page_title: Invasive species
section_title: Ecosystem-based mechanisms
hierarchical_section_title: Invasive species / Causes / Ecosystem-based mechanisms
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: In ecosystems, the amount of available resources and the extent to which those resources are used by organisms determine the effects of additional species on the ecosystem. In stable ecosystems, equilibrium exists in the use of available resources. These mechanisms describe a situation in which the ecosystem has suffered a disturbance, which changes the fundamental nature of the ecosystem.
  When changes such as a forest fire occur, normal succession favors native grasses and forbs. An introduced species that can spread faster than natives can use resources that would have been available to native species, squeezing them out. Nitrogen and phosphorus are often the limiting factors in these situations.
  Every species occupies a niche in its native ecosystem; some species fill large and varied roles, while others are highly specialized. Some invading species fill niches that are not used by native species, and they also can create new niches. An example of this type can be found within the Lampropholis delicata species of skink. Invasion is more likely in ecosystems that are similar to the one in which the potential invader evolved. 
  Ecosystem changes can alter species' distributions. For example, edge effects describe what happens when part of an ecosystem is disturbed as when land is cleared for agriculture. The boundary between remaining undisturbed habitat and the newly cleared land itself forms a distinct habitat, creating new winners and losers and possibly hosting species that would not thrive outside the boundary habitat.
  In 1958, Charles S. Elton claimed that ecosystems with higher species diversity were less subject to invasive species because of fewer available niches. Other ecologists later pointed to highly diverse, but heavily invaded ecosystems and argued that ecosystems with high species diversity were more susceptible to invasion.
  This debate hinged on the spatial scale at which invasion studies were performed, and the issue of how diversity affects susceptibility remained unresolved as of 2011. Small-scale studies tended to show a negative relationship between diversity and invasion, while large-scale studies tended to show the reverse. The latter result may be a side-effect of invasives' ability to capitalize on increased resource availability and weaker species interactions that are more common when larger samples are considered. However, this spatial scale dependent pattern of the effects of invasion on diversity does not seem to hold true when the invader is a vertebrate.
  Island ecosystems may be more prone to invasion because their species face few strong competitors and predators, or because their distance from colonizing species populations makes them more likely to have "open" niches. An example of this phenomenon is the decimation of native bird populations on Guam by the invasive brown tree snake. Conversely, invaded ecosystems may lack the natural competitors and predators that check invasives' growth in their native ecosystems.
  On small islands, native birds may have become flightless because of the absence of predators prior to introductions. These birds cannot readily escape the danger brought to them by introduced predators. The tendency of rails in particular to evolve flightless forms on islands has made them vulnerable and has led to the disproportionate number of extinctions in that family.
  The islands of Hawaii have many invasive species affecting the islands' native plants and animals. Invasive insects, plants, hoofed animals such as deer, goats and pigs endanger native plants, rosy wolfsnails from Africa feed on the island's native snails, and plants such as Australian tree fern and Miconia calvescens shade out native plants. Populations of introduced little fire ants in Hawaii can have major negative impacts on animals, crops, and humans. The veiled chameleon and the Jackson's chameleon have a great impact on the ecology of Hawaii.
  In New Zealand the first invasive species were the dogs and rats brought by Polynesian settlers around 1300. Cats, brought later by Europeans, have had a dev
caption: The brown tree snake (Boiga irregularis)
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