Invasive species, also called invasive exotics or simply 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 introduced species (also called "non-indigenous" or "non-native") that adversely affect the habitats and bioregions they invade economically, environmentally, and/or ecologically. Such invasive species may be either plants or animals and may disrupt by dominating a region, wilderness areas, particular habitats, or wildland-urban interface land from loss of natural controls (such as predators or herbivores). This includes non-native invasive plant species labeled as exotic pest plants and invasive exotics growing in native plant communities. It has been used in this sense by government organizations as well as conservation groups such as the International Union for Conservation of Nature (IUCN) and the California Native Plant Society. The European Union defines "Invasive Alien Species" as those that are, firstly, outside their natural distribution area, and secondly, threaten biological diversity. It is also used by land managers, botanists, researchers, horticulturalists, conservationists, and the public for noxious weeds. 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 non-indigenous species. This one can be too broad, as not every nonindigenous or "introduced" species has an adverse effect on a non-indigenous 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.
Because of the variability of its definition, and because definitions are often from a socioeconomic perspective, the phrase invasive species is often criticized as an imprecise term for the scientific field of ecology. This article concerns the first two definitions; for the third, see Introduced species.
- 1 Causes
- 2 Ecology
- 3 Effects
- 4 Study
- 5 See also
- 6 References
- 7 External links
Scientists include species- and ecosystem factors among the mechanisms that when combined, establish invasiveness in a newly introduced species.
While all species compete to survive, invasive species appear to have specific traits or specific combinations of traits that allow them to outcompete native species. In some cases the competition is about rates of growth and reproduction. In other cases species interact with each other more directly.
Researchers disagree about the usefulness of traits as invasiveness markers. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone. Another study found invasive species tended only to have a small subset of the presumed traits, and that many similar traits were found in noninvasive species, requiring other explanations. Common invasive species traits include:
- Fast growth
- Rapid reproduction
- High dispersal ability
- Phenotypic plasticity (the ability to alter growth form to suit current conditions)
- Tolerance of a wide range of environmental conditions (Ecological competence)
- Ability to live off of a wide range of food types (generalist)
- Association with humans
- Prior successful invasions
Typically, an introduced species must survive at low population densities before it becomes invasive in a new location. 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 reach a location multiple times before it becomes established. Repeated patterns of human movement, such as ships sailing to and from ports or cars driving up and down highways offer repeated opportunities for establishment (also known as a high propagule pressure).
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, then the new environment may host fewer able competitors, allowing the invader to proliferate quickly. Ecosystems in which 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. 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 that were 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 Magnesium/Calcium 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 adapted poorly to serpentine soils.
Ecological facilitation occurs when a species alters its environment by using chemicals or manipulating abiotic factors, allowing the species to thrive while making the environment less favorable to competitors. One such facilitative mechanism is allelopathy, also known as chemical competition or interference competition, where a plant secretes chemicals that 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 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 it. Such co-evolved native plants have also evolved defenses. C. diffusa and C. solstitialis do not appear in their native habitats to be overwhelmingly successful competitors. Success or lack of success in one habitat does not necessarily imply success in others. Conversely, examining habitats in which a species is less successful can reveal novel weapons to defeat invasiveness.
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 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.
Facilitation also occurs where one species so physically modifies a habitat in ways that are 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.
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.
When changes such as a forest fire occur, normal succession favors native grasses and forbs. An introduced species that can spread faster than natives can use resources that would have been available to native species, squeezing them out. Nitrogen and phosphorus are often the limiting factors in these situations.
Every species occupies a niche in its native ecosystem; some species fill large and varied roles, while others are highly specialized. Some invading species fill niches that are not used by native species, and they also can create new niches.
Ecosystem changes can alter species' distributions. For example edge effects describe what happens when part of an ecosystem is disturbed as when land is cleared for agriculture. The boundary between remaining undisturbed habitat and the newly cleared land itself forms a distinct habitat, creating new winners and losers and possibly hosting species that would not thrive outside the boundary habitat.
Although invasive species have typically been introduced to a habitat, some native species can, under the influence of events, such as long-term rainfall changes or human modifications to the habitat, increase in number and range and become invasive by expanding into new areas and disturbing the balance of species in the new area.
All species experience increases and decreases in numbers, in many cases accompanied by expansion or contraction of range. For example, the Monterey cypress is an endangered endemic, 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 natural habitat.
Traits of invaded ecosystems
In 1958, Charles S. Elton claimed that ecosystems with higher species diversity were less subject to invasive species because of fewer available niches. Other ecologists later pointed to highly diverse, but heavily invaded ecosystems and argued that ecosystems with high species diversity were more susceptible to invasion.
This debate hinged on the spatial scale at which invasion studies were performed, and the issue of how diversity affects susceptibility remained unresolved as of 2011. Small-scale studies tended to show a negative relationship between diversity and invasion, while large-scale studies tended to show the reverse. The latter result may be a side-effect of invasives' ability to capitalize on increased resource availability and weaker species interactions that are more common when larger samples are considered.
Invasion was more likely in ecosystems that were similar to the one in which the potential invader evolved. Island ecosystems may be more prone to invasion because their species faced few strong competitors and predators, or because their distance from colonizing species populations makes them more likely to have "open" niches. An example of this phenomenon was the decimation of native bird populations on Guam by the invasive brown tree snake. Conversely, invaded ecosystems may lack the natural competitors and predators that check invasives' growth in their native ecosystems, a factor that affected Guam snake populations.
Invaded ecosystems may have experienced disturbance, typically human-induced. Such a disturbance may give invasive species a chance to establish themselves with less competition from natives less able to adapt to a disturbed ecosystem.
Non-native species have many vectors, including biogenic vectors, but most invasions 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 humans typically carry specimens greater distances than natural forces.
Vectors include plants or seeds imported for horticulture. The pet trade moves animals across borders, where they can escape and become invasive. Organisms stow away on transport vehicles. Ballast water taken up at sea and released in port by transoceanic vessels is the largest vector for non-native aquatic species invasions. Around the world on the average day, more than 3,000 different species of aquatic life may be transported on these vessels. For example, freshwater zebra mussels, native to the Black, Caspian and Azov seas, probably reached the Great Lakes via ballast water from a transoceanic vessel. Although the zebra mussel invasion was first noted in 1988, and a mitigation plan was successfully implemented shortly thereafter, the plan had (and continued to have as of 2005) a serious flaw or loophole, whereby ships that are loaded with cargo when they reach the Seaway need not be tested, but all the same they transfer ballast 'puddles' between Seaway ports, according to Professor Anthony Ricciardi.
Species have also been introduced intentionally. For example, to feel more "at home", American colonists formed "Acclimation Societies" that repeatedly imported birds that were native to Europe to North America and other distant lands. 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. Agricultural Research Service entomologists identified them as rhinoceros beetle, hercules beetle, and king stag beetle. Because these species were not native to the U.S., they could have threatened native ecosystems. To prevent exotic species from becoming a problem in the U.S., special handling and permits are required when living materials are shipped from foreign countries. USDA programs such as Smuggling Interdiction and Trade Compliance (SITC) attempt to prevent exotic species outbreaks in America.
Economics plays a major role in exotic species introduction. High 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 can sterilize soils, while adding a variety of nutrients. In the resulting free-for-all, formerly entrenched species lose their advantage, leaving more room for invasives. In such circumstances plants that can regenerate from their roots have an advantage. Non-natives with this ability can benefit from a low intensity fire burns that removes surface vegetation, leaving natives that rely on seeds for propagation to find their niches occupied when their seeds finally sprout.
Impact of wildfire suppression on spreading
Wildfires often occur in remote areas, needing fire suppression crews to travel through pristine forest to reach the site. The crews can bring invasive seeds with them. If any of these stowaway seeds become established, a thriving colony of invasives can erupt in as few as six weeks, after which controlling the outbreak can need years of continued attention to prevent further spread. Also, disturbing the soil surface, such as cutting firebreaks, destroys native cover, exposes soil, and can accelerate invasions. In suburban and wildland-urban interface areas, the vegetation clearance and brush removal ordinances of municipalities for defensible space can result in excessive removal of native shrubs and perennials that exposes the soil to more light and less competition for invasive plant species.
Fire suppression vehicles are often major culprits in such outbreaks, as the vehicles are often 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, washing stations "decontaminate" vehicles before engaging in suppression activities. Large wild fires attract firefighters from remote places, further increasing the potential for seed transport.
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 Hawaii (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.
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. 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. For example, hybridization with introduced cordgrass, Spartina alterniflora, threatens the existence of California cordgrass (Spartina foliosa) in San Francisco Bay. Invasive species cause competition for native species and because of this 400 of the 958 endangered species under the Endangered Species Act are at risk
Non-native species can have benefits. Asian oysters, for example, filter water pollutants better than native[clarification needed] oysters. They also grow faster and withstand disease better than natives. Biologists are currently considering releasing this mollusk in the Chesapeake Bay to help restore oyster stocks and remove 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.
Economic costs from invasive species can be separated into direct costs through production loss in agriculture and forestry, and management costs. Estimated damage and control cost of invasive species in the U.S. alone amount to more than $138 billion annually. Economic losses can also occur through loss of recreational and tourism revenues. 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. The following examples from different sectors of the economy demonstrate the impact of biological invasions.
Some invasions offer potential commercial benefits. For instance, silver carp and common carp can be harvested for human food and exported to markets already familiar with the product, or processed into pet foods, or mink feed. Vegetative invasives such as water hyacinth can be turned into fuel by methane digesters.
Weeds reduce yield in agriculture, though they may provide essential nutrients. Some deep-rooted weeds can "mine" nutrients (see dynamic accumulator) from the subsoil and deposit them on the topsoil, while others provide habitat for beneficial insects and/or provide foods for pest species. Many weed species are accidental introductions that accompany seeds and imported plant material. Many introduced weeds in pastures compete with native forage plants, threaten young cattle (e.g., leafy spurge, Euphorbia esula) or are 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. A decline in pollinator services and loss of fruit production has been caused by honey bees infected by the invasive varroa mite. Introduced rats (Rattus rattus and R. norvegicus) have become serious pests on farms, destroying stored grains.
Invasive plant pathogens and insect vectors for plant diseases can also suppress agricultural yields and nursery stock. Citrus greening is a bacterial disease vectored by the invasive Asian citrus psyllid (ACP). Because of the impacts of this disease on citrus crops, citrus is under quarantine and highly regulated in areas where ACP has been found.
The unintentional introduction of forest pest species and plant pathogens can change forest ecology and damage the timber industry. The Asian long-horned beetle (Anoplophora glabripennis) was first introduced into the U.S. in 1996, and was expected to infect and damage millions of acres of hardwood trees. As of 2005 thirty million dollars had been spent in attempts to eradicate this pest and protect millions of trees in the affected regions.
Garlic mustard, Alliaria petiolata, is one of the most problematic invasive plant species in eastern North American forests. The characteristics of garlic mustard are slightly different from those of the surrounding native plants, which results in a highly successful species that is altering the composition and function of the native communities it invades. When garlic mustard invades the understory of a forest, it effects the growth rate of tree seedlings, which is likely to alter forest regeneration of impact forest composition in the future.
Tourism and recreation
Invasive species can impact outdoor recreation, such as fishing, hunting, hiking, wildlife viewing, and water-based activities. They can damage a wide array of environmental services that are important to recreation, including, but not limited to, water quality and quantity, plant and animal diversity, and species abundance. Eiswerth states, "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 complicating fishing and boating. The very loud call of the introduced common coqui depresses real estate values in affected neighborhoods of Hawaii.
Encroachment of humans into previously remote ecosystems has exposed exotic diseases such as AIDS virus to the wider population. 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 afflictions. The introduced Chinese mitten crabs are carriers of Asian lung fluke. Throughout recorded history, epidemics of human diseases, such as malaria, yellow fever, typhus, and bubonic plague, spread via these vectors. A recent example of an introduced disease is the spread of the West Nile virus, which killed humans, birds, mammals, and reptiles. Waterborne disease agents, such as cholera bacteria (Vibrio cholerae), and causative agents of harmful algal blooms are often transported via ballast water. Invasive species and accompanying control efforts can have long term public health implications. For instance, pesticides applied to treat a particular pest species could pollute soil and surface water.
Biotic invasion is considered one of the five top drivers for global biodiversity loss and is increasing because of tourism and globalization. This may be particularly true in inadequately regulated fresh water systems, though quarantines and ballast water rules have improved the situation.
Invasive species may drive local native species to extinction via competitive exclusion, niche displacement, or hybridisation with related native species. 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. Nevertheless, it is difficult to unequivocally attribute extinctions to a species invasion, and the few scientific studies that have done so have been with animal 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.
Native species can be threatened with extinction through the process of genetic pollution. Genetic pollution is unintentional 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. Genetic pollution can operate either through introduction or through habitat modification, bringing previously isolated species into contact. Hybrids resulting from rare species that interbreed with abundant species can swamp the rarer species' gene pool. This is not always apparent from morphological observations alone. Some degree of gene flow is normal, and preserves constellations of genes and genotypes. An example of this is the interbreeding of migrating coyotes with the red wolf, in areas of eastern North Carolina where the red wolf was reintroduced.
|0||Propagules residing in a donor region|
|III||Localized and numerically rare|
|IVa||Widespread but rare|
|IVb||Localized but dominant|
|V||Widespread and dominant|
While the study of invasive species can be done within many subfields of biology, the majority of research on invasive organisms has been within the field of ecology where the issue of biological invasions is especially important. Much of the study of invasive species has been influenced by Charles Elton's 1958 book The Ecology of Invasion by Animals and Plants which drew upon the limited amount of research done within disparate fields to create a generalized picture of biological invasions. Studies on invasive species remained sparse until the 1990s when research in the field experienced a large amount of growth which continues to this day. This research, which has largely consisted of field observational studies, has disproportionately been concerned with terrestrial plants. The rapid growth of the field has driven a need to standardize the language used to describe invasive species and events. Despite this, little standard terminology exists within the study of invasive species which itself lacks any official designation but is commonly referred to as "Invasion ecology" or more generally "Invasion biology". This lack of standard terminology is a significant problem, and has largely arisen due to the interdisciplinary nature of the field which borrows terms from numerous disciplines such as agriculture, zoology, and pathology, as well as due to studies on invasive species being commonly performed in isolation of one another.
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 proposed a new nomenclature system based on biogeography rather than on taxa.
By discarding taxonomy, human health, and economic factors, this model focused only on ecological factors. The model evaluated individual populations rather than entire species. It classified each population based on its success in that environment. This model applied equally to indigenous and to introduced species, and did not automatically categorize successful introductions as harmful.
- Applied ecology
- Ballast water discharge and the environment
- Beaver eradication in Tierra del Fuego
- Interplanetary contamination (invasives species of microorganisms on other planets)
- Genetic pollution
- Global Invasive Species Information Network
- Introduced mammals on seabird breeding islands
- Introduced species
- Invader potential
- Invasion biology terminology for a review of the terminology used in invasion biology.
- Invasive earthworms of North America
- Island restoration
- List of invasive species
- List of the world's 100 worst invasive species
- Noxious weed
- Pheromone trap
- Invasive species by country
This article incorporates CC-BY-3.0 text from the reference
- Exotic Pest Plant Council. 'Exotic Pest Plants of Greatest Ecological Concern in California' accessed 4/10/2010.
- (September 21, 2006). National Invasive Species Information Center - What is an Invasive Species?. United States Department of Agriculture: National Agriculture Library. Retrieved on September 1, 2007.
- USA (1999). Executive Order 13112 of February 3, 1999: Invasive Species. Federal Register 64(25), 6183-6186.
- Colautti, Robert I.; MacIsaac, Hugh J.; MacIsaac, Hugh J. (2004). "A neutral terminology to define 'invasive' species" (PDF). Diversity and Distributions 10 (2): 135–141. doi:10.1111/j.1366-9516.2004.00061.x. Retrieved 2007-07-11.
- "Communication From The Commission To The Council, The European Parliament, The European Economic And Social Committee And The Committee Of The Regions Towards An EU Strategy On InvasFpollive Species" (PDF). Retrieved 2011-05-17.
- Exotic Pest Plant Council. p. 1. accessed 4/10/2010.
- Kolar, C.S.; D.M. Lodge (2001). "Progress in invasion biology: predicting invaders". Trends in Ecology & Evolution 16 (4): 199–204. doi:10.1016/S0169-5347(01)02101-2. PMID 11245943.
- Thebaud, C.; A.C. Finzi, L. Affre, M. Debussche, J. Escarre (1996). "Assessing why two introduced Conyza differ in their ability to invade Mediterranean old fields". Ecology (Ecology, Vol. 77, No. 3) 77 (3): 791–804. doi:10.2307/2265502. JSTOR 2265502.
- Reichard, S.H.; C. W. Hamilton (1997). "Predicting invasions of woody plants introduced into North America". Conservation Biology 11 (1): 193–203. doi:10.1046/j.1523-1739.1997.95473.x.
- Williams, J.D.; G. K. Meffe (1998). "Nonindigenous Species". Status and Trends of the Nation's Biological Resources. Reston, Virginia: United States Department of the Interior, Geological Survey 1.
- Ewell, J.J.; D.J. O’Dowd, J. Bergelson, C.C. Daehler, C.M. D’Antonio, L.D. Gomez, D.R. Gordon, R.J. Hobbs, A. Holt, K.R. Hopper, C.E. Hughes, M. LaHart, R.R.B. Leakey, W.G. Wong, L.L. Loope, D.H. Lorence, S.M. Louda, A.E. Lugo, P.B. McEvoy, D.M. Richardson, and P.M. Vitousek (1999). "Deliberate introductions of species: Research needs - Benefits can be reaped, but risks are high". BioScience (BioScience, Vol. 49, No. 8) 49 (8): 619–630. doi:10.2307/1313438. JSTOR 1313438.
- Tilman, D. (2004). "Niche tradeoffs, neutrality, and community structure: A stochastic theory of resource competition, invasion, and community assembly". Proceedings of the National Academy of Sciences 101 (30): 10854–10861. doi:10.1073/pnas.0403458101. PMC 503710. PMID 15243158.
- Verling, E.; G.M. Ruiz, L.D. Smith, B. Galil, A.W. Miller, and K.R. Murphy (2005). "Supply-side invasion ecology: characterizing propagule pressure in coastal ecosystems". Proceedings of the Royal Society B 272 (1569): 1249–1256. doi:10.1098/rspb.2005.3090. PMC 1564104. PMID 16024389.
- Stohlgren, T.J.; D. Binkley, G.W. Chong, M.A. Kalkhan, L.D. Schell, K.A. Bull, Y. Otsuki, G. Newman, M. Bashkin, and Y. Son (1999). "Exotic plant species invade hot spots of native plant diversity". Ecological Monographs 69: 25–46. doi:10.1890/0012-9615(1999)069[0025:EPSIHS]2.0.CO;2.
- Sax, D.F.; S. D. Gaines and J. H. Brown (2002). "Species Invasions Exceed Extinctions on Islands Worldwide: A Comparative Study of Plants and Birds". American Naturalist 160 (6): 766–783. doi:10.1086/343877. PMID 18707464.
- Huenneke, L.; S. Hamburg; R. Koide; H. Mooney; P. Vitousek (1990). "Effects of soil resources on plant invasion and community structure in California (USA) serpentine grassland". Ecology (Ecology, Vol. 71, No. 2) 71 (2): 478–491. doi:10.2307/1940302. JSTOR 1940302.
- Hierro, J.L.; R.M. Callaway (2003). "Allelopathy and exotic plant invasion". Plant and Soil 256 (1): 29–39. doi:10.1023/A:1026208327014.
- Vivanco, J.M.; H.P. Bais, F.R. Stermitz, G.C. Thelen, R.M. Callaway (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.
- Brooks, M.L.; C. M. D’Antonio, D. M. Richardson, J. B. Grace, J. E. Keeley, J. M. DiTomaso, R. J. Hobbs, M. Pellant, and D. Pyke (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.
- Silver Botts, P.; B. A. Patterson and D. Schlosser (1996). "Zebra mussel effects on benthic invertebrates: Physical or biotic?". Journal of the North American Benthological Society (15): 179–184.
- 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.
- Davis, M.A.; J.P. Grime, K. Thompson (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.
- Smith, J. P., Jr.; K. Berg (1988). Inventory of rare and endangered vascular plants of California. Sacramento, California: California Native Plant Society. ISBN 0-943460-14-X.
- Elton, C.S. (2000) . The Ecology of Invasions by Animals and Plants. Foreword by Daniel Simberloff. Chicago: University of Chicago Press. p. 196. ISBN 0-226-20638-6.
- Stohlgren, T.J.,; D. Binkley, G.W. Chong, M.A. Kalkhan, L.D. Schell, K.A. Bull, Y. Otsuki, G. Newman, M. Bashkin, and Y. Son (1999). "Exotic plant species invade hot spots of native plant diversity". Ecological Monographs 69: 25–46. doi:10.1890/0012-9615(1999)069[0025:EPSIHS]2.0.CO;2.
- Byers, J.E.; E.G. Noonburg (2003). "Scale dependent effects of biotic resistance to biological invasion". Ecology 84 (6): 1428–1433. doi:10.1890/02-3131.
- Levine, J. M. (2000). "Species diversity and biological invasions: Relating local process to community pattern". Science 288 (5467): 852–854. doi:10.1126/science.288.5467.852. PMID 10797006.
- Williams, J.D.; G. K. Meffe (1998). "Nonindigenous Species". Status and Trends of the Nation's Biological Resources. Reston, Virginia: United States Department of the Interior, Geological Survey 1.
- Stachowicz, J.J.; D. Tilman (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. Species Invasions: Insights into Ecology, Evolution, and Biogeography. Sunderland, Massachusetts: Sinauer Associates. ISBN 0-87893-811-7.
- Fritts, T.H.; D. Leasman-Tanner (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.
- Cassey, P; T.M. Blackburn, R.P. Duncan and S.L. Chown (2005). "Concerning Invasive Species: Reply to Brown and Sax". Austral Ecology 30 (4): 475. doi:10.1111/j.1442-9993.2005.01505.x.
- Matisoo-Smith, E.; R.M. Roberts, G.J. Irwin, J.S. Allen, D. Penny, and D.M. Lambert (1998). "Patterns of prehistoric human mobility in Polynesia indicated by mtDNA from the Pacific rat". Proceedings of the National Academy of Sciences of the United States of America 95 (25): 15145–15150. doi:10.1073/pnas.95.25.15145. PMC 24590. PMID 9844030.
- F. Moretzsohn, J.A. Sánchez Chávez, and J.W. Tunnell, Jr. (ed.). "Invasive Species". GulfBase: Resource Database for Gulf of Mexico Research. Harte Research Institute for Gulf of Mexico Studies at Texas A&M University-Corpus Christi. Retrieved March 19, 2013.
- Aquatic invasive species. A Guide to Least-Wanted Aquatic Organisms of the Pacific Northwest. 2001. University of Washington 
- jsonline.com: "Noxious cargo" 31 Oct 2005
- Leung, B.; N.E. Mandrak (2007). "The risk of establishment of aquatic invasive species: joining invasibility and propagule pressure". Proceedings of the Royal Society B 274 (1625): 2733–2739. doi:10.1098/rspb.2007.0841. PMC 2275890. PMID 17711834.
- "Our Invaluable Invertebrate Collections". Ars.usda.gov. Retrieved 2011-05-17.
- Grosholz, E.D. (2005). "Recent biological invasion may hasten invasional meltdown by accelerating historical introductions". Proceedings of the National Academy of Sciences 102 (4): 1088–1091. doi:10.1073/pnas.0308547102. PMC 545825. PMID 15657121.
- Mack, R.; D. Simberloff, W.M. Lonsdale, H. Evans, M. Clout, and F.A. Bazzazf (2000). "Biotic invasions: Causes, epidemiology, global consequences, and control". Ecological Applications 10 (3): 689–710. doi:10.1890/1051-0761(2000)010[0689:BICEGC]2.0.CO;2.
- Hawkes, C.V.; I.F. Wren, D.J. Herman, and M.K. Firestone (2005). "Plant invasion alters nitrogen cycling by modifying the soil nitrifying community". Ecology Letters 8 (9): 976–985. doi:10.1111/j.1461-0248.2005.00802.x.
- Rhymer, J. M.; Simberloff, D. (1996). "Extinction by hybridization and introgression". Annual Review of Ecology and Systematics 27 (27): 83–109. doi:10.1146/annurev.ecolsys.27.1.83.
- Ayres, D.; et al. (2004). "Spread of exotic cordgrasses and hybrids (Spartina sp.) in the tidal marshes of San Francisco Bay, California". USA Biological Invasions 6 (2): 221–231. doi:10.1023/B:BINV.0000022140.07404.b7.
- Primtel, David (2005). "Update on the environmental and economic costs associated with alien-invasive species in the United States". Ecological Economics 52 (4): 1088–1091. doi:10.1073/pnas.0308547102. PMC 545825. PMID 15657121.
- Tom Pelton, Baltimore Sun, May 26, 2006.
- Pimentel, D.; R. Zuniga and D., Morrison (2005). "Update on the environmental and economic costs associated with alien-invasive species in the United States". Ecological Economics 52 (3): 273–288. doi:10.1016/j.ecolecon.2004.10.002.
- Simberloff, D. (2001). "Biological invasions - How are they affecting us, and what can we do about them?". Western North American Naturalist 61: 308–315.
- 2013. "Citrus Greening." Invasive Species Program - Pest Alerts. Clemson University - DPI. http://www.clemson.edu/public/regulatory/plant_industry/invasive_exotic_programs/Pest%20Alerts/citrus_greening.html. Accessed 24 May 2013.
- Azevedo-Santos, V.M.; O. Rigolin-Sá; F.M. Pelicice (2011). "Growing, losing or introducing? Cage aquaculture as a vector for the introduction of non-native fish in Furnas Reservoir, Minas Gerais, Brazil". Neotropical Ichthyology 9 (4): 915–9. doi:10.1590/S1679-62252011000400024. ISSN 1679-6225.
- Balsam woolly aphid Adelges piceae (Ratzeburg) ForestPests.org (March 3, 2005) Retrieved on September 1, 2007.
- Scott E. Schlarbaum, Frederick Hebard, Pauline C. Spaine, and Joseph C. Kamalay (1997). "Three American Tragedies: Chestnut Blight, Butternut Canker and Dutch Elm Disease". (originally published via: Proceedings: Exotic Pests of Eastern Forests; (1997 April 8–10); Nashville, TN. Tennessee Exotic Pest Plant Council: 45-54.). Southern Research Station, Forest Service, United States Department of Agriculture. Retrieved June 22, 2012.
Alternative link and additional publication citation information: Tree Search, US Forest Service, USDA. http://www.treesearch.fs.fed.us/pubs/745
- Rodger, Vikki,Stinson, Kristin & Finzi, Adrian (2008) "Ready or Not, Garlic Mustard Is Moving In: Alliaria petiolata as a Member of Eastern North American Forests" BioScience Vol.58 No.5.
- Eiswerth, M.E.; Darden, Tim D.; Johnson, Wayne S.; Agapoff, Jeanmarie; Harris, Thomas R. (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.
- Sin, Hans; Adam Radford (2007). "Coqui frog research and management efforts in Hawaii". Managing Vertebrate Invasive Species: Proceedings of an International Symposium (G. W. Witmer, W. C. Pitt, K. A. Fagerstone, Eds). USDA/APHIS/WS, National Wildlife Research Center, Fort Collins, CO. Retrieved 2013-06-26.
- Aquatic invasive species. A Guide to Least-Wanted Aquatic Organisms of the Pacific Northwest. 2001. University of Washington
- Lanciotti, R.S.; Roehrig, JT; Deubel, V; Smith, J; Parker, M; Steele, K; Crise, B; Volpe, KE et al. (1999). "Origin of the West Nile virus responsible for an outbreak of encephalitis in the northeastern United States". Science 286 (5448): 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.
- Odendaal L. J., Haupt T. M. & Griffiths C. L. (2008). "The alien invasive land snail Theba pisana in the West Coast National Park: Is there cause for concern?". Koedoe - African Protected Area Conservation and Science 50(1): 93-98. abstract, doi:10.4102/koedoe.v50i1.153.
- Mooney, HA; Cleland, EE (2001). "The evolutionary impact of invasive species". Proceedings of the National Academy of Sciences of the United States of America (date=) 98 (10): 5446–51. doi:10.1073/pnas.091093398. PMC 33232. PMID 11344292.
- "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". Nativeseednetwork.org. Retrieved 2011-05-17.
- 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, JSTOR: Annual Review of Ecology and Systematics, Vol. 27 (1996), pp. 83-109
- 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
- Lockwood, Julie L.; Martha F. Hoopes; Michael P. Marchetti (2007). Invasion Ecology. Blackwell Publishing. p. 7. Retrieved 21 January 2014.
- Lowry, E; Rollinson, EJ; Laybourn, AJ; Scott, TE; Aiello-Lammens, ME; Gray, SM; Mickley, J; Gurevitch, J (2012). "Biological invasions: A field synopsis, systematic review, and database of the literature". Ecology and evolution 3 (1): 182–96. doi:10.1002/ece3.431. PMC 3568853. PMID 23404636.
- Colautti, Robert I.; Hugh J. MacIsaac (2004). "A neutral terminology to define 'invasive' species" (PDF). Diversity and Distributions 10 (2): 135–141. doi:10.1111/j.1366-9516.2004.00061.x. Retrieved 2007-09-01.
- Further reading
- Derickx, Lisa; Pedro M. Antunes (2013). A guide to the identification and control of exotic invasive species in Ontario's hardwood forests. Invasive Species Research Institute - Algoma University. p. 294. ISBN 978-0-9291-0021-0.
- Baskin, Yvonne (2003). A Plague of Rats and Rubbervines: The Growing Threat Of Species Invasions. Island Press. p. 377. ISBN 978-1-55963-051-1.
- Burdick, Alan (2006) . 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 0-19-921876-5.
- Elton, Charles S. (2000) [First published 1958]. The Ecology of Invasions by Animals and Plants. University of Chicago Press. p. 196. ISBN 978-0-226-20638-7.
- Lockwood, Julie; Martha Hoopes; Michael Marchetti (2007) . Invasion Ecology. Blackwell Publishing. p. 304. ISBN 978-1-4051-1418-9.
- McNeeley, Jeffrey A. (2001). The Great Reshuffling: Human Dimensions Of Invasive Alien Species. World Conservation Union (IUCN). p. 109. ISBN 978-2-8317-0602-3.
- Terrill, Ceiridwen (2007). Unnatural Landscapes: Tracking Invasive Species. University of Arizona Press. p. 240. ISBN 0-8165-2523-4.
- Van Driesche, Jason; Roy Van Driesche (2004). Nature Out of Place: Biological Invasions In The Global Age. Island Press. p. 377. ISBN 978-1-55963-758-9.
- Invasive species at Encyclopædia Britannica
- North American Invasive Species Network, a consortium that uses a coordinated network to advance science-based understanding and enhance management of non-native, invasive species.
- Invasive Species Compendium, An encyclopaedic resource that draws together scientific information on all aspects of invasive species.
- Invasive Species, National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for invasive species.
- Schierenbeck, Kristina A.; Lee, Carol Eunmi; Holt, Robert D. (February 26, 2010). "EDITORIAL: Synthesizing ecology and evolution for the study of invasive species". Evolutionary Applications (John Wiley & Sons) 3 (2): 96. doi:10.1111/j.1752-4571.2010.00123.x. ISSN 1752-4571. OCLC 769072511. Archived from the original on March 30, 2014. Retrieved March 30, 2014.
- Invasive Species Specialist Group - global invasive species database
- Pacific Island Ecosystems at Risk project (PIER)
- Hawaiian Ecosystems at Risk project (HEAR)
- www.invadingspecies.com of the Ontario Ministry of Natural Resources and Ontario Federation of Anglers and Hunters
- Aquatic invasive species in Ireland, Inland Fisheries Ireland
- The Nature Conservancy's Great Lakes Project- Aquatic Invasive Species
- Invasive alien species in Belgium Belgian Forum on Invasive Species (BFIS)
- "Invasive species" from the Global Legal Information Network Subject Term Index
- Don't Move Firewood - Part of the Continental Dialogue on Non-Native Forest Insects and Diseases
- Dentinger, Rachel (17 Jan 2012). "Reconsidering Non-Native Species: Ecologists challenge the categories that identify some species as natives and others as invaders". The Naked Scientists. Retrieved 16 July 2013.