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Invasive species

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Kudzu, an invasive species from Japan, growing in Atlanta, Georgia
See also: Introduced species, Non-indigenous species, and Weed

Invasive species is a phrase with several definitions, but the term is most often used to describe non-indigenous species (i.e. plants or animals) that adversely effect the habitats they invade either economically, environmentally or otherwise. It has been used in this sense by government organizations[1][2] as well as conservation groups such as the IUCN.[3] However, the term can also be extended to include both native and non-native species that heavily colonize a particular habitat. Yet another use of the word is for a widespread non-indigenous species, which is particularly misleading as not all non-indigenous species necessarily have an adverse effect on their adopted environment. An example of this use includes claims that the common goldfish (Carassius auratus) is invasive, but while it is common outside of its range globally, it almost never appears in harmful densities. Due to the ambiguity of the phrase, it 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, most likely, it is 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 out compete native species. Sometimes they just have ability to grow and reproduce more rapidly than native species, often times it's more complex, involving a multiplex of traits and interactions.

Several traits have been singled out by researchers as predictors of invasive ability. For example in plants, the ability to reproduce both asexually (vegetatively) as well as sexually, rapid growth, early sexual maturity, high reproductive output, the ability to disperse young widely, tolerance of a broad range of environmental conditions, high phenotypic plasticity and allelopathy are all abilities that might aid an invasive plant in establishing and proliferating in a new environment.

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 [citation needed]. Another study found that invasive species tended to only have a small subset of the invasive traits, and that many of these invasive traits were found in non-invasive species as well [citation needed] indicating that invasiveness involves complex interaction not easily categorized. (Kolar and Lodge 2001, Thebaud et al. 1996, Reichard and Hamilton 1997). Common invasive species traits include 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, ability to live off of a wide range of food types, asexual reproduction, and association with humans (Williams and Meffee 1998). The single best predictor of invasiveness, however, is whether or not the species has been invasive elsewhere (Ewel et al. 1999). Typically an introduced species must survive at low population densities before it becomes invasive in a new location.(Tilman 2004)At low population densisties, it is often difficult for the introduced species to reproduce and maintain its self in a new location, but often do to human actions a species might be transported to a location a number of times before it become established. Our repeated patterns of 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”) (e.g., Verling et al. 2005).

An introduced species might become invasive if in can out compete native species for resources such as nutrients, light, physical space, water or food. Some species when introduced into a new environment lack the competition and predation they evolved under in their native environments freeing them to proliferate quickly. Ecosystems where all available resources are being used to their full 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 instant, equivalent extinction of native species with increased populations of the invader) is not the rule (Stolgren 2003, Sax et al. 2002). Invasive species often coexist with native species for an extended time and gradually the superior competitive ability of an invasive species become apparent when its population grows larger and denser often after it adapts to its new location.

Lantana growing in abandoned citrus plantation; Moshav Sdey Hemed, Israel

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, barb goatgrass (Aegilops triuncialis), can be found in its introduced range in California on serpentine soils, which have low water-holding capacity, low nutrients, 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. (Huenneke et al. 1990).

Facilitation is the mechanism by which some species can alter their environment through chemicals or manipulation of abiotic factors, usually to make it less favorable for other species to compete against them, allowing the species to grow or reproduction. One such facilitative mechanism is allelopathy, also known as chemical competition. In allelopathy a plant or in Interference Competition a bacteria will secrete chemicals which make the surrounding soil uninhabitable, or at least inhibitory, to other 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 result shows how difficult it can be to predict whether 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 (Vivanco et al. 2004, Hierro and Callaway 2003).

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 (Brooks et al. 2004).

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 nooks and crannies 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 (Silver Botts et al. 1996).

Ecosystem based mechanisms

Unused resources

When examining an ecosystem, it is important to look at two things: the amount of resources available, and how much of those resources are being used. In a stable ecosystem, all of the available resources are used up, and all of the species there have enough to continue to survive.

Yet what happens if spare resources happen to appear in an ecosystem? For example, lets say a forest fire has cleared an area, leaving only hardy brush and established trees. The resources used by the cleared species (e.g. grasses) are no longer being utilized, and may be taken up by other organisms. While these resources are most likely to be used by the nearby species, there is an opportunity for exotic species to establish themselves. The data shows that nitrogen and phosphorus are often the limiting factors for a situation such as this (Davis et al 2000).

Unfilled niches

Every species has a role to play in its native ecosystem; some are general while others are highly specialized. These roles are known as niches. This mechanism describes a situation where the invaded ecosystem in question has unfilled niches, which are promptly filled by an invader.

Ecosystem instability

Imagine a situation involving a group of people who have mastered a game, and are able to beat anyone outside of their group. They know the rules inside and out, and are constantly perfecting their strategies to win. Few, if any, have a chance against this group. Then suddenly the rules change, and this group is at a loss, since their strategies no longer work the way they used to, and other players with vastly different strategies are able to come in, and win where they could not before. This is how the familiar ecosystem mechanism works.

This mechanism describes a situation where the ecosystem in question has suffered a disturbance of some sort, which changes the fundamental nature of the ecosystem (Byers 2002).

Ecology of invasive species

Ecological circumstances of invasive species

Pied Currawong

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 longterm rainfall changes or human modifications to the habitat, increase in numbers and become invasive. The Pied Currawong of south-east Australia is an example; as a result of human changes to the landscape, Pied Currawongs increased greatly in range during the 20th century and have caused substantial declines in the populations of the smaller birds whose nestlings they prey on[citation needed].

All species on Earth go through periods of increasing and decreasing population numbers, in many cases accompanied by expansion and contraction of range. Human “alterations” on the landscape are especially significant. 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 because the range expansion results in the species occurring where it was not before native. 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 is sufficient reason to regard a native species as "invasive" requires a broad definition of the term—but it seems reasonable to consider that some native species in disrupted ecosystems can spread widely and cause harm and in that sense become invasive. For example the Monterey Cypress is a rare and endangered endemic (Smith & Berg 1988) naturally occurring only in two small stands in California. They are being exterminated as exotic invasive species less than 50 miles from their native home. Naturalizing the cypress outside it's native range could be considered a valuable preservation tool.


Traits of invaded ecosystems

In 1958, Charles S. Elton 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 (Stohlgren et al. 1999). In the end, 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 (Levine 2000, Byers and Noonberg 2003)

The brown tree snake (Boiga irregularis)

Invasion is more likely if an ecosystem is similar to the one that the potential invader evolved (Williams and Meffee 1998). 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 (Stachowicz and Tilman 2005). An example of this phenomenon is the decimation of the native bird populations on Guam by the invasive brown tree snake (Fritts and Leasman-Tanner 2001). 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 (Williams and Meffee 1998). 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 (Tilman 2004).

Vectors

Non-native species have many vectors, including many natural ones, but most of the species that we consider "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 (Cassey et al. 2005).

One of the earliest human influenced introductions involves prehistoric humans introducing the Pacific rat (Rattus exulans) to Polynesia (Matisoo-Smith et al. 1998). Today, non-native species come from horticultural plants either in the form of the plants themselves or animals and seeds carried with them, from animals and plants released through the pet trade. Invasives also come from organisms stowed away on every type of transport vehicle imaginable, to name a few unintentional vectors. For example, ballast water taken up at sea and released in port is a major source of exotic marine life.

Chinese mitten crab (Eriocheir sinensis)

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.

Impacts of invasive species

Ecological impacts

Biological species invasions can negatively or positively impact ecological systems in a multitude of ways. Worldwide an estimated 80% of endangered species could suffer losses due to competition with or predation by invasive species (Pimentel et al. 2005). Pimentel also reports that Introduced species, such as corn, wheat, rice, and other food crops, and cattle, poultry, and other livestock, now provide more than 98% of the U.S. food system at a value of approximately $800 billion per year." 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 and disturbed habitat is often 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 waterbird refuges because it quickly covers shallow mudflats established for endangered Hawaiian stilt (Himantopus mexicanus knudseni), making these undesirable feeding areas for the birds. Sometimes, multiple successive introductions of different nonnative species can have interactive effects, where 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 (Grosholz, 2005).

Invasive plants can arise from human clearing (such as slash-and-burn) or cattle grazing actions, such that the altered land is more hospitable to the invasive species than the original plant palette. The invasive plant can even be a pre-existing species that has attained dominance from the disturbance. For example, 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 invasives; 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.

This native Canadian Eastern Cottontail rabbit among non-native plants Garlic Mustard, Mugwort, and Burdock.

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 (Tamarisk) in native ecosystems (Mack et al. 2000). Invasive species that are closely related with rare native species have the potential to hybridize with native species. Harmful effects of hybridization have lead to a decline and even extinction of native species (Hawkes et al. 2005, Rhymer and Simberloff 1998). For example, hybridization with introduced cordgrass, Spartina alterniflora, threatens the existence of California cordgrass (Spartina foliosa) in San Francisco Bay (Ayres et al 2004).

Economic impacts

Economic costs due to invasive species can be separated into direct costs due to 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 (Pimentel et al. 2005). In addition to these costs, economic losses can occur due to loss from recreational and tourism revenues (Simberloff 2001). 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 (Pimentel et al. 2005). The following examples from different sectors of the economy demonstrate the impact of biological invasions.

Agriculture

Agricultural weeds cause an overall reduction in yield. Most 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 cattle or non palatable due to thorns and spines (e.g., yellow star thistle, Centaurea solstitialis). Forage loss due to invasive weeds on pastures amounts to nearly $1 billion in the U.S. alone (Pimentel et al. 2005). A decline in pollinator services and loss of fruit production has been observed due to 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 (Pimentel et al. 2005).

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 (Pimentel et al. 2005).

The woolly adelgid inflicts damage on old growth spruce fir forests and negatively impacts the Christmas tree industry (Forest Pests: Insects, Diseases & Other Damage Agents 2005). 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 (Eiswerth 2005).

Health impacts

An increasing threat of exotic diseases exists due to increased transportation and encroachment of humans into previously remote ecosystems that can lead to new associations between a disease and a human host (e.g., AIDS virus in human host; Pimentel et al. 2005). 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. Throughout recorded history epidemics of human diseases such as malaria, yellow fever, typhus, and plague have been associated with these vectors (Elton 1958). A recent example of an introduced disease is the spread of the West Nile virus across North America resulting in human deaths and in the deaths of many birds, mammals, and reptiles (Lanciotti 1999). Waterborne disease agents, such as Cholera bacteria (Vibrio cholerae), and causative agents of harmful algal blooms are often transported via ballast water (Hallegraeff 1998). 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 (Pimentel et al. 2005).

The threat to global biodiversity

Main article: Biodiversity

The impact on global biodiversity of human introduction of non native species that have subsequently become invasive is subjective. Climate change and the movement of the continents through the ages have created divisions and changes to species over the long history of this planet. Limited information on the circumstances and impact at the time makes it difficult to directly compare this to the advent of international travel for people and goods which has made the introduction of new species increasingly easy, and the direct ecological impacts are now more measurable.

Historically the deliberate introduction of non native species has been done with little or no consideration of the impact outside of having a favored animal, fish, or plant available locally, or perhaps an ill-conceived attempt to control a native pest. In areas with highly endemic, specialised and isolated flora and fauna such as Australia, New Zealand, Madagascar, the Hawaiian Archipelago, and the Galapagos Islands, introduced species that successfully establish themselves in habitats utilized by natives compete for limited resources or prey on the native species, some of which are unable to adapt to the more competitive environment and gradually die out.

As more adaptable and generalized species are introduced to environments impacted adversely by human activities, some native species may be put at a disadvantage to survive while others thrive in the modified ecosystem. One of the primary threats to biodiversity is the spread of humanity into what were once isolated areas with land clearing and habitation putting significant pressure on local species. Agriculture, livestock and fishing can also introduce changes to local populations of indigenous species which may result in a previously innocuous native species becoming a pest due to a reduction of natural predators.

Control of invasive species

The control of invasive species can involve their eradication or their containment within a specified area. In both cases, the goal is to prevent further spread to un-invaded systems (National Biological Information Infrastructure 2004). This type of management can be implemented at several scales, from a homeowner working in his or her own backyard to large government agencies taking a national approach. The decision to eradicate a species versus contain it can depend on several factors, including, but not limited to, the type of habitat, characteristics of the organism, the spatial dimensions of the spread, time available to dedicate to control, and cost. These factors also play a role in determining which specific control technique(s) to utilize.

Mechanical control

Mechanical control involves the removal of invasive species by hand or with machines. Often, these methods are effective in controlling small populations and can be target specific, minimizing harm to non-invasive plants and animals (The Nature Conservancy 2005). Mechanical control, however, is extremely labor intensive and requires a large time investment, as treatments must often be applied several times to ensure success (The Nature Conservancy 2005). Commonly implemented control methods for plants include hand pulling, mowing, girdling, and burning (The Nature Conservancy 2005). For invasive animal control, techniques such as hunting, trapping, and the construction of physical barriers like fences or nets, are used (Hengeveld 1989).

Chemical control

Chemical compounds can be used to prevent the spread of invasive species. This method of control can be very effective in both large and small areas, but is often criticized due to the possible contamination of land and water resources and a lack of target specificity that can result in the killing of desirable plant and animal species (National Park Service 2004). Also, the target species may develop a resistance to the chemicals over time, rendering this method ineffective. Herbicides are chemicals used to control invasive plants and, depending on the target species, can be applied directly to a plant, in the soil at a plant’s base, or even to the soil before seeds develop (The Nature Conservancy 2005). For animals, other pesticides are used to restrict growth and reproduction or to kill invasive insect pests (National Biological Information Infrastructure 2004). Another form of chemical control is the use of attractant pheromones to lure mate-seeking insects into traps (Myers et al. 2000).

Biological control

Biological control involves the release of a specific species to restrict the spread of the invasive species. With the proper research, this method of control can be both environmentally safe and successful. However, it can be ineffective if the released species do not survive or if their impact on the invasive species is not as great as predicted (The Nature Conservancy 2005). Also, the species chosen for release is not always a native organism, increasing the possibility of even more invasive species. Predatory insects, called weed feeders, can be released to control invasive plants. Similarly, plants can be infected by disease causing organisms, such as fungi, bacteria, and viruses, killing them or reducing their reproductive output (Cornell University 2005). Invasive animals can be controlled with the release of predatory or parasitic organisms (especially in the case of invasive insects) or with the transmission of diseases in a similar manner as with plants (Cornell University 2005). Additionally, sterile insect or fish males of the invasive species can be released so that after mating, a female will lay “dead” eggs or eggs that will develop into sterile adults (Myers et al. 2000). There are instances in the past when biological controls specifically introduced to an area to control a nonnative species has had a negative effect on the ecosystem. A good example of this is when the mongoose was used as a biological predator in Hawaii to control the introduced rat population, unfortunately the mongoose also preys on native bird species and their eggs.

Prevention

Preventing the establishment of invasive species is always the best method of control (The Nature Conservancy 2005). Stopping harmful species at this stage can be difficult. Many governments try to limit the entry of invasive species into their lands with thorough inspections of international shipments, customs checks, and proper quarantine regulations. The creation of a list of safe and potentially harmful species can be helpful in regulation. The general public can also participate in invasive species prevention by educating themselves about invasive species and by making informed decisions.

Opposing Views

David Theodoropoulos in his book 'Invasion Biology: Critique of a Pseudoscience' argues that 'Invasion Biology' is a pseudoscience, and that its main tenet that "non-native = bad" cannot be proven scientifically. He claims that movement and change are a natural part of every ecosystem, and that all ecosystems are accustomed to receiving new members. He asserts that newcomers are rapidly incorporated into the receiving biota, are ecologically indistinguishable from natives, and have many beneficial, diversifying effects, and therefore there is no credible evidence for an "invasive species crisis". According to Theodoropoulos, the herbicide and regulatory industries are exploiting invader fears for profit, and extermination projects are causing widespread ecological, psychological, political, and cultural harm. He asserts that Alpha, Beta, Gamma, and Omega biodiversity are all increased when a new species enters an ecosystem.

A book review criticizes Theodoropoulos's book as being "a polemic", claiming that despite its 600 cited references it does not provide convincing evidence to support its premise. The review also notes that Theodoropoulos, a "conservation biologist", has not published in peer-reviewed journals.[4]

Attempting a Scientific Definition

In an attempt to avoid the subjective and pejorative vocabulary that so often accompanies discussion of invasive species, Colautti and MacIsaac[3] have proposed a new nomenclature system based on biogeography rather than on taxa. Their system categorizes a species into the following stages:

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

By removing taxonomy 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 badness to invasive species and goodness to native species. It merely classifies a species in a particular location based on it's growth patterns in that particular microenvironment.

Sources

  1. ^ National Invasive Species Information Center - What is an Invasise Species?
  2. ^ USA (1999) Executive Order 13112 of February 3, 1999: Invasive Species. Federal Register 64(25), 6183-6186.
  3. ^ a b c Colautti, Robert I.; MacIsaac, Hugh J. (2004), "A neutral terminology to define 'invasive' species" (PDF), Diversity and Distributions, 10: 135–141, retrieved 2007-07-11 {{citation}}: Check date values in: |accessdate= (help)CS1 maint: multiple names: authors list (link)
  4. ^ Raven, J. 2004. "Review of "Invasion Biology: Critique of a Pseudoscience" Annals of Botany 94(1): 196.

See also

References

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