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"Invasive species" is a phrase with several definitions. The first definition expresses the phrase in terms of non-indigenous species (e.g. plants or animals) that adversely affect the habitats they invade economically, environmentally or ecologically. It has been used in this sense by government organizations^[1]^[2] as well as conservation groups such as the IUCN (International Union for Conservation of Nature).^[3]

The second definition broadens the boundaries to include both native and non-native species that heavily colonize a particular habitat.

The third definition is an expansion of the first and defines an invasive species as a widespread non-indigenous species.^[3] This last definition is arguably too broad as not all non-indigenous species necessarily have an adverse effect on their adopted environment. An example of this broader use would include the claim that the common goldfish (Carassius auratus) is invasive. Although it is common outside its range globally, it almost never appears in harmful densities.^[3]

Because of the ambiguity of its definition, the phrase invasive species is often criticized as an imprecise term within the field of ecology.^[3] 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 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's more complex, involving a multiplex of traits and interactions.

Studies seem to indicate that 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.^[4] Another study found that invasive species tended only to have a small subset of the invasive traits and that many of these invasive traits were found in non-invasive species as well indicating that invasiveness involves complex interaction not easily categorized.^[4]^[5]^[6] Common invasive species traits include:

The ability to reproduce both asexually as well as sexually
Fast growth
Rapid reproduction
High dispersal ability
Phenotypic plasticity (the ability to alter one’s growth form to suit current conditions)
Tolerance of a wide range of environmental conditions (generalist)
Ability to live off of a wide range of food types (generalist)
Association with humans^[7]
Other successful invasions^[8]

Typically an introduced species must survive at low population densities before it becomes invasive in a new location.^[9] 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 become 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).^[10]

An introduced species might become invasive if it can out-compete 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.^[11]^[12] 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.^[13]

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.

One example of this is the knapweed Centaurea diffusa. This Eastern European weed has spread its way through the western United 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 does not appear in its native habitat to be an overwhelmingly successful competitor. This shows how difficult it can be to predict if a species will be invasive just from looking at its behavior in its native habitat, and demonstrates the potential for novel weapons to aid in invasiveness.^[14]^[15]

Changes in fire regimes are another form of facilitation. Bromus tectorum, originally from Eurasia, is highly fire-adapted. It not only spreads rapidly after burning, but actually 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 regime so much that native plants cannot survive the frequent fires, allowing B. tectorum to further extend and maintain dominance in its introduced range.^[16]

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 crevases 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.^[17]

Ecosystem-based mechanisms

In ecosystems, the amount of available resources and the extent to which those resources are utilized by organisms determines the effects of additional species on the ecosystem. In stable ecosystems, equilibrium exists in the utilization of available resources. These mechanisms describe a situation in which the ecosystem has suffered a disturbance which changes the fundamental nature of the ecosystem.^[18] 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 utilized by an invader. This impacts 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.^[19]

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 utilized 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 utilize 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.

Ecology

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 landscape alterations 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^[20] 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.

Traits of invaded ecosystems

In 1958, Charles S. Elton^[21] argued that 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 that ecosystems with high species diversity seem to be more susceptible to invasion.^[11] 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.^[22]^[23]

Invasion is more likely if an ecosystem is similar to the one in which the potential invader evolved.^[7] 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.^[24] An example of this phenomenon is the decimation of the native bird populations on Guam by the invasive brown tree snake.^[25] Alternately, 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.^[7] 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.^[9]

Vectors

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 that species can travel to colonize are also often much greater with human agency.^[26]

Chinese mitten crab (*Eriocheir sinensis*)

One of the earliest human influenced introductions involves prehistoric humans introducing the Pacific rat (Rattus exulans) to Polynesia.^[27] 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.^[28] The arrival of invasive propagules to a new site is a function of the site's invasibility.^[29]

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.

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.

Impact

Ecological impacts

Biological species invasions alter ecosystems in a multitude of ways. Worldwide, an estimated 80% of endangered species could suffer losses by competition with, or predation by, invasive species.^[30] Pimentel also reports that introduced species, such as corn, wheat, rice, cattle, and poultry, provide more than 98% of the U.S. food system at a value of approximately $800 billion per year.^[30] As highly adaptable and generalized species are introduced to environments already impacted by human activities, some native species may be put at a disadvantage to survive while other species survival is enhanced.

Land clearing and human habitation put significant pressure on local species. This disturbed habitat is 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 nonnative 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.^[31]

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 regime (cheatgrass, Bromus tectorum), nutrient cycling (smooth cordgrass Spartina alterniflora), and hydrology (Tamarix) in native ecosystems.^[32] Invasive species that are closely related with 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.^[33]^[34] For example, hybridization with introduced cordgrass, Spartina alterniflora, threatens the existence of California cordgrass (Spartina foliosa) in San Francisco Bay.^[35]

Genetic pollution

Natural, wild species can be threatened with extinction^[36] 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.^[37] 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, and all constellations of genes and genotypes cannot be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.^[38]^[39]

Economic impacts

Benefits

Often overlooked, economic benefits from so-called "invasive" species should also be accounted. The wide range of benefits from many "invasives" is both well-documented and under-reported. 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.^[40]

Costs

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.^[30] In addition to these costs, economic losses can occur through loss of recreational and tourism revenues.^[41] Economic costs of invasions, when calculated as production loss and management costs, are low because they do not usually consider environmental damages. If monetary values could be assigned to the extinction of species, loss in biodiversity, and loss of ecosystem services, costs from impacts of invasive species would drastically increase.^[30] The following examples from different sectors of the economy demonstrate the impact of biological invasions.

Agriculture

Weeds cause an overall reduction in yield, though they often provide essential nutrients for subsistence 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 non-palatable because of thorns and spines (e.g., Yellow Starthistle, Centaurea solstitialis). Forage loss from invasive weeds on pastures amounts to nearly $1 billion in the U.S. alone.^[30] A decline in pollinator services and loss of fruit production has been observed to cause the infection of honey bees (Apis mellifera another invasive species to the Americas) by the invasive varroa mite. Introduced rodents (rats, Rattus rattus and R. norvegicus) have become serious pests on farms destroying stored grains.^[30]

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

Forestry

The unintentional introduction of forest pest species and plant pathogens can change forest ecology and negatively impact 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.^[30]

The woolly adelgid inflicts damage on old growth spruce fir forests and negatively impacts the Christmas tree industry.^[42] 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.^[43] 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.^[44]

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^[30]). Introduced birds (e.g. pigeons), rodents and insects (e.g. mosquitoes, fleas, lice and tsetse fly) can serve as vectors and reservoirs of human diseases. The introduced Chinese mitten crabs are carriers of the Asian lung fluke.^[45] Throughout recorded history epidemics of human diseases such as malaria, yellow fever, typhus, and bubonic plague have been associated with these vectors.^[21] 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.^[46] Waterborne disease agents, such as Cholera bacteria (Vibrio cholerae), and causative agents of harmful algal blooms are often transported via ballast water.^[47] 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.^[30]

Threat to global biodiversity

Biotic invasion is one of the five top drivers for global biodiversity loss and is increasing because of tourism and globalization.^{[citation needed]} It poses a particular risk to inadequately regulated fresh water systems, though quarantines and ballast water rules have improved the situation.^[48]

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, Colautti and MacIsaac^[3] 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.

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^ Ayres, D. (2004). "Spread of exotic cordgrasses and hybrids (Spartina sp.) in the tidal marshes of San Francisco Bay, California". USA Biological Invasions. 6: 221–231. doi:10.1023/B:BINV.0000022140.07404.b7. {{cite journal}}: Italic or bold markup not allowed in: |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
^ Hybridization and Introgression; Extinctions; from "The evolutionary impact of invasive species; by H. A. Mooney and E. E. Cleland" Proc Natl Acad Sci U S A. 2001 May 8; 98(10): 5446–5451. doi: 10.1073/pnas.091093398. Proc Natl Academy Sci U S A, v.98(10); May 8, 2001, The National Academy of Sciences
^ Glossary: definitions from the following publication: Aubry, C., R. Shoal and V. Erickson. 2005. Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. USDA Forest Service. 44 pages, plus appendices.; Native Seed Network (NSN), Institute for Applied Ecology, 563 SW Jefferson Ave, Corvallis, OR 97333, USA
^ EXTINCTION BY HYBRIDIZATION AND INTROGRESSION; by Judith M. Rhymer , Department of Wildlife Ecology, University of Maine, Orono, Maine 04469, USA; and Daniel Simberloff, Department of Biological Science, Florida State University, Tallahassee, Florida 32306, USA; Annual Review of Ecology and Systematics, November 1996, Vol. 27, Pages 83-109 (doi: 10.1146/annurev.ecolsys.27.1.83), [2]
^ Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program; by Brad M. Potts, Robert C. Barbour, Andrew B. Hingston; September 2001; RIRDC Publication No 01/114; RIRDC Project No CPF - 3A; ISBN 0 642 58336 6; ISSN 1440-6845; Australian Government, Rural Industrial Research and Development Corporation
^ Tom Pelton, Baltimore Sun, May 26, 2006.
^ Simberloff, D. (2001). "Biological invasions - How are they affecting us, and what can we do about them?". Western North American Naturalist. 61: 308–315.
^ (March 3, 2005). Balsam woolly aphid Adelges piceae (Ratzeburg). ForestPests.org. Retrieved on September 1 2007.
^ Eiswerth, M.E. (2005). "Input-output modeling, outdoor recreation, and the economic impacts of weeds". Weed Science. 53: 130–137. doi:10.1614/WS-04-022R.
^ Eurasian Watermilfoil in the Great Lakes Region. GreatLakes.net. Retrieved on September 1 2007.
^ Aquatic invasive species. A Guide to Least-Wanted Aquatic Organisms of the Pacific Northwest. 2001. University of Washington
^ Lanciotti, R.S. (1999). "Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States". Science. 286: 2333–2337. doi:10.1126/science.286.5448.2333. PMID 10600742.
^ Hallegraeff, G.M. (1998). "Transport of toxic dinoflagellates via ships' ballast water: Bioeconomic risk assessment and efficacy of possible ballast water management strategies". Marine Ecology Progress Series. 168: 297–309. doi:10.3354/meps168297.
^ Millennium Ecosystem Assessment (2005). "Ecosystems and Human Well-being: Biodiversity Synthesis" (PDF). World Resources Institute.

External links

CISR Center for Invasive Species Research
Invading Species.com Ontario Ministry of Natural Resources and the Ontario Federation of Anglers and Hunters.
Invasive Species of Oregon - Statesman Journal Stories, videos, photos, and a comprehensive database of invasive species found in Oregon.
Invasive Species U.S. Fish and Wildlife Service
INVASIVE .org The Source for Information and Images of Invasive & Exotic Species
Global Invasive Species Database
- 100 Worst Alien Invasive Species
Aliens Among Us: A round table with James H. Brown and Dov F. Sax, Daniel Simberloff, and Mark Sagoff
City forests in peril: Invasive plants are killing native trees
Hawaiian Ecosystems at Risk project (HEAR)
Invasive Species Specialist Group
Invasive Species Weblog
National Invasive Species Information Center
Pacific Island Ecosystems at Risk project (PIER)
The Nature Conservancy's Global Invasive Species Team
U.S. Coast Guard Ballast Water Management Program
USDA NRCS Plant Materials Program
UK Plant Health service
Alien Plants in Ireland Database of Alien and invasive plants in Ireland
Invasive alien species in Belgium The Belgian Forum on Invasive Species (BFIS) is an informal structure animated by the Belgian Biodiversity Platform where in scientists interested in biological invasions are involved.
Invasive Species in Ireland project
Don't Move Firewood project Information on the effects of moving firewood and how it contributes to the spread of invasive insects and diseases.
CFIA's Don't Move Firewood Campaign
REEF Invasive Pacific Lionfish in Caribbean and Atlantic waters.
Cape of Fire Identifying Invasive Species - Alien Mug Shots.

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[2] USA (1999). Executive Order 13112 of February 3, 1999: Invasive Species. Federal Register 64(25), 6183-6186.

[neutral-3] Colautti, Robert I.; MacIsaac, Hugh J. (2004), "A neutral terminology to define 'invasive' species" (PDF), Diversity and Distributions, 10: 135–141, doi:10.1111/j.1366-9516.2004.00061.x, retrieved 2007-07-11{{citation}}: CS1 maint: multiple names: authors list (link) Cite error: The named reference "neutral" was defined multiple times with different content (see the help page).

[kolar-4] Kolar, C.S. (2001). "Progress in invasion biology: predicting invaders". Trends in Ecology & Evolution. 16 (4): 199–204. doi:10.1016/S0169-5347(01)02101-2. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[theb-5] Thebaud, C. (1996). "Assessing why two introduced Conyza differ in their ability to invade Mediterranean old fields". Ecology. 77 (3): 791–804. doi:10.2307/2265502. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

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[williams-7] Williams, J.D. (1998). "Nonindigenous Species". Status and Trends of the Nation’s Biological Resources. Reston, Virginia: United States Department of the Interior, Geological Survey. 1. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) Cite error: The named reference "williams" was defined multiple times with different content (see the help page).

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[stoh-11] Stohlgren, T.J. (1999). "Exotic plant species invade hot spots of native plant diversity". Ecological Monographs. 69: 25–46. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help) Cite error: The named reference "stoh" was defined multiple times with different content (see the help page).

[sax-12] Sax, D.F. (2002). "Species Invasions Exceed Extinctions on Islands Worldwide: A Comparative Study of Plants and Birds". American Naturalist. 160: 766–783. doi:10.1086/343877. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[13] Huenneke, L. (1990). "Effects of soil resources on plant invasion and community structure in California (USA) serpentine grassland". Ecology. 71: 478–491. doi:10.2307/1940302. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[14] Hierro, J.L. (2003). "Allelopathy and exotic plant invasion". Plant and Soil. 256 (1): 29–39. doi:10.1023/A:1026208327014. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[vivan-15] Vivanco, J.M. (2004). "Biogeographical variation in community response to root allelochemistry: Novel weapons and exotic invasion". Ecology Letters. 7 (4): 285–292. doi:10.1111/j.1461-0248.2004.00576.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[16] Brooks, M.L. (2004). "Effects of invasive alien plants on fire". BioScience. 54 (54): 677–688. doi:10.1641/0006-3568(2004)054[0677:EOIAPO]2.0.CO;2. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[silv-17] Silver Botts, P. (1996). "Zebra mussel effects on benthic invertebrates: Physical or biotic?". Journal of the North American Benthological Society (15): 179–184. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[18] Byers, J.E. (2002). "Impact of non-indigenous species on natives enhanced by anthropogenic alteration of selection regimes". Oikos. 97 (3): 449–458. doi:10.1034/j.1600-0706.2002.970316.x.

[19] Davis, M.A. (2000). "Fluctuating resources in plant communities: A general theory of invisibility". Journal of Ecology. 88 (3): 528–534. doi:10.1046/j.1365-2745.2000.00473.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[smith-20] Smith, J. P., Jr. (1988). Inventory of rare and endangered vascular plants of California. Sacramento, California: California Native Plant Society. ISBN 0-943460-14-X. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: authors list (link)

[elton-21] Elton, C.S. (2000) [1958]. The Ecology of Invasions by Animals and Plants. Foreword by Daniel Simberloff. Chicago: University of Chicago Press. p. 196. ISBN 0-226-20638-6.

[22] Byers, J.E. (2003). "Scale dependent effects of biotic resistance to biological invasion". Ecology. 84: 1428–1433. doi:10.1890/02-3131. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[23] Levine, J. M. (2000). "Species diversity and biological invasions: Relating local process to community pattern". Science. 288: 852–854. doi:10.1126/science.288.5467.852. PMID 10797006.

[stach-24] Stachowicz, J.J. (2005). "Species invasions and the relationships between species diversity, community saturation, and ecosystem functioning". In D.F. Sax, J.J. Stachowicz, and S.D. Gaines (ed.). Species Invasions: Insights into Ecology, Evolution, and Biogeography. Sunderland, Massachusetts: Sinauer Associates. ISBN 0878938117. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: multiple names: editors list (link)

[25] Fritts, T.H. (2001). "The Brown Treesnake on Guam: How the arrival of one invasive species damaged the ecology, commerce, electrical systems, and human health on Guam: A comprehensive information source". Retrieved 2007-09-01. {{cite journal}}: Cite journal requires |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[26] Cassey, P (2005). "Concerning Invasive Species: Reply to Brown and Sax". Austral Ecology. 30: 475. doi:10.1111/j.1442-9993.2005.01505.x. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[27] Matisoo-Smith, E. (1998). "Patterns of prehistoric human mobility in Polynesia indicated my mtDNA from the Pacific rat". Proceedings of the National Academy of the Sciences USA. 95: 15145–15150. doi:10.1073/pnas.95.25.15145. PMID 9844030. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[28] Aquatic invasive species. A Guide to Least-Wanted Aquatic Organisms of the Pacific Northwest. 2001. University of Washington. [1]

[29] Leung, B. (2007). "The risk of establishment of aquatic invasive species: joining invasibility and propagule pressure". Proceedings of the Royal Society B. 274: 2733–2739. doi:10.1098/rspb.2007.0841. {{cite journal}}: Cite has empty unknown parameter: |month= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[pimental-30] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Pimentel, D. (2005). "Update on the environmental and economic costs associated with alien-invasive species in the United States". Ecological Economics. 52: 273–288. doi:10.1016/j.ecolecon.2004.07.013. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[31] Grosholz, E.D. (2005). "Recent biological invasion may hasten "invasion meltdown" by accelerating historical introductions". Proceedings of the National Academy of Sciences. 102: 1088–1091. doi:10.1073/pnas.0308547102. PMID 15657121.

[32] Mack, R. (2000). "Biotic invasions: Causes, epidemiology, global consequences, and control". Ecological Applications. 10: 689–710. doi:10.1890/1051-0761(2000)010[0689:BICEGC]2.0.CO;2. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[33] Hawkes, C.V. (2005). "Plant invasion alters nitrogen cycling by modifying the soil nitrifying community". Ecology Letters. 8: 976–985. doi:10.1111/j.1461-0248.2005.00802.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[rhymer-34] Rhymer, J. M. (1996). "Extinction by hybridization and introgression". Annual Review of Ecology and Systematics. 27 (27): 83–109. doi:10.1146/annurev.ecolsys.27.1.83. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)

[ayres-35] Ayres, D. (2004). "Spread of exotic cordgrasses and hybrids (Spartina sp.) in the tidal marshes of San Francisco Bay, California". USA Biological Invasions. 6: 221–231. doi:10.1023/B:BINV.0000022140.07404.b7. {{cite journal}}: Italic or bold markup not allowed in: |journal= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)

[36] Hybridization and Introgression; Extinctions; from "The evolutionary impact of invasive species; by H. A. Mooney and E. E. Cleland" Proc Natl Acad Sci U S A. 2001 May 8; 98(10): 5446–5451. doi: 10.1073/pnas.091093398. Proc Natl Academy Sci U S A, v.98(10); May 8, 2001, The National Academy of Sciences

[37] Glossary: definitions from the following publication: Aubry, C., R. Shoal and V. Erickson. 2005. Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest. USDA Forest Service. 44 pages, plus appendices.; Native Seed Network (NSN), Institute for Applied Ecology, 563 SW Jefferson Ave, Corvallis, OR 97333, USA

[38] EXTINCTION BY HYBRIDIZATION AND INTROGRESSION; by Judith M. Rhymer , Department of Wildlife Ecology, University of Maine, Orono, Maine 04469, USA; and Daniel Simberloff, Department of Biological Science, Florida State University, Tallahassee, Florida 32306, USA; Annual Review of Ecology and Systematics, November 1996, Vol. 27, Pages 83-109 (doi: 10.1146/annurev.ecolsys.27.1.83), [2]

[39] Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program; by Brad M. Potts, Robert C. Barbour, Andrew B. Hingston; September 2001; RIRDC Publication No 01/114; RIRDC Project No CPF - 3A; ISBN 0 642 58336 6; ISSN 1440-6845; Australian Government, Rural Industrial Research and Development Corporation

[40] Tom Pelton, Baltimore Sun, May 26, 2006.

[simb-41] Simberloff, D. (2001). "Biological invasions - How are they affecting us, and what can we do about them?". Western North American Naturalist. 61: 308–315.

[42] (March 3, 2005). Balsam woolly aphid Adelges piceae (Ratzeburg). ForestPests.org. Retrieved on September 1 2007.

[43] Eiswerth, M.E. (2005). "Input-output modeling, outdoor recreation, and the economic impacts of weeds". Weed Science. 53: 130–137. doi:10.1614/WS-04-022R.

[44] Eurasian Watermilfoil in the Great Lakes Region. GreatLakes.net. Retrieved on September 1 2007.

[45] Aquatic invasive species. A Guide to Least-Wanted Aquatic Organisms of the Pacific Northwest. 2001. University of Washington

[46] Lanciotti, R.S. (1999). "Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States". Science. 286: 2333–2337. doi:10.1126/science.286.5448.2333. PMID 10600742.

[47] Hallegraeff, G.M. (1998). "Transport of toxic dinoflagellates via ships' ballast water: Bioeconomic risk assessment and efficacy of possible ballast water management strategies". Marine Ecology Progress Series. 168: 297–309. doi:10.3354/meps168297.

[48] Millennium Ecosystem Assessment (2005). "Ecosystems and Human Well-being: Biodiversity Synthesis" (PDF). World Resources Institute.

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