An invasive species is a species introduced outside its native geographical range that establishes and spreads and causes negative ecological, economic or social impacts. The term is more specific than non native, introduced or alien species. A species can be introduced to a new region without becoming invasive. It may fail to establish, remain confined to human managed environments or coexist without significant harmful effects. The invasion process generally involves transport, introduction, establishment and subsequent spread.
Invasive species are important because they can alter native communities, ecosystems and ecological processes and can contribute to biodiversity loss. IPBES identifies invasive alien species as one of the five major direct global drivers of biodiversity loss and reports that more than 37,000 alien species have been recorded worldwide, including more than 3,500 invasive alien species.
The distinction between native, alien and invasive species is fundamental. A native species occurs naturally in a region without human mediated introduction from elsewhere. An alien species, also called introduced, non native or exotic in some contexts, occurs outside its historical native range because of human activity or other mechanisms. An alien species becomes invasive when it establishes and spreads and produces harmful effects.
Therefore, not every alien species is invasive and not every introduced organism automatically represents an ecological threat. This terminology matters because labelling all non native species as invasive would incorrectly treat harmless introduced organisms and damaging invaders as equivalent. Scientific assessments generally consider evidence of establishment, spread and impact. The definition can also vary slightly among legal and ecological frameworks, particularly concerning whether demonstrated harm is required or whether potential harmfulness is sufficient for regulatory purposes.
Human activity is the dominant pathway for many long distance biological introductions. International trade, shipping, aviation, agriculture, horticulture, forestry, aquaculture, tourism and transport can move organisms across natural geographical barriers. Species may be transported intentionally as crops, ornamental plants, pets, livestock or biological control agents, or accidentally in soil, ballast water, cargo, packaging, vehicles and contaminated equipment.
Once introduced, establishment depends on environmental compatibility, population size, reproduction, food availability, absence of effective predators and competitors and the ability to tolerate local conditions. Climate change can modify these conditions by making previously unsuitable environments more favourable. IPBES describes biological invasions as a process involving transport, introduction, establishment and spread, with multiple human activities facilitating movement between regions.
Invasive species can harm native biodiversity through several mechanisms. Competition occurs when invaders use resources also required by native species, such as food, light, water, nesting sites or nutrients. Predation can be especially severe where native organisms have no evolutionary history with the introduced predator. Invasive plants can alter vegetation structure, fire regimes, soil chemistry or water availability. Introduced pathogens and parasites can infect species that lack effective defences. Hybridisation can alter the genetic composition of native populations.
Some invaders act as ecosystem engineers by physically changing habitats, while others modify nutrient cycles or food webs. The consequences can range from reduced population sizes to local extirpation and, in severe cases, global extinction. IPBES notes that biological invasions can produce dramatic and sometimes irreversible changes and can affect economies, food security and human health as well as biodiversity.
Islands are often particularly susceptible to biological invasions because their native communities evolved under unusual combinations of isolation, limited predator diversity and specialised ecological relationships. A frequently cited example is the brown tree snake on Guam. Introduced accidentally during the twentieth century, the snake encountered abundant prey and few effective natural predators and subsequently spread across the island. It contributed to the decline and extinction of numerous native forest bird species. The example illustrates why ecological impacts depend on context.
A predator that is relatively ordinary within its native ecosystem can become highly disruptive after introduction into an ecosystem whose species evolved without it. Island vulnerability does not mean continental ecosystems are immune. Continental environments can also experience major invasions, particularly when human disturbance creates opportunities for establishment or when introduced species possess traits that allow rapid reproduction and dispersal.
Some invasive species become successful because they possess ecological characteristics such as rapid reproduction, broad diets, high dispersal ability, tolerance of environmental variation or strong competitive abilities. Disturbed environments can provide opportunities because native communities may already be weakened by habitat loss, pollution or land use change. The enemy release hypothesis proposes that introduced organisms may sometimes benefit from escaping predators, parasites or pathogens that regulate them in their native ranges, although this mechanism does not explain every invasion.
Climate change can further influence invasion by shifting temperature and precipitation conditions and allowing some established alien species to expand into new areas. The relationship between disturbance, climate, resource availability and invasion is therefore complex. Invasive species should not be understood as inherently biologically superior organisms. Their success depends on interactions between species traits and the ecological conditions of the recipient environment.
Managing invasive species can involve prevention, early detection, eradication, containment, control and long term ecosystem restoration. Prevention is often the most efficient strategy because eliminating an invasive population becomes increasingly difficult after it spreads across a large area. Measures can include quarantine, inspection, cleaning of equipment, regulation of trade, ballast water management and restrictions on high risk species. Early detection allows rapid response before populations become widespread.
Established invasives can sometimes be removed mechanically, chemically or biologically, although each method carries ecological and practical risks. Biological control introduces natural enemies under carefully assessed conditions, but poorly designed biological control programmes can themselves create new ecological problems. Restoration after removal may also be necessary because altered habitats can remain vulnerable to reinvasion. Effective management therefore requires knowledge of species biology, pathways of introduction, ecosystem conditions, costs and potential unintended consequences.
The term invasive species should consequently be interpreted as an ecological and management category rather than simply an emotional label for a non native organism. The central concepts are introduction outside the native range, establishment, spread and harmful impact. Invasive species are important because they can interact with the other major drivers of biodiversity loss, including habitat change, exploitation, pollution and climate change.
They can disrupt food webs, alter habitats and ecological processes, compete with native species and contribute to extinctions. At the same time, the impacts of individual introduced species vary considerably, so assessment should be based on evidence rather than on origin alone. Modern invasion biology increasingly considers the entire invasion pathway from prevention and transport through establishment, spread, impact and control. This systems perspective is essential because preventing introduction can be far easier than reversing ecological changes after an invasive population has become widespread.
