Nonnatives, or exotics, are species that have been intentionally or unintentionally introduced to an area outside of their native range as a result of human activity. These species are not natural parts of our ecological communities, which includes plants and animals. Native ecological communities have evolved together over time as species adapted to new pressures and natural processes. Some species became extinct or extirpated (locally extinct) while others colonized the communities. As these changes took place, the original community was lost over time. This process is slow, but can be accelerated by fire, flood, and sudden climate change.
The introduction of nonnative species can disrupt the native ecological community which has developed over time. Because nonnative species are not a natural part of our ecological systems, they have not evolved with native species and lack natural checks on their populations, such as predators, competitors, and parasites. Nonnatives are typically adaptable and tolerant to a wide variety of conditions and are able to take over an area, sometimes forming a monoculture (an area dominated by a single species). Though some nonnative plants do not reproduce without human help (e.g., crops and ornamental plants), some spread very quickly and aggressively. Nonnatives can replace native species, which degrades the integrity and diversity of an area. These species disrupt the complex native ecological communities and the natural evolution of those communities, degrading native habitats and polluting gene pools by hybridization. The presences of nonnative species can also alter fire, water, nutrient cycles, and food chains.