Invasive species

Invasive species
North American beavers constitute an invasive species in Tierra del Fuego, where they have a substantial impact on landscape and local ecology through their dams.
Kudzu, a Japanese vine species invasive in the southeast United States, growing in Atlanta, Georgia

"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions.

  • The first definition, the most used, applies to non-indigenous species, or "non-native", plants or animals that adversely affect the habitats and bioregions they invade economically, environmentally, and/or ecologically. They disrupt by dominating a region, wilderness areas, particular habitats, and/or wildland-urban interface land from loss of natural controls (i.e.: predators or herbivores). This includes non-native invasive plant species labeled as exotic pest plants and invasive exotics, in restoration parlance, growing in native plant communities.[1] It has been used in this sense by government organizations[2][3] as well as conservation groups such as the International Union for Conservation of Nature (IUCN) and the California Native Plant Society.[4] The European Union defines "Invasive Alien Species" as those that are, firstly, outside their natural distribution area, and secondly, threaten biological diversity.[5] It is also used by land managers, botanists, researchers, horticulturalists, conservationists, and the public for noxious weeds.[6] The kudzu vine (Pueraria lobata), Andean Pampas grass (Cortaderia jubata), and yellow starthistle (Centaurea solstitialis) are examples.
  • The second definition includes the first, but broadens the boundaries to include indigenous or native species, with the non-native ones, that disrupt by a dominant colonization of a particular habitat or wildlands area from loss of natural controls (i.e.: predators or herbivores). Deer are an example, considered to be overpopulating their native zones and adjacent suburban gardens, by some in the Northeastern and Pacific Coast regions of the United States.
  • The third definition identifies invasive species as a widespread nonindigenous species.[4] This one can be too broad, as not every nonindigenous or "introduced" species has an adverse effect on a nonindigenous environment. A nonadverse example is the common goldfish (Carassius auratus), though common outside its native range globally, it is rarely in harmful densities to a native habitat.[4]

Because of the variability of its definition, and because definitions are often from a socio-economic perspective, the phrase invasive species is often criticized as an imprecise term for the scientific field of ecology.[4] This article concerns the first two definitions; for the third, see Introduced species.


Conditions that lead to invasion

Scientists propose several mechanisms to explain invasive species, including species-based mechanisms and ecosystem-based mechanisms. It is most likely a combination of several mechanisms that cause an invasive situation to occur, since most introduced plants, biotic and animals do not become invasive.

Species-based mechanisms

Species-based characteristics focus on competition. While all species compete to survive, invasive species appear to have specific traits or combinations of specific traits that allow them to outcompete native species. Sometimes they just have the ability to grow and reproduce more rapidly than native species; other times it is more complex, involving a number of traits and interactions.

Studies seem to indicate certain traits mark a species as potentially invasive. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone.[7] Another study found invasive species tended only to have a small subset of the invasive traits, and that many of these invasive traits were found in noninvasive species, as well indicating that invasiveness involves complex interaction not easily categorized.[7][8][9] Common invasive species traits include:

Typically an introduced species must survive at low population densities before it becomes invasive in a new location.[12] At low population densities, it can be difficult for the introduced species to reproduce and maintain itself in a new location, so a species might be transported to a location a number of times before it becomes established. Repeated patterns of human movement from one location to another, such as ships sailing to and from ports or cars driving up and down highways, allow for species to have multiple opportunities for establishment (also known as a high propagule pressure).[13]

An introduced species might become invasive if it can outcompete native species for resources, such as nutrients, light, physical space, water or food. If these species evolved under great competition or predation, the new environment may allow them to proliferate quickly. Ecosystems in which all available resources are being used to their fullest capacity by native species can be modeled as zero-sum systems, where any gain for the invader is a loss for the native. However, such unilateral competitive superiority (and extinction of native species with increased populations of the invader) is not the rule.[14][15] Invasive species often coexist with native species for an extended time, and gradually the superior competitive ability of an invasive species becomes apparent as its population grows larger and denser and it adapts to its new location.

An invasive species might be able to use resources previously unavailable to native species, such as deep water sources accessed by a long taproot, or an ability to live on previously uninhabited soil types. For example, barbed goatgrass (Aegilops triuncialis) was introduced to California on serpentine soils, which have low water-retention, low nutrient levels, a high Mg/Ca ratio, and possible heavy metal toxicity. Plant populations on these soils tend to show low density, but goatgrass can form dense stands on these soils, crowding out native species that have not adapted well to growing on serpentine soils.[16]

Facilitation is the mechanism by which some species can alter their environment using chemicals or manipulating abiotic factors, allowing the species to thrive, while making the environment less favorable to other species with which it competes.[citation needed] One such facilitative mechanism is allelopathy, also known as chemical competition or interference competition. In allelopathy, a plant will secrete chemicals which make the surrounding soil uninhabitable, or at least inhibitory, to competing species.

Examples of this in Centaurea are Centaurea solstitialis (yellow starthistle) and Centaurea diffusa (diffuse knapweed). These Eastern European noxious weeds have spread their way through the western and West Coast states. Experiments show that 8-hydroxyquinoline, a chemical produced at the root of C. diffusa, has a negative effect only on plants that have not co-evolved with C. diffusa. Such co-evolved native plants have also evolved defenses, and C. diffusa and C. solstitialis do not appear in their native habitats to be overwhelmingly successful competitors. This shows how difficult it can be to predict if a species will be invasive just from evaluating its behavior in its native habitat, and demonstrates the potential for novel weapons to aid in invasiveness.[17][18]

Changes in fire regimens are another form of facilitation. Bromus tectorum, originally from Eurasia, is highly fire-adapted. It not only spreads rapidly after burning, but also increases the frequency and intensity (heat) of fires, by providing large amounts of dry detritus during the dry fire season in western North America. In areas where it is widespread, it has altered the local fire regimen so much that native plants cannot survive the frequent fires, allowing B. tectorum to further extend and maintain dominance in its introduced range.[19]

Facilitation also occurs when one species physically modifies a habitat and that modification is advantageous to other species. For example, zebra mussels increase habitat complexity on lake floors, providing crevices in which invertebrates live. This increase in complexity, together with the nutrition provided by the waste products of mussel filter-feeding, increases the density and diversity of benthic invertebrate communities.[20]

Ecosystem-based mechanisms

In ecosystems, the amount of available resources and the extent to which those resources are used by organisms determines the effects of additional species on the ecosystem. In stable ecosystems, equilibrium exists in the use of available resources. These mechanisms describe a situation in which the ecosystem has suffered a disturbance which changes the fundamental nature of the ecosystem.[21] When changes occur in an ecosystem, like forest fires in an area, normal succession would favor certain native grasses and forbs. With the introduction of a species that can multiply and spread faster than the native species, the balance is changed and the resources that would have been used by the native species are now used by an invader. This has an impact on the ecosystem and changes its composition of organisms and their use of available resources. Nitrogen and phosphorus are often the limiting factors in these situations.[22]

Every species has a role to play in its native ecosystem; some species fill large and varied roles, while others are highly specialized. These roles are known as niches. Some invading species are able to fill niches that are not used by native species, and they also can create niches that did not exist.

When changes occur to ecosystems, conditions change that impact the dynamics of species interaction and niche development. This can cause once rare species to replace other species, because they now can use greater available resources that did not exist before; an example would be the edge effect. The changes can favor the expansion of a species that would not have been able to colonize areas and niches that did not exist before.


Monterey cypress

Although an invasive species is often defined as an introduced species that has spread widely and causes harm, some species native to a particular area can, under the influence of natural events, such as long-term rainfall changes or human modifications to the habitat, increase in numbers and become invasive.

All species go through changes in population numbers, in many cases accompanied by expansion or contraction of range. Human alterations of the natural landscape are especially significant. This anthropogenic alteration of an environment may enable the expansion of a species into a geographical area where it had not been seen before, and thus that species could be described as invasive. In essence, one must define "native" with care, as it refers to some natural geographic range of a species, and is not coincident with human political boundaries. Whether noticed increases in population numbers and expanding geographical ranges is sufficient reason to regard a native species as "invasive" requires a broad definition of the term, but some native species in disrupted ecosystems can spread widely and cause harm, and in that sense become invasive. For example, the Monterey cypress is an endangered endemic,[23] naturally occurring only in two small stands in California. They are being exterminated as exotic invasive species less than 50 miles (80 km) from their native home.[citation needed]

Traits of invaded ecosystems

In 1958, Charles S. Elton[24] argued ecosystems with higher species diversity were less subject to invasive species because of fewer available niches. Since then, other ecologists have pointed to highly diverse, but heavily invaded ecosystems and have argued ecosystems with high species diversity seem to be more susceptible to invasion.[25] This debate seems largely to hinge on the spatial scale at which invasion studies are performed, and the issue of how diversity affects community susceptibility to invasion remains unresolved. Small-scale studies tend to show a negative relationship between diversity and invasion, while large-scale studies tend to show a positive relationship. The latter result may be an artifact of invasive or non-native species capitalizing on increased resource availability and weaker overall species interactions that are more common when larger samples are considered.[26][27]

The brown tree snake (Boiga irregularis)

Invasion is more likely if an ecosystem is similar to the one in which the potential invader evolved.[28] Island ecosystems may be prone to invasion because their species are “naïve” and have faced few strong competitors and predators throughout their existence, or because their distance from colonizing species populations makes them more likely to have “open” niches.[29] An example of this phenomenon is the decimation of the native bird populations on Guam by the invasive brown tree snake.[30] Alternatively, invaded ecosystems may lack the natural competitors and predators that keep introduced species in check in their native ecosystems, a point that is also seen in the Guam example. Lastly, invaded ecosystems have often experienced disturbance, usually human-induced.[10] This disturbance may give invasive species, which are not otherwise co-evolved with the ecosystem, a chance to establish themselves with less competition from more adapted species.[12]


Non-native species have many vectors, including many biogenic ones, but most species considered "invasive" are associated with human activity. Natural range extensions are common in many species, but the rate and magnitude of human-mediated extensions in these species tend to be much larger than natural extensions, and the distances species can travel to colonize are also often much greater with human agency.[31]

Chinese mitten crab (Eriocheir sinensis)

One of the earliest human-influenced introductions involved prehistoric humans introducing the Pacific rat (Rattus exulans) to Polynesia.[32] Today, non-native species come from horticultural plants either in the form of the plants themselves or animals and seeds carried with them, and from animals and plants released through the pet trade. Invasive species also come from organisms stowed away on every type of transport vehicle. For example, ballast water taken up at sea and released in port is a major source of exotic marine life. The invasive freshwater zebra mussels, native to the Black, Caspian and Azov seas, were probably transported to the Great Lakes via ballast water from a transoceanic vessel.[33] The arrival of invasive propagules to a new site is a function of the site's invasibility.[34]

Species have also been introduced intentionally. For example, to feel more "at home", American colonists formed "Acclimation Societies" that repeatedly released birds that were native to Europe until they finally established along the east coast of North America. In 2008, U.S. postal workers in Pennsylvania noticed noises coming from inside a box from Taiwan; the box contained more than two dozen live beetles. U.S. Customs and Border Protection sent the beetles to the Agricultural Research Service (ARS) to be expertly identified. The ARS entomologists identified them as rhinoceros beetle, hercules beetle, and king stag beetle.[35] Because these beetles are not native to the U.S., they could pose a threat to native ecosystems, agriculture, and the environment. To prevent exotic species from becoming a problem in the U.S., special handling and permits are needed when insects and other living materials are shipped from foreign countries. Programs such as Smuggling Interdiction and Trade Compliance (SITC) have also been set up by the USDA in an effort to prevent exotic species outbreaks in America.

Economics play a major role in exotic species introduction. The scarcity and demand for the valuable Chinese mitten crab is one explanation for the possible intentional release of the species in foreign waters.

Impacts of wildfire

Invasive species often exploit disturbances to an ecosystem (wildfires, roads, foot trails) to colonize an area. Large wildfires are capable of sterilizing soils and removing any trace of life from their path, while adding a variety of nutrients to the soil.[22] In the resulting ecological free-for-all, invasive species can easily dominate native plants, and quickly become established.

Many invasive plant species have the ability to regenerate from their roots. This means if a low intensity fire burns through an area and removes surface vegetation, native species will have to rely on seeds for propagation, while a well-established invasive species with intact roots can regrow as soon as the ecosystem recovers from the fire, and often completely shade out any native vegetation.[19]

Impact of wildfire suppression on spreading

Wildfires often occur in remote areas, requiring fire suppression crews to travel through pristine forest to reach the site. The crews can unwittingly be the bearers of invasive seeds. Should any of these stowaway seeds become established along the way, a new thriving concentration of invasive weeds can be present in as few as six weeks, at which point controlling the outbreak will require years of continued attention to prevent further spread. Also, the disturbance of the soil surface, such as firebreaks for fire prevention, destroying the native cover and exposing open soil, can accelerate 'invasive exotic' plants spreading. In suburban and wildland-urban interface areas, the vegetation clearance and brush removal ordinances of municipalities for defensible space can result in excessive clear-cutting of native shrubs and perennials that exposes the soil to more light and less competition for invasive plant species.

Fire suppression vehicles are often the major culprits of such outbreaks, as the vehicles are frequently driven on back roads often overgrown with invasive plant species. The undercarriage of the vehicle becomes a prime vessel of transport. In response, on large fires, vehicle washing stations are set up, and it is required that vehicles be "decontaminated" prior to engaging in suppression activities. In addition when suppressing large wild fires, personnel from around the country are often used, further increasing the potential for transport of seeds across the country, thus showing the importance of "cleaning stations".


Ecological impacts

Land clearing and human habitation put significant pressure on local species. Disturbed habitats are prone to invasions that can have adverse effects on local ecosystems, changing ecosystem functions. A species of wetland plant known as ʻaeʻae in Hawaiʻi (the indigenous Bacopa monnieri) is regarded as a pest species in artificially manipulated water bird refuges because it quickly covers shallow mudflats established for endangered Hawaiian stilt (Himantopus mexicanus knudseni), making these undesirable feeding areas for the birds.

Multiple successive introductions of different non-native species can have interactive effects; the introduction of a second non-native species can enable the first invasive species to flourish. Examples of this are the introductions of the amethyst gem clam (Gemma gemma) and the European green crab (Carcinus maenas). The gem clam was introduced into California's Bodega Harbor from the East Coast of the United States a century ago. It had been found in small quantities in the harbor but had never displaced the native clam species (Nutricola spp.). In the mid 1990s, the introduction of the European green crab, found to prey preferentially on the native clams, resulted in a decline of the native clams and an increase of the introduced clam populations.[36]

In the Waterberg region of South Africa, cattle grazing over the past six centuries has allowed invasive scrub and small trees to displace much of the original grassland, resulting in a massive reduction in forage for native bovids and other grazers. Since the 1970s, large scale efforts have been underway to reduce invasive species; partial success has led to re-establishment of many species that had dwindled or left the region. Examples of these species are giraffe, blue wildebeest, impala, kudu and white rhino.

Invasive species can change the functions of ecosystems. For example, invasive plants can alter the fire regimen (cheatgrass, Bromus tectorum), nutrient cycling (smooth cordgrass Spartina alterniflora), and hydrology (Tamarix) in native ecosystems.[37] Invasive species that are closely related to rare native species have the potential to hybridize with the native species. Harmful effects of hybridization have led to a decline and even extinction of native species.[38][39] For example, hybridization with introduced cordgrass, Spartina alterniflora, threatens the existence of California cordgrass (Spartina foliosa) in San Francisco Bay.[40]

Genetic pollution

Natural, wild species can be threatened with extinction[41] through the process of genetic pollution. Genetic pollution is uncontrolled hybridization and introgression, which leads to homogenization or replacement of local genotypes as a result of either a numerical or fitness advantage of the introduced species.[42] Genetic pollution can bring about a form of extinction either through purposeful introduction or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones where the abundant ones can interbreed with them, creating hybrids and swamping the entire rarer gene pool, thus driving the native species to extinction. Attention has to be focused on the extent of this problem, it is not always apparent from morphological observations alone. Some degree of gene flow may be a normal, evolutionarily constructive process, all constellations of genes and genotypes can be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.[43][44] An example of this is the interbreeding of migrating coyotes with the red wolf, in areas of eastern North Carolina where the red wolf has been reintroduced.

Economic impacts


Non-native species can have benefits. Asian oysters, for example, are better at filtering out water pollutants than native oysters. They also grow faster and withstand disease better than natives. Biologists are currently considering releasing the mollusk in the Chesapeake Bay to help restore oyster stocks and clean up the bay's pollution. A recent study by the Johns Hopkins School of Public Health found the Asian oyster could significantly benefit the bay's deteriorating water quality.[45]


Economic costs from invasive species can be separated into direct costs through production loss in agriculture and forestry, and management costs of invasive species. Estimated damage and control cost of invasive species in the U.S. alone amount to more than $138 billion annually.[46] In addition to these costs, economic losses can occur through loss of recreational and tourism revenues.[47] When economic costs of invasions are calculated as production loss and management costs, they are low because they do not consider environmental damage; if monetary values were assigned to the extinction of species, loss in biodiversity, and loss of ecosystem services, costs from impacts of invasive species would drastically increase.[46] The following examples from different sectors of the economy demonstrate the impact of biological invasions.

Economic opportunities

For many invasive species, there are commercial benefits, either existent or capable of being developed. For instance, silver carp and common carp, where heavy metals are not excessive in their flesh, can be harvested for human food and exported to markets already familiar with the product, or into pet foods, or mink feed. Numerous vegetative 'invasives' like water hyacinth, when in sufficient quantities to be harvestable, can be turned into fuel by methane digesters if no other better use can be determined. The depletion or exploitation of any unwanted species is dependent on officials who recognize the need for a solution. Commercial enterprises need assurances the exploitation can continue long enough for a reasonable profit to be generated and that taxation of the 'resource' is given a sufficiently long period of grace that an enterprise is attracted to the proposition.[citation needed]


Weeds cause an overall reduction in yield, though they often provide essential nutrients for sustenance farmers. Weeds can have other useful purposes: some deep-rooted weeds can "mine" nutrients from the subsoil and bring them to the topsoil, while others provide habitat for beneficial insects and/or provide alternative foods for pest species. Many weed species are accidental introductions with crop seeds and imported plant material. Many introduced weeds in pastures compete with native forage plants, are toxic (e.g., leafy spurge, Euphorbia esula) to young cattle (older animals will avoid them) or unpalatable because of thorns and spines (e.g., yellow starthistle). Forage loss from invasive weeds on pastures amounts to nearly US$1 billion in the U.S. alone.[46] A decline in pollinator services and loss of fruit production has been caused by the infection of honey bees (Apis mellifera, another invasive species to the Americas) by the invasive varroa mite. Introduced rats (Rattus rattus and R. norvegicus) have become serious pests on farms, destroying stored grains.[46]

In many cases, one could consider the overabundant invasive plant species as a ready source of biomass in the perspective of biogas production. See Eichhornia crassipes.


The unintentional introduction of forest pest species and plant pathogens can change forest ecology and negatively affect the timber industry. The Asian long-horned beetle (Anoplophora glabripennis) was first introduced into the U.S. in 1996, and is expected to infect and damage millions of acres of hardwood trees. Thirty million dollars have already been spent in attempts to eradicate this pest and protect millions of trees in the affected regions.[46]

The woolly adelgid inflicts damage on old-growth spruce fir forests and negatively affects the Christmas tree industry.[48] The chestnut blight fungus (Cryphonectria parasitica) and Dutch elm disease (Ophiostoma novo-ulmi) are two plant pathogens with serious impacts on forest health.

Tourism and recreation

Invasive species can have impacts on recreational activities, such as fishing, hunting, hiking, wildlife viewing, and water-based recreation. They negatively affect a wide array of environmental attributes that are important to support recreation, including, but not limited to, water quality and quantity, plant and animal diversity, and species abundance.[49] Eiswerth goes on to say that "very little research has been performed to estimate the corresponding economic losses at spatial scales such as regions, states, and watersheds." Eurasian Watermilfoil (Myriophyllum spicatum) in parts of the US, fill lakes with plants making fishing and boating difficult.[50]

Health impacts

An increasing threat of exotic diseases exists because of increased transportation and encroachment of humans into previously remote ecosystems. This can lead to new associations between a disease and a human host (e.g., AIDS virus[46]). Introduced birds (e.g. pigeons), rodents and insects (e.g. mosquito, flea, louse and tsetse fly pests) can serve as vectors and reservoirs of human diseases. The introduced Chinese mitten crabs are carriers of the Asian lung fluke.[51] Throughout recorded history, epidemics of human diseases, such as malaria, yellow fever, typhus, and bubonic plague, have been associated with these vectors.[24] A recent example of an introduced disease is the spread of the West Nile virus across North America, resulting in the deaths of humans, birds, mammals, and reptiles.[52] Waterborne disease agents, such as cholera bacteria (Vibrio cholerae), and causative agents of harmful algal blooms are often transported via ballast water.[53] The full range of impacts of invasive species and their control goes beyond immediate effects and can have long term public health implications. For instance, pesticides applied to treat a particular pest species could pollute soil and surface water.[46]

Threat to global biodiversity

Biotic invasion is considered one of the five top drivers for global biodiversity loss and is increasing because of tourism and globalization.[citation needed] This may be particularly true in inadequately regulated fresh water systems, though quarantines and ballast water rules have improved the situation.[54]

As noted above, invasive species may drive local native species to extinction via competitive exclusion, niche displacement, or hybridisation with related native species.[55] Therefore, besides their economic ramifications, alien invasions may result in extensive changes in the structure, composition and global distribution of the biota of sites of introduction, leading ultimately to the homogenisation of the world’s fauna and flora and the loss of biodiversity.[55] Nevertheless, it is difficult to unequivocally attribute e.g., extinctions, to an invasive species, and the few scientific studies that have done so have been with animal (see examples above) rather than plant taxa. Concern over the impacts of invasive species on biodiversity must therefore consider the actual evidence (either ecological or economic), in relation to the potential risk.

Scientific definition

Stage Characteristic
0 Propagules residing in a donor region
I Traveling
II Introduced
III Localized and numerically rare
IVa Widespread but rare
IVb Localized but dominant
V Widespread and dominant

In an attempt to avoid the ambiguous, subjective, and pejorative vocabulary that so often accompanies discussion of invasive species even in scientific papers, e.g., Colautti and MacIsaac[56] have proposed a new nomenclature system based on biogeography rather than on taxa.

By removing taxonomy, human health, and economic factors from consideration, this model focuses only on ecological factors. The model evaluates individual populations, and not entire species. This model does not attribute detrimentality to invasive species and beneficiality to native species. It merely classifies a species in a particular location based on its growth patterns in that particular microenvironment. This model could be applied equally to indigenous and to non-native species.

See also

Invasive species by country


This article incorporates CC-BY-3.0 text from the reference[55]

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Further reading
  • Baskin, Yvonne (2003). A Plague of Rats and Rubbervines: The Growing Threat Of Species Invasions. Island Press. p. 377. ISBN 978-1559630511. 
  • Burdick, Alan (2006) [2005]. Out of Eden: An Odyssey of Ecological Invasion. Farrar Straus and Giroux. p. 336. ISBN 0-374-53043-2. 
  • Davis, Mark A. (2009). Invasion Biology. Oxford University Press. p. 243. ISBN 0199218765. 
  • Elton, Charles S. (2000) [First published 1958]. The Ecology of Invasions by Animals and Plants. University of Chicago Press. p. 196. ISBN 978-0226206387. 
  • Lockwood, Julie; Martha Hoopes, Michael Marchetti (2007) [2006]. Invasion Ecology. Blackwell Publishing. p. 304. ISBN 978-1405114189. 
  • McNeeley, Jeffrey A. (2001). The Great Reshuffling: Human Dimensions Of Invasive Alien Species. World Conservation Union (IUCN). p. 109. ISBN 978-2831706023. 
  • Terrill, Ceiridwen (2007). Unnatural Landscapes: Tracking Invasive Species. University of Arizona Press. p. 240. ISBN 0-816-52523-4. 
  • Van Driesche, Jason; Roy Van Driesche (2004). Nature Out of Place: Biological Invasions In The Global Age. Island Press. p. 377. ISBN 978-1559637589. 

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