An invasive species is harmful, it means that produces important changes in the composition, structure or processes of ecosystems natural or semi endangering the biodiversity native (in diversity of species , diversity within populations or diverse ecosystems ). Due to their impacts on the ecosystems where these species have been introduced, they are considered ecosystem engineers .
Natural or human-caused changes in ecosystems around the globe have redistributed plant and animal species accidentally or voluntarily. As a consequence of these changes, certain species have an invasive behavior in their natural or introductory locality, with altered or degraded habitats being more susceptible . These invasions are associated with several problems:
At the ecological level, the loss of native diversity and the degradation of the invaded habitats stand out.
Economically, direct effects on agricultural activities and public health are important.
Once the invasion is detected, its control and eradication are costly and not always possible. Identifying potential invaders and avoiding their establishment is the best way to stop a problem that increases at the same pace as globalization.
Effects
Ecological
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 regime (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.
Geomorphological
Primary geomorphological effects of invasive plants are bioconstruction and bioprotection. For example, Kudzu Pueraria montana, a vine native to Asia was widely introduced in the southeastern USA in the early 20th century to control soil erosion. While primary effects of invasive animals are bioturbation, bioerosion, and bioconstruction. For example, invasion of Chinese mitten crab Eriocheir sinensis have resulted in higher bioturbation and bioerosion rates.
Economic
Some invaders cause negative benefits towards the economy of the local area. For example, in the Great Lakes Region the sea lamprey is an invasive species that acts as a predator. In its original habitat, the sea lamprey used co-evolution to act as a parasite without killing the host organism. 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. Sea lampreys prey on all types of large fish such as lake trout and salmon. The sea lampreys' destructive effects towards large fish negatively affects the fishing industry and has helped collapse the population of some economy dependent species.
Economic opportunities
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. Water hyacinth can be turned into fuel by methane digesters, and other invasive plants can also be harvested and utilized as a source of bioenergy.
Benefits
Although most people focus on the negative effects of invasive and non-native species, they can actually be harmless or even beneficial in some cases. Ecosystems thrive because of biodiversity and some need non-native species in order to succeed. There are four major ways that non-natives can be very beneficial for an ecosystem. The first is that they can provide a suitable habitat or food source for other organisms. In areas where a native has become extinct or reached a point that it cannot be restored, non-native species can fill their role. A good example of this is the Tamarisk, a non-native woody plant, and the Southwestern Willow Flycatcher, an endangered bird. 75% of Southwestern Willow Flycatcher were found to nest in these plants and their success was the same as the flycatchers that had nested in native plants. The removal of Tamarisk would be detrimental to Southwestern Willow Flycatcher as their native nesting sites are unable to be restored. The second way that non-native species can be beneficial is that they act as catalysts for restoration. This is because the presence of non-native species increases the heterogeneity and biodiversity in an ecosystem. This increase in heterogeneity can create microclimates in sparse and eroded ecosystems, which then promotes the growth and reestablishment of native species. Another benefit of non-native species is that they can act as a substitute for an existing ecosystem engineer. In many cases, non-native species can be introduced to fill a niche that had previously been occupied by a native species. Many non-native species have similar characteristics and functions and can keep an ecosystem functioning properly without collapse. An example of this is the Aldabra giant tortoises, which were introduced on several small islands and have successfully taken over the roles of herbivore and seed disperser. The last benefit of non-native species is that they provided ecosystem services. There are many examples of this. The major one being pollinators. The American Honey bee was introduced in the rainforest to pollinate fragmented landscapes that native species cannot. Also, non-native species can function as biocontrol agents to limit the effects of invasive species. Such as the use of non-native species to control agricultural pests.
Non-native species can have other 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. Additionally, some species have invaded an area so long ago that they have found their own beneficial niche in the environment, a term referred to as naturalisation. For example, L. leucozonium, shown by population genetic analysis to be an invasive species in North America, has become an important pollinator of caneberry as well as cucurbit, apple trees, and blueberry bushes.
Invasivorism
Invasive species are flora and fauna whose introduction into a habitat disrupts the native eco-system. In response, Invasivorism is a movement that explores the idea of eating invasive species in order to control, reduce, or eliminate their populations. Chefs from around the world have begun seeking out and using invasive species as alternative ingredients. Miya's of New Haven, Connecticut created the first invasive species menu in the world. Skeptics point out that once a foreign species has entrenched itself in a new place—such as the Indo-Pacific lionfish that has now virtually taken over the waters of the Western Atlantic, Caribbean and Gulf of Mexico—eradication is almost impossible. Critics argue that encouraging consumption might have the unintended effect of spreading harmful species even more widely.
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. Proponents of invasivorism also point to the success that Jamaica has had in significantly decreasing the population of lionfish by encouraging the consumption of the fish.
Costs
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.
It is often argued that the key to reducing the costs of invasive species damage and management is early detection and rapid response, 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.
Plant industry
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 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.
Aquaculture
Aquaculture is a very common vector of species introductions – mainly of species with economic potential (e.g., Oreochromis niloticus).
Forestry
The unintentional introduction of forest pest species and plant pathogens can change forest ecology and damage the timber industry. Overall, forest ecosystems in the U.S. are widely invaded by exotic pests, plants, and pathogens.
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. The woolly adelgid has inflicted damage on old-growth spruce, fir and hemlock forests and damages the Christmas tree industry. And the chestnut blight fungus (Cryphonectria parasitica) and Dutch elm disease (Ophiostoma novo-ulmi) are two plant pathogens with serious impacts on these two species, and forest health. 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 affects 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.
Health
Encroachment of humans into previously remote ecosystems has exposed exotic diseases such as HIV 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.
Biodiversity
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.
Genetic pollution
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 occurs either through introduction or through habitat modification, where previously isolated species are brought into contact with the new genotypes. Invading species have been shown to adapt to their new environments in a remarkably short amount of time. The population size of invading species may remain small for a number of years and then experience an explosion in population, a phenomenon known as "the lag effect".
Hybrids resulting from invasive species interbreeding with native species can incorporate their genotypes into the gene pool over time through introgression. Similarly, in some instances a small invading population can threaten much larger native populations. For example, Spartina alterniflora was introduced in the San Francisco Bay and hybridized with native Spartina foliosa. 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. Reduction in fitness 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. The end result was a decrease in stable breeding pairs of red wolf, which may further complicate the social stability of packs and reintroduction efforts.
Invasive exotic diseases
History is rife with the spread of exotic diseases, such as the introduction of smallpox into the indigenous peoples of the Americas by the Spanish, where it obliterated entire populations of indigenous civilizations before they were ever even seen by Europeans.
Problematic exotic disease introductions in the past century or so include the chestnut blight which has almost eliminated the American chestnut tree from its forest habitat. Responses to increase the population of the American chestnut include creating blight resistant trees that can be reintroduced. This displays both the positive and negative aspects of introduced species.
Another example is the Dutch elm disease, which has severely reduced the American elm trees in forests and cities.
Diseases may also be vectored by invasive insects such as the Asian citrus psyllid and the bacterial disease citrus greening.
But in recent years some argue that some introduced species may have a positive ecological impact on an environment.
Study and eradication
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. 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.
Introduced species on islands
Perhaps the best place to study problems associated with introduced species is on islands. 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. Often this can mean that no natural predator of an introduced species is present, and the non-native spreads uncontrollably into open or occupied niche.
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. The tendency of rails in particular to evolve flightless forms on islands making them vulnerable has led to the disproportionate number of extinctions in that family.
The field of island restoration has developed as a field of conservation biology and ecological restoration, a large part of which deals with the eradication of invasive species.
New Zealand
In New Zealand the largest commercial crop is Pinus radiata, the native Californian Monterey pine tree, which grows as well in New Zealand as in California. However, the pine forests are also occupied by deer from North America and Europe and by possums from Australia. All are exotic species and all have thrived in the New Zealand environment. The pines are seen as beneficial while the deer and possums are regarded as serious pests.
Common gorse, originally a hedge plant in Britain, was introduced to New Zealand for the same purpose. Like the Monterey pine, it has shown a favour to its new climate. 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.
Rabbits, introduced as a food source by sailors in the 1800s, have become a severe nuisance to farmers, notably in the South Island. The myxomatosis virus was illegally imported and illegally released, but it had little lasting effect upon the rabbit population other than to make it more resistant to the virus.
Cats, brought later by Europeans, have had a devastating effect upon the native birdlife, particularly as many New Zealand birds are flightless. Feral cats and dogs which were originally brought as pets are also known to kill large numbers of birds. A recent (2006) study in the South Island has shown that even domestic cats with a ready supply of food from their owners may kill hundreds of birds in a year, including natives.
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.
In much of New Zealand, the Australian black swan has effectively eliminated the existence of the previously introduced mute swan.
Two notable varieties of spiders have also been introduced: the white tail spider and the redback spider. Both may have arrived inside shipments of fruit. Until then, the only spider (and the only venomous animal) dangerous to humans was the native katipo, which is very similar to the redback and interbreed with the more aggressive Australian variety.
South Georgia Island
In 2018, the South Georgia Island was declared free of invasive rodents after a multi-year extermination effort.
Relationship or cost-benefit analysis
This analysis is the basis of the management decision to be followed with the invasive alien species in terms of its eradication or control, based on the relationship between the costs of its damage and the benefits it generates. To this end, the most complete theory in this regard was introduced by the Cuban mathematician Javier Pérez Capdevila, who introduced the concepts of individual benefit of an invasive exotic species and collective benefit, hence deriving two processes of cost-benefit analysis, an a priori where an initial decision is foreseen that can be modified by another ex post analysis.
The most effective way to reduce costs is early detection followed by prompt action.
Source from Wikipedia
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