Exotic or alien species are those that do not occur naturally, but have been introduced to an area and become established. Exotic species compete for territories (food, habitat or nutrients) with endemic (naturally occurring) species, often very successfully. These newcomers may lack predators, parasites, or other natural controls and find abundant disturbed habitat to occupy, resulting in the limitation or even replacement of native species. There are many documented accounts of exotic species having a major impact, either physical or biological, or both, on natural ecosystems. Purple loose-strife (Lythrum salicaria), for example, may be so invasive in undisturbed riparian habitats that it can completely destroy native wetlands fish and wildlife habitat. (Harding, 1994). Another example is knapweed, especially the Spotted Knapweed (Centaurea maculosa) which has invaded grasslands in the southern interior including the Columbia Basin. Diffuse and spotted knapweeds cause environmental deterioration and major losses in terms of beef production, recreation and wildlife habitat. Costs of control are estimated at greater than $1 million per year in the province (Harding, 1994).

Notwithstanding the environmental implications, some exotic species may be considered beneficial. Many introduced wildflowers are aesthetically pleasing, as well as adding nitrogen to the soil and contributing to soil stability and development. They also provide seeds for birds and small mammals and nectar for hummingbirds and insects (Harding, 1994). Some insects have been introduced successfully for the purposes of biological control.