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Invasive species

An invasive species is a species that is not native to a specific location (an introduced species), and that has a tendency to spread to a degree believed to cause damage to the environment, human economy or human health.[1]

The term as most often used applies to introduced species (also called 'non-indigenous' or 'non-native') that adversely affect the habitats and bioregions they invade economically, environmentally, 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.[3]

Charles Darwin, a British naturalist, performed many experiments to better understand long distance seed dispersal, and was able to germinate seeds from insect frass, faeces of waterfowl, dirt clods on the feet of birds, all of which may have traveled significant distances under their own power, or be blown off course by thousands of miles.

Invasion of long-established ecosystems by organisms from distant bio-regions is a natural phenomenon, which has likely been accelerated via hominid-assisted migration although this has not been adequately directly measured.

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.

Another study found invasive species tended to have only a small subset of the presumed traits and that many similar traits were found in noninvasive species, requiring other explanations.[12][13][14]

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).[18]

If these species evolved under great competition or predation, then the new environment may host fewer able competitors, allowing the invader to proliferate quickly.

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 and crowd out native species that have adapted poorly to serpentine soils.[21]

Invasive species might alter their environment by releasing chemical compounds, modifying abiotic factors, or affecting the behaviour of herbivores, creating a positive or negative impact on other species.

Some species, like Kalanchoe daigremontana, produce allelopathic compounds, that might have an inhibitory effect on competing species, and influence some soil processes like carbon and nitrogen mineralization.[22]

Other species like Stapelia gigantea facilitates the recruitment of seedlings of other species in arid environments by providing appropriate microclimatic conditions and preventing herbivory in early stages of development.[23]

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.

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.[27]

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.[31]

One interesting finding in studies of invasive species has shown that introduced populations have great potential for rapid adaptation and this is used to explain how so many introduced species are able to establish and become invasive in new environments.

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.[38][39]

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.[41]

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.[43]

Although the zebra mussel invasion was first noted in 1988, and a mitigation plan was successfully implemented shortly thereafter, the plan had a serious flaw or loophole, whereby ships loaded with cargo when they reached the Seaway were not tested because their ballast water tanks were empty.

However, even in an empty ballast tank, there remains a puddle of water filled with organisms that could be released at the next port (when the tank is filled with water after unloading the cargo, the ship takes on ballast water which mixes with the puddles and then everything including the living organisms in the puddles is discharged at the next port).[55]

Even though ballast water regulations are in place to protect against potentially invasive species, there exists a loophole for organisms in the 10-50 micron size class.

For organisms between 10 and 50 microns, such as certain types of phytoplankton, current regulations allow less than 10 cells per milliliter be present in discharge from treatment systems.[57]

Since many species of phytoplankton are less than 10 microns in size and reproduce asexually, only one cell released into the environment could exponentially grow into many thousands of cells over a short amount of time.

Fortunately, human deaths related to domoic acid poisoning have been prevented because of stringent monitoring programs that arose after a domoic acid outbreak in Canada in 1987.[58]

Their results suggest that heat challenges organisms face during transport may enhance the stress tolerance of species in their non-native range by selecting for genetically adapted genotypes that will survive a second applied heat stress, such as increased ocean temperature in the founder population.[62]

Since some studies have suggested increased temperature tolerance of “hijackers” on ships’ hulls or in ballast water, it is necessary to develop more comprehensive fouling and ballast water management plans in an effort to prevent against future possible invasions as environmental conditions continue to change around the world.

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.[26]

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.

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.[citation needed]

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.

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.[63]

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.

For example, invasive plants can alter the fire regime (cheatgrass, Bromus tectorum), nutrient cycling (smooth cordgrass Spartina alterniflora), and hydrology (Tamarix) in native ecosystems.[64]

However, in the Great Lakes Region, this co-evolutionary link is non existent, so the sea lamprey acts as a predator, and can consume up to 40 pounds of fish in its 12-18 month feeding period.[70]

The sea lampreys' destructive effects towards large fish negatively affects the fishing industry and has helped collapse the population of some economy dependent species.[70]

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.

Proponents of invasivorism argue that humans have the ability to eat away any species that it has an appetite for, pointing to the many animals which humans have been able to hunt to extinction—such as the Dodo bird, the Caribbean monk seal, and the passenger pigeon.

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.[80]

meaning that incurring an initial cost of searching for and finding an invasive species and quickly controlling it, while the population is small, is less expensive that managing the invasive population when it is widespread and already causing damage.

However, an intense search for the invader is only important to reduce costs in cases where the invasive species is (1) not frequently reintroduced into the managed area and (2) cost effective to search for and find.[83]

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 or provide foods for pest species.

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).

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 affects the growth rate of tree seedlings, which is likely to alter forest regeneration of impact forest composition in the future.[90]

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.[91]

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.[97]

The higher pollen count and male fitness of the invading species resulted in introgression that threatened the native populations due to lower pollen counts and lower viability of the native species.[103]

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 and geography 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.[109][110]

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'.[109][110]

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.[109]

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.[111]

Depending upon the isolation (how far an island is located from continental biotas), native island biological communities may be poorly adapted to the threat posed by exotic introductions.

An additional problem is that birds native to small islands may have become flightless because of the absence of predators prior to introductions and cannot readily escape the danger brought to them by introduced predators.

It is, however, regarded as a noxious plant that threatens to obliterate native plants in much of the country and is hence routinely eradicated, though it can also provide a nursery environment for native plants to reestablish themselves.

Sparrows, which were brought to control insects upon the introduced grain crops, have displaced native birds as have rainbow lorikeets and cockatoos (both from Australia) which fly free around areas west of Auckland City such as the Waitakere Ranges.

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