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 as well as conservation groups such as the IUCN.
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. 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.
Because of the ambiguity of its definition, the phrase invasive species is often criticized as an imprecise term within the field of ecology. This article concerns the first two definitions; for the third, see introduced species.
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. 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 indicating that invasiveness involves complex interaction not easily categorized. 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. The single best predictor of invasiveness, however, is whether or not the species has been invasive elsewhere. Typically an introduced species must survive at low population densities before it becomes invasive in a new location. At low population densities, it is often difficult for the introduced species to reproduce and maintain its self in a new location, but often because of human actions a species might be transported to a location a number of times before it become established. Humans 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”).
An introduced species might become invasive if it 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. 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.
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. Invasive species are either plant or animal, from another area and over-compete other native species living in the area.
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 reproduce. One such facilitative mechanism is allelopathy, also known as chemical competition. In allelopathy a plant or in Interference Competition a bacterium 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 ).
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.
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.
In ecosystems the amount of resources available and how much of those resources are utilized by organisms, determine the effects of new additions to the ecosystem. In stable ecosystems equilibrium exists in the utilization of available resources.
When changes occur in an ecosystem, like forest fires removing large stands of vegetation in an area, normal succession would favor certain native grasses and forbs, but with the introduction of a species that can multiply and spread faster on open ground 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 thus impacting the ecosystem and changing its composition of organisms and their use of available resources. The data shows that nitrogen and phosphorus are often the limiting factors for a situation such as this.
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 without the change would not be able to colonize areas and niches that did not exist before.
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.
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 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. 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 a rare and endangered endemic naturally occurring only in two small stands in California. They are being exterminated as exotic invasive species less than from their native home.
Invasion is more likely if an ecosystem is similar to the one in which the potential invader evolved. 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. An example of this phenomenon is the decimation of the native bird populations on Guam by the invasive brown tree snake. 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. 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
One of the earliest human influenced introductions involves prehistoric humans introducing the Pacific rat (Rattus exulans) to Polynesia. 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 imaginable. 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.
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.
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 aeae 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. 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.
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. Invasive species that are closely related with rare native species have the potential to hybridize with 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.
Purebred naturally evolved region specific wild species can be threatened with extinction through the process of genetic pollution i.e. uncontrolled hybridization, introgression and genetic swamping which leads to homogenization or replacement of local genotypes as a result of either a numerical and/or fitness advantage of introduced plant or animal. Non-native species can bring about a form of extinction of native plants and animals by hybridization and introgression either through purposeful introduction by humans 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 swamping the entire rarer gene pool creating hybrids thus driving the entire original purebred native stock to complete extinction. Attention has to be focused on the extent of this under appreciated problem that is not always apparent from morphological (outward appearance) 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.
The woolly adelgid inflicts damage on old growth spruce fir forests and negatively impacts the Christmas tree industry. The chestnut blight fungus (Cryphonectria parasitica) and Dutch elm disease (Ophiostoma novo-ulmi) are two plant pathogens with serious impacts on forest health.
Biotic invasion is one of the five top drivers for global biodiversity loss and is increasing because of tourism and globalization. It poses a particular risk to inadequately regulated fresh water systems, though quarantines and ballast water rules have improved the situation in other respects.
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, specialized 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 because of a reduction of natural predators.
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 have proposed a new nomenclature system based on biogeography rather than on taxa. Their system categorizes a species into the following stages:
|0||Propagules residing in a donor region|
|III||Localized and numerically rare|
|IVa||Widespread but rare|
|IVb||Localized but dominant|
|V||Widespread and dominant|
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 badness to invasive species and goodness 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.