An invasive species is a species introduced outside its natural range whose establishment or spread causes harm. The Convention on Biological Diversity defines invasive alien species by their threat to biological diversity, while the United States definition also encompasses actual or likely economic and human-health harm. Invasive organisms include plants, animals, fungi, and microorganisms. Their effects depend on the receiving ecosystem: an organism invasive in one region may be native, or harmless when introduced, elsewhere. (cbd.int)
Terminology and scope
“Alien,” “introduced,” and “non-native” describe organisms moved by human activity beyond their natural past or present distributions. These terms do not themselves imply harmful effects. Many introduced crops and other useful organisms are not considered invasive. Introduction can occur within a country as well as across international borders; the relevant boundary is a natural geographic range, not necessarily a political one. (invasivespeciesinfo.gov)
Establishment means that introduced organisms reproduce successfully with the likelihood of continued survival. It is distinct from arrival, and neither arrival nor establishment automatically demonstrates invasiveness. This distinction separates the presence of an unfamiliar organism from evidence that it threatens native species or ecosystem integrity. Seeds, eggs, and other reproductive material are included in definitions of alien species because they can initiate new populations. (cbd.int)
Introduction pathways and establishment
Human-mediated pathways include deliberate releases, escapes from cultivation or captivity, contaminants in traded goods, and organisms transported unintentionally as stowaways. Horticulture, aquaculture, forestry, and the pet trade can move organisms beyond their original ranges. Nursery material, timber, soil, and shipping containers can carry unintended passengers. Increasing international trade and transport create additional opportunities for these movements. (cbd.int)
Shipping ballast water provides a well-documented aquatic pathway. Zebra mussels probably reached North America’s Great Lakes in water discharged by ships from Europe. After an initial introduction, further movement can distribute organisms through connected waterways or additional human transport. Thus, prevention addresses both entry into a new region and subsequent spread within it. (usgs.gov)
Successful establishment depends partly on whether the receiving habitat provides suitable conditions. Rapid reproduction, effective dispersal, flexible responses to environmental conditions, and broad food requirements are common among invasive species, but they do not provide a universal diagnostic rule. A history of invasion elsewhere is a useful predictor. Impacts may also become apparent long after introduction, so an initially inconspicuous population is not necessarily without future consequences. (cbd.int)
Ecological and economic effects
Invasive species can compete with native organisms for resources, consume them, introduce pathogens, and alter ecosystem composition or structure. These changes can reduce ecosystem services and affect food production and economic activities. Impacts are especially consequential where introduced organisms threaten species with restricted distributions, including those on islands. Invasion is therefore a major concern in conservation biology, not merely a question of whether a species is geographically unfamiliar. (cbd.int)
Changes to feeding relationships can reorganize a food web. Zebra mussels, for example, filter algae needed by native organisms and attach to native mussels, impairing them. They also clog water intakes, connecting ecological effects with damage to infrastructure. These impacts illustrate how one introduced organism can affect both biological communities and industrial operations. (usgs.gov)
The 2023 assessment by IPBES documented more than 37,000 established alien species worldwide, including more than 3,500 invasive alien species. The assessment estimated that global economic costs exceeded US$423 billion in 2019. These figures describe the assessment’s evidence base and historical cost estimate, rather than a complete inventory or a current annual total. (ipbes.net)
Documented examples
The zebra mussel (Dreissena polymorpha) is native to Eurasian freshwater systems and was first detected in the Great Lakes in 1988. Its likely transport in ballast tanks, subsequent spread, effects on native mussels, and obstruction of water intakes make it a prominent example of an aquatic invasion with ecological and economic consequences. (pubs.usgs.gov)
The brown tree snake (Boiga irregularis) was accidentally transported to Guam after World War II, probably in cargo. Predation severely affected the island’s birds: a study published in 2003 found that 22 resident bird species were severely affected and that 12 were probably eliminated as breeding residents on the main island. Local disappearance, or extirpation, must be distinguished from global extinction. The snakes have also caused electrical outages. (pubs.usgs.gov)
Detection, management, and international policy
Management distinguishes prevention, early detection and rapid response, and action against established populations. Border inspection, quarantine, pathway management, and pre-introduction risk assessment seek to reduce arrival and establishment. Once a species is established, the appropriate response depends on its distribution, impacts, and the affected ecosystem; eradication and continuing control are different management objectives. (cbd.int)
Surveillance increasingly uses environmental DNA, genetic material released into the environment through skin cells, mucus, feces, and other sources. Testing for DNA can help detect invasive organisms present at low abundance and difficult to find through conventional sampling. Standardized sampling and laboratory procedures support reliable interpretation and management responses. (usgs.gov)
Climate change interacts with land-use change and biological invasions. Warming can permit some established alien species to expand toward higher latitudes or elevations, or survive outdoors where climate previously restricted them. Internationally, Target 6 of the Kunming–Montreal Global Biodiversity Framework calls for preventing priority invasions, reducing introduction and establishment rates of other known or potential invasive alien species by at least 50 percent by 2030, and eradicating or controlling invasive species, especially at priority sites such as islands. (ipbes.net)