ECOLOGY
Introduced species are organisms moved by human activity beyond their native geographic range, whether deliberately or accidentally. Some establish without major disruption, while a smaller subset becomes invasive by spreading and causing ecological, economic, or health harms. The category includes plants, animals, fungi, and microorganisms, and is defined by origin and movement rather than by whether the species is beneficial or damaging.
Introduced species are organisms transported by people into regions where they did not occur naturally. Movement may be intentional, as with crops, livestock, ornamental plants, game animals, or biological-control agents, or accidental, as with organisms carried in cargo, soil, nursery stock, ships, and aircraft.1 The terms non-native, alien, and exotic are often used similarly, although technical definitions vary by country and discipline. An introduction can occur across continents or across ecological boundaries within one country.
Transport is only the first stage. A species must survive arrival, reproduce, and spread to become established. Many introductions fail because of unsuitable climate, small founding populations, predators, disease, or limited food. Others remain localized for years before a population increase produces a recognizable invasion, a delay sometimes called an invasion lag.2
“Invasive species” describes an introduced species whose spread produces significant adverse effects, not every organism living outside its native range. Effects may include predation, competition, herbivory, disease transmission, habitat alteration, hybridization, and disruption of nutrient or fire regimes. Impacts can also reach agriculture, fisheries, forestry, infrastructure, tourism, and human health, although their magnitude depends strongly on location and context.
Introductions sometimes provide food, timber, erosion control, pollination, or other ecosystem services. The same species can be valuable in one setting and harmful in another, and benefits may accrue to people while costs fall on native species or neighboring communities. Island ecosystems, freshwater systems, and habitats already disturbed by land-use change are often especially vulnerable, but no environment is uniformly protected. Genetic effects, including hybridization with native relatives, can occur even when population numbers appear small.
Prevention and early detection are generally more effective and less costly than controlling a widespread introduction. Risk assessment can examine a species’ biology, likely pathways, climate suitability, and potential impacts before importation or release. Measures include quarantine, inspection of traded goods, cleaning equipment, regulating ornamental and live-animal trade, and treating ships’ ballast water, a major pathway for aquatic organisms.
Once a species is established, managers may eradicate small populations, contain expanding fronts, suppress populations, or accept coexistence while protecting high-value sites. Methods include physical removal, targeted chemicals, habitat restoration, and carefully evaluated biological control. Removal can have unintended effects if predators, food webs, or sediment processes have changed. The Convention on Biological Diversity promotes prevention, control, and eradication within broader national biodiversity strategies.1 Long-term monitoring is necessary because recolonization and repeated introductions can undermine short campaigns.
Introductions often involve several interacting species rather than a single isolated arrival. Cargo networks can create repeated “propagule pressure,” in which frequent arrivals and larger founding populations increase the chance of establishment.2 Some organisms arrive in stages: a plant may first be cultivated, later escape into surrounding habitat, and finally spread through roads, rivers, or altered disturbance regimes.
Not every non-native population is easy to classify. Urban parks, reservoirs, farms, and other human-made environments may support novel ecosystems containing mixtures of native and introduced organisms. In freshwater science, records such as the United States Geological Survey’s Nonindigenous Aquatic Species database show how detection depends on sampling effort and reporting practices.3 A less visible dimension is disease: introduced pathogens and parasites can transform native populations even when the carrier species itself attracts little public attention. Effective decisions therefore weigh ecological evidence, social priorities, reversibility, and uncertainty rather than relying on origin alone.
Terminology differs among jurisdictions: some authorities use “introduced,” “alien,” “exotic,” and “non-native” interchangeably, while “invasive” generally adds an impact or spread criterion.
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