The study of the establishment, spread, and ecological impact of species translocated from one region or continent to another by humans. Biological invasions have gained attention as a tool for basic research, used to study the ecology and evolution of populations and of novel biotic interactions; and as a conservation issue tied to the preservation of biodiversity. The invasion of nonindigenous (also called exotic, alien, or nonnative) species is a serious concern for those charged with managing and protecting natural as well as managed ecosystems. See Ecology, Population ecology
Ecologists make a distinction between introduced species, meaning any species growing outside its natural habitat including cultivated or domesticated organisms, and invasive species, meaning the subset of introduced species that establish free-living populations in the wild. The great majority of introduced species (approximately 90% as estimated from some studies) do not become invasive. While certain problem invaders, such as the zebra mussel (Dreissena polymorpha), exact enormous economic and ecological costs, other introduced species are generally accepted as beneficial additions, such as most major food crops.
Intentional plant introductions have been promoted primarily by the horticulture industry to satisfy the public's desire for novel landscaping. However, plants have also been introduced for agriculture, for silviculture, and for control of soil erosion. Intentional animal introductions include game species brought in for sport hunting or fishing. Unlike these examples, intentional introductions can also include species that are not necessarily intended to form self-sustaining populations, such as those promoted by the aquarium or pet trade.
Species introduced accidentally are “hitchhikers.” Shipping ballast has been a major vector, first in the form of soil carrying terrestrial invertebrates and plant seeds or rhizomes, and more recently in the form of ballast water carrying planktonic larvae from foreign ports. While many species are introduced in ballast or by similar means, hitchhikers can also be unwanted parasites that bypass importation and quarantine precautions. For example, many nonindigenous agricultural weeds have been imported in contaminated seed lots.
Certain types of habitats seem to have higher numbers of established nonindigenous species than others. The characteristics that make a site open to invasion must be determined. For example, islands are notably vulnerable to invasions. Islands usually have fewer resident species to begin with, leading to the conjecture that simpler systems have less biotic resistance to invaders. That is, an introduced species is less likely to be met by a resident competitor, predator, or pathogen capable of excluding it. The idea of biotic resistance is also consistent with the idea that complexity confers stability in natural systems. See Invasion ecology
A second generalization about invasibility is that ecosystems with high levels of anthropogenic disturbance, such as agricultural fields or roadsides, seem to be more invaded. Increased turnover of open space in these sites could provide more opportunities for the establishment of new species. An alternative explanation is that many species that adapted to anthropogenic habitats in Europe simply tagged along as humans re-created those habitats in new places. Those species would naturally have an advantage over native species at exploiting human disturbances. A final suggestion by proponents of ecosystem management is that disturbance (including, in this context, a disruption of natural disturbance regimes, for example, fire suppression) weakens the inherent resistance of ecosystems and promotes invasion.
Invasive species can have several different types of impacts. First, they can affect the traits and behavior of resident organisms (for example, causing a shift in diet, size, or shape of the native species they encounter). Second, impacts can occur at the level of the population, either by changing the abundance of a native population or by changing its genetic composition. Hybridization between an invader and a closely related native can result in introgression and genetic pollution. The endpoint can be the de facto extinction of the native species when the unique aspects of its genome are overwhelmed. Third, impacts can occur at the level of ecological communities. When individual populations are reduced or even driven extinct by competition or predation by an invasive species, the result is a decrease in the overall biodiversity of the invaded site. Finally, invaders can impact not only other species but the physical characteristics of an ecosystem as well.
There are two main contributing factors in determining which species have the biggest impacts: abundance and special characteristics. Invaders that reach extremely high density simply overwhelm all other organisms. Other species have special traits that result in an impact out of proportion to their numbers.
Because of the economic and conservation importance of nonindigenous species, much of invasion ecology focuses on the prevention, eradication, and control of invaders, and the restoration of sites after control. Research has emphasized the importance of early detection and eradication of problem species. Biological control has been touted as an environmentally friendly alternative to herbicides and pesticides. See Allelopathy, Ecological communities, Ecological succession, Speciation, Species concept