, colonization is the initial interaction between the invasive species and the habitat. Whether or not the species survives is up to the species persistence and chance. Sometimes, successful colonization involves repeated introductions to overcome natural and human boundaries. The third stage of invasion, establishment means the species successfully colonized into the environment and is able to reproduce. The first three stages usually requires human intervention but after the species disperses into the environment the species no longer needs assistance. The fourth and fifth stages of invasion is the dispersal of the new species and formation of populations across the environment. The United States Department of Agriculture offers detailed information on invasive species at the National Invasive Species Information Center: http://www.invasivespeciesinfo.gov/whatis.shtml