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An invasive species otherwise known as an alien is an introduced organism that becomes
overpopulated Overpopulation or overabundance is a phenomenon in which a species' population becomes larger than the carrying capacity of its environment. This may be caused by increased birth rates, lowered mortality rates, reduced predation or large scale ...
and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species adversely affect
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
s and
bioregion A bioregion is an ecologically and geographically defined area that is smaller than a biogeographic realm, but larger than an ecoregion or an ecosystem, in the World Wide Fund for Nature classification scheme. There is also an attempt to use the ...
s, causing ecological, environmental, and/or economic damage. The term can also be used for native species that become harmful to their native environment after human alterations to its
food web A food web is the natural interconnection of food chains and a graphical representation of what-eats-what in an ecological community. Another name for food web is consumer-resource system. Ecologists can broadly lump all life forms into one o ...
for example the purple sea urchin (''
Strongylocentrotus purpuratus ''Strongylocentrotus purpuratus'', the purple sea urchin, lives along the eastern edge of the Pacific Ocean extending from Ensenada, Mexico, to British Columbia, Canada. This sea urchin species is deep purple in color, and lives in lower int ...
'') which has decimated kelp forests along the northern California coast due to overharvesting of its natural predator, the California sea otter (''
Enhydra lutris The sea otter (''Enhydra lutris'') is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between , making them the heaviest members of the weasel family, but among the small ...
''). Since the 20th century, invasive species have become a serious economic, social, and environmental threat. Invasion of long-established ecosystems by organisms is a natural phenomenon, but human-facilitated introductions have greatly increased the rate, scale, and geographic range of invasion. For millennia, humans have served as both accidental and deliberate dispersal agents, beginning with their earliest migrations, accelerating in the
age of discovery The Age of Discovery (or the Age of Exploration), also known as the early modern period, was a period largely overlapping with the Age of Sail, approximately from the 15th century to the 17th century in European history, during which seafa ...
, and accelerating again with
international trade International trade is the exchange of capital, goods, and services across international borders or territories because there is a need or want of goods or services. (see: World economy) In most countries, such trade represents a significa ...
. Notable examples of invasive plant species include the
kudzu Kudzu (; also called Japanese arrowroot or Chinese arrowroot) is a group of climbing, coiling, and trailing deciduous perennial vines native to much of East Asia, Southeast Asia, and some Pacific islands, but invasive in many parts of the wor ...
vine, Andean pampas grass,
English ivy English usually refers to: * English language * English people English may also refer to: Peoples, culture, and language * ''English'', an adjective for something of, from, or related to England ** English national id ...
, Japanese knotweed, and
yellow starthistle ''Centaurea solstitialis'', the yellow star-thistle, is a species of thorny plant in the genus ''Centaurea'', which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other p ...
. Examples of invasive animals include the
New Zealand mud snail The New Zealand mud snail (''Potamopyrgus antipodarum'') is a species of very small freshwater snail with a gill and an operculum. This aquatic gastropod mollusk is in the family Tateidae. It is native to New Zealand, where it is found throug ...
,
feral pig The feral pig is a domestic pig which has gone feral, meaning it lives in the wild. They are found mostly in the Americas and Australia. Razorback and wild hog are Americanisms applied to feral pigs or boar-pig hybrids. Definition A feral ...
,
European rabbit The European rabbit (''Oryctolagus cuniculus'') or coney is a species of rabbit native to the Iberian Peninsula (including Spain, Portugal, and southwestern France), western France, and the northern Atlas Mountains in northwest Africa. It has b ...
,
grey squirrel The eastern gray squirrel (''Sciurus carolinensis''), also known, particularly outside of North America, as simply the grey squirrel, is a tree squirrel in the genus ''Sciurus''. It is native to eastern North America, where it is the most prodi ...
,
domestic cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members o ...
,
carp Carp are various species of oily freshwater fish from the family Cyprinidae, a very large group of fish native to Europe and Asia. While carp is consumed in many parts of the world, they are generally considered an invasive species in parts of ...
, and
ferret The ferret (''Mustela furo'') is a small, domesticated species belonging to the family Mustelidae. The ferret is most likely a domesticated form of the wild European polecat (''Mustela putorius''), evidenced by their interfertility. Other mus ...
. Some popular reference sources now name ''
Homo sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture ...
'', especially modern-age humans, as an invasive species, but broad appreciation of human learning capacity and their behavioral potential and plasticity argues against any such fixed categorization.


Terminology

Alien or naturalized species are those species which are not native to an area but established, and those that are a threat to native species and biodiversity are often called invasive species. The term "invasive" is poorly defined and often very subjective, Invasive species may be plants, animals, fungi, and microbes; some also include native species that have invaded human habitats such as farms and landscapes. Some broaden the term to include indigenous or "native" species that have colonized natural areas. The definition of "native" is also sometimes controversial. For example, the ancestors of '' Equus ferus'' (modern horses)
evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variati ...
in
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...
and radiated to
Eurasia Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipelag ...
before becoming locally extinct. Upon returning to North America in 1493, during their human-assisted migration, it is debatable as to whether they were native or exotic to the continent of their evolutionary ancestors. 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 Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...
and
geography Geography (from Greek: , ''geographia''. Combination of Greek words ‘Geo’ (The Earth) and ‘Graphien’ (to describe), literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, an ...
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 plant A terrestrial plant is a plant that grows on, in, or from land. Other types of plants are aquatic (living in water), epiphytic (living on trees) and lithophytic (living in or on rocks). The distinction between aquatic and terrestrial plants i ...
s. 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 Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
,
zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the animal kingdom, including the structure, embryology, evolution, classification, habits, an ...
, and
pathology Pathology is the study of the causes and effects of disease or injury. The word ''pathology'' also refers to the study of disease in general, incorporating a wide range of biology research fields and medical practices. However, when used in ...
, as well as due to studies on invasive species being commonly performed in isolation of one another. Invasive species often thrive because there are no predators that hunt them in their new locations. Many invasive species are destroying habitats where plants and animals naturally live. 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 Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, ...
rather than on
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
. By discarding taxonomy,
human health Health, according to the World Health Organization, is "a state of complete physical, mental and social well-being and not merely the absence of disease and infirmity".World Health Organization. (2006)''Constitution of the World Health Organiza ...
, 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. The USDA's National Invasive Species Information Center defines invasive species very narrowly. According to Executive Order 13112, Invasive species' means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health."


Causes

Typically, an introduced species must survive at low population densities before it becomes invasive in a new location. At low population densities, it can be difficult for the introduced species to reproduce and maintain itself in a new location, so a species might reach a location multiple times before it becomes established. Repeated patterns of human movement, such as ships sailing to and from ports or cars driving up and down highways offer repeated opportunities for establishment (also known as a high propagule pressure). Scientists include ecosystem and species factors among the mechanisms that, when combined, establish invasiveness in a newly introduced species.


Ecosystem-based mechanisms

In
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s, the amount of available resources and the extent to which those resources are used by organisms determine the effects of additional species on the ecosystem. In stable ecosystems, equilibrium exists in the use of available resources. These mechanisms describe a situation in which the ecosystem has suffered a disturbance, which changes the fundamental nature of the ecosystem. When changes such as a
forest fire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
occur, normal
succession Succession is the act or process of following in order or sequence. Governance and politics *Order of succession, in politics, the ascension to power by one ruler, official, or monarch after the death, resignation, or removal from office of ...
favors native
grasses Poaceae () or Gramineae () is a large and nearly ubiquitous family of monocotyledonous flowering plants commonly known as grasses. It includes the cereal grasses, bamboos and the grasses of natural grassland and species cultivated in lawns ...
and
forb A forb or phorb is an herbaceous flowering plant that is not a graminoid (grass, sedge, or rush). The term is used in biology and in vegetation ecology, especially in relation to grasslands and understory. Typically these are dicots without woo ...
s. An introduced species that can spread faster than natives can use resources that would have been available to native species, squeezing them out.
Nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
and
phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ea ...
are often the limiting factors in these situations. Every species occupies a
niche Niche may refer to: Science *Developmental niche, a concept for understanding the cultural context of child development * Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ecology, the ...
in its native ecosystem; some species fill large and varied roles, while others are highly specialized. Some invading species fill niches that are not used by native species, and they also can create new niches. An example of this type can be found within the '' Lampropholis delicata'' species of skink. Invasion is more likely in ecosystems that are similar to the one in which the potential invader evolved. Ecosystem changes can alter species' distributions. For example,
edge effect In ecology, edge effects are changes in population or community structures that occur at the boundary of two or more habitats. Areas with small habitat fragments exhibit especially pronounced edge effects that may extend throughout the range. ...
s describe what happens when part of an ecosystem is disturbed as when land is cleared for
agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people ...
. 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. In 1958,
Charles S. Elton Charles Sutherland Elton (29 March 1900 – 1 May 1991) was an English zoologist and animal ecologist. He is associated with the development of population and community ecology, including studies of invasive organisms. Personal life Charles S ...
claimed that ecosystems with higher
species diversity Species diversity is the number of different species that are represented in a given community (a dataset). The effective number of species refers to the number of equally abundant species needed to obtain the same mean proportional species abundan ...
were less subject to invasive species because of fewer available niches. Other ecologists later pointed to highly diverse, but heavily invaded ecosystems and argued that ecosystems with high species diversity were more susceptible to invasion. This debate hinged on the
spatial scale Spatial scale is a specific application of the term scale for describing or categorizing (e.g. into orders of magnitude) the size of a space (hence ''spatial''), or the extent of it at which a phenomenon or process occurs. For instance, in phy ...
at which invasion studies were performed, and the issue of how diversity affects susceptibility remained unresolved as of 2011. Small-scale studies tended to show a negative relationship between
diversity Diversity, diversify, or diverse may refer to: Business *Diversity (business), the inclusion of people of different identities (ethnicity, gender, age) in the workforce *Diversity marketing, marketing communication targeting diverse customers * ...
and invasion, while large-scale studies tended to show the reverse. The latter result may be a side-effect of invasives' ability to capitalize on increased resource availability and weaker species interactions that are more common when larger samples are considered. However, this spatial scale dependent pattern of the effects of invasion on diversity does not seem to hold true when the invader is a vertebrate. Island ecosystems may be more prone to invasion because their species face few strong competitors and predators, or because their distance from colonizing species populations makes them more likely to have "open" niches. An example of this phenomenon is the decimation of native bird populations on
Guam Guam (; ch, Guåhan ) is an organized, unincorporated territory of the United States in the Micronesia subregion of the western Pacific Ocean. It is the westernmost point and territory of the United States (reckoned from the geographic cent ...
by the invasive
brown tree snake The brown tree snake (''Boiga irregularis''), also known as the brown catsnake, is an arboreal rear-fanged colubrid snake native to eastern and northern coastal Australia, eastern Indonesia (Sulawesi to Papua), Papua New Guinea, and many islands ...
. Conversely, invaded ecosystems may lack the natural competitors and predators that check invasives' growth in their native ecosystems. On small islands, native birds may have become flightless because of the absence of predators prior to introductions. These birds cannot readily escape the danger brought to them by introduced predators. The tendency of
rail Rail or rails may refer to: Rail transport *Rail transport and related matters *Rail (rail transport) or railway lines, the running surface of a railway Arts and media Film * ''Rails'' (film), a 1929 Italian film by Mario Camerini * ''Rail'' ( ...
s in particular to evolve flightless forms on islands has made them vulnerable and has led to the disproportionate number of extinctions in that family. The islands of
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
have many invasive species affecting the islands' native plants and animals. Invasive insects, plants, hoofed animals such as deer, goats and pigs endanger native plants, rosy wolfsnails from the southeastern
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
feed on the island's native snails, and plants such as
Australian tree fern Australian tree fern may refer to any species of tree fern native to Australia, most commonly referring to: *''Cyathea australis'' *'' Cyathea cooperi'', native to New South Wales and Queensland *''Dicksonia antarctica ''Dicksonia antarctica ...
and '' Miconia calvescens'' shade out native plants. Populations of introduced
little fire ants The little fire ant (''Wasmannia auropunctata''), also known as the electric ant, is a small (approx long), light to golden brown (ginger) social ant native to Central and South America, now spread to parts of Africa (including Gabon and Camer ...
in Hawaii can have major negative impacts on animals, crops, and humans. The
veiled chameleon The veiled chameleon (''Chamaeleo calyptratus'') is a species of chameleon ( family Chamaeleonidae) native to the Arabian Peninsula in Yemen and Saudi Arabia. Other common names include cone-head chameleon and Yemen chameleon. They are born pa ...
and the
Jackson's chameleon Jackson's chameleon (''Trioceros jacksonii''), also known as Jackson's horned chameleon, three-horned chameleon or Kikuyu three-horned chameleon, is a species of chameleon (family Chamaeleonidae) native to East Africa, but also introduced to Haw ...
have a great impact on the ecology of Hawaii. In
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...
the first invasive species were the dogs and rats brought by Polynesian settlers around 1300. Cats, brought later by Europeans, have had a devastating effect upon the native birdlife, particularly as many New Zealand birds are flightless.
Rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit sp ...
s, introduced as a food source by sailors in the 1800s, have become a severe nuisance to farmers, notably in the
South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman ...
. Common
gorse ''Ulex'' (commonly known as gorse, furze, or whin) is a genus of flowering plants in the family Fabaceae. The genus comprises about 20 species of thorny evergreen shrubs in the subfamily Faboideae of the pea family Fabaceae. The species are ...
, originally a hedge plant native to
Western Europe Western Europe is the western region of Europe. The region's countries and territories vary depending on context. The concept of "the West" appeared in Europe in juxtaposition to "the East" and originally applied to the ancient Mediterranean ...
, was introduced to New Zealand for the same purpose but grows aggressively and threatens to obliterate native plants in much of the country and is hence routinely eradicated. The native forests are heavily impacted by several species of
deer Deer or true deer are hoofed ruminant mammals forming the family Cervidae. The two main groups of deer are the Cervinae, including the muntjac, the elk (wapiti), the red deer, and the fallow deer; and the Capreolinae, including the re ...
from
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...
and
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
and by the Australian
brushtail possum The brushtail possums are the members of the genus ''Trichosurus'' in the Phalangeridae, a family of marsupials. They are native to Australia (including Tasmania) and some small nearby islands. Unique among marsupials, they have shifted the hy ...
. These exotic species have all thrived in the New Zealand environment. The colonization of the island of
Madagascar Madagascar (; mg, Madagasikara, ), officially the Republic of Madagascar ( mg, Repoblikan'i Madagasikara, links=no, ; french: République de Madagascar), is an island country in the Indian Ocean, approximately off the coast of East Afric ...
has introduced exotic plant and animal species which have significantly altered the island's landscape. This is a result of man-made disturbances to the ecosystems present. The most well-known disturbance is extensive logging. This allows the invasion of non-native species as they establish in the spaces created. Some of the invasive plant species in Madagascar include prickly pear (''Opuntia'' spp.) and silver wattle (''
Acacia dealbata ''Acacia dealbata'', the silver wattle, blue wattle or mimosa, is a species of flowering plant in the legume family Fabaceae, native to southeastern Australia in New South Wales, Victoria, Tasmania, and the Australian Capital Territory, and widel ...
)''. The water hyacinth (''
Eichhornia crassipes ''Pontederia crassipes'' (formerly ''Eichhornia crassipes''), commonly known as common water hyacinth is an aquatic plant native to South America, naturalized throughout the world, and often invasive outside its native range. However, this plant is now being used in fertilizers and paper bags and for cleaning up biological waste. Invaded ecosystems may have experienced disturbance, typically human-induced. Such a disturbance may give invasive species a chance to establish themselves with less competition from natives less able to adapt to a disturbed ecosystem. Primary geomorphological effects of invasive plants are bioconstruction and bioprotection. For example, kudzu (''
Pueraria montana ''Pueraria montana'' is a species of plant in the botanical family Fabaceae. At least three sub-species (alternatively called varieties) are known. It is closely related to other species in the genus '' Pueraria'' (''P. edulis'' and '' P. phaseol ...
''), a vine native to Asia, was widely introduced in the
southeastern United States The Southeastern United States, also referred to as the American Southeast or simply the Southeast, is a geographical List of regions in the United States, region of the United States. It is located broadly on the eastern portion of the south ...
in the early 20th century to control
soil erosion Soil erosion is the denudation or wearing away of the upper layer of soil. It is a form of soil degradation. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, a ...
. The primary geomorphological effects of invasive animals are
bioturbation Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a ...
,
bioerosion Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and ...
, and bioconstruction. For example, invasions of the Chinese mitten crab (''
Eriocheir sinensis The Chinese mitten crab ('; ,  "big sluice crab"), also known as the Shanghai hairy crab (, p ''Shànghǎi máoxiè''), is a medium-sized burrowing crab that is named for its furry claws, which resemble mittens. It is native to river ...
'') have resulted in higher bioturbation and bioerosion rates.


Species-based mechanisms

While all species compete to survive, invasive species appear to have specific traits or specific combinations of traits that allow them to outcompete
native species In biogeography, a native species is indigenous to a given region or ecosystem if its presence in that region is the result of only local natural evolution (though often popularised as "with no human intervention") during history. The term is e ...
. In some cases, the competition is about rates of growth and reproduction. In other cases, species interact with each other more directly. Researchers disagree about the usefulness of traits as invasiveness markers. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone. Another study found invasive species tended to have only a small subset of the presumed traits and that many similar traits were found in noninvasive species, requiring other explanations. Common invasive species traits include the following: * Fast growth * Rapid
reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproduction is a fundamental feature of all known life; each individual or ...
* High dispersal ability * Phenotype plasticity (the ability to alter growth form to suit current conditions) * Tolerance of a wide range of environmental conditions (
Ecological competence Ecological competence is a term that has several different meanings that are dependent on the context it is used. The term "Ecological competence" can be used in a microbial sense, and it can be used in a sociological sense. Microbiology Ecologic ...
) * Ability to live off of a wide range of food types ( generalist) * Association with humans * Prior successful invasions An introduced species might become invasive if it can outcompete native species for resources such as
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...
s, light, physical space, water, or food. If these species evolved under great
competition Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, ind ...
or
predation Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...
, then the new environment may host fewer able competitors, allowing the invader to proliferate quickly.
Ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...
s which are being used to their fullest capacity by native species can be modeled as
zero-sum Zero-sum game is a mathematical representation in game theory and economic theory of a situation which involves two sides, where the result is an advantage for one side and an equivalent loss for the other. In other words, player one's gain is e ...
systems in which any gain for the invader is a loss for the native. However, such
unilateral __NOTOC__ Unilateralism is any doctrine or agenda that supports one-sided action. Such action may be in disregard for other parties, or as an expression of a commitment toward a direction which other parties may find disagreeable. As a word, ''un ...
competitive superiority (and extinction of native species with increased populations of the invader) is not the rule. Invasive species often coexist with native species for an extended time, and gradually, the superior competitive ability of an invasive species becomes apparent as its population grows larger and denser and it adapts to its new location. An invasive species might be able to use resources that were previously unavailable to native species, such as deep water sources accessed by a long
taproot A taproot is a large, central, and dominant root from which other roots sprout laterally. Typically a taproot is somewhat straight and very thick, is tapering in shape, and grows directly downward. In some plants, such as the carrot, the taproo ...
, or an ability to live on previously uninhabited soil types. For example, barbed goatgrass ('' Aegilops triuncialis'') was introduced to
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
on
serpentine soil Serpentine soil is an uncommon soil type produced by weathered ultramafic rock such as peridotite and its metamorphic derivatives such as serpentinite. More precisely, serpentine soil contains minerals of the serpentine subgroup, especially anti ...
s, which have low water-retention, low nutrient levels, a high
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...
/
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
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. Invasive species might alter their environment by releasing chemical compounds, modifying
abiotic In biology and ecology, abiotic components or abiotic factors are non-living chemical and physical parts of the environment that affect living organisms and the functioning of ecosystems. Abiotic factors and the phenomena associated with them under ...
factors, or affecting the behaviour of
herbivore A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthpar ...
s, 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. 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. Other examples are ''
Centaurea solstitialis ''Centaurea solstitialis'', the yellow star-thistle, is a species of thorny plant in the genus '' Centaurea'', which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other ...
'' (yellow starthistle) and ''Centaurea diffusa'' (
diffuse knapweed ''Centaurea diffusa'', also known as diffuse knapweed, white knapweed or tumble knapweed, is a member of the genus ''Centaurea'' in the family Asteraceae. This species is common throughout western North America but is not actually native to the ...
). These
Eastern Europe Eastern Europe is a subregion of the European continent. As a largely ambiguous term, it has a wide range of geopolitical, geographical, ethnic, cultural, and socio-economic connotations. The vast majority of the region is covered by Russia, whi ...
an
noxious weed A noxious weed, harmful weed or injurious weed is a weed that has been designated by an agricultural or other governing authority as a plant that is injurious to agricultural or horticultural crops, natural habitats or ecosystems, or humans or li ...
s have spread through the
western Western may refer to: Places *Western, Nebraska, a village in the US *Western, New York, a town in the US *Western Creek, Tasmania, a locality in Australia *Western Junction, Tasmania, a locality in Australia *Western world, countries that id ...
and West Coast states. Experiments show that 8-hydroxyquinoline, a chemical produced at the root of ''C. diffusa'', has a negative effect only on plants that have not co-evolved with it. Such co-evolved native plants have also evolved defenses. ''C. diffusa'' and ''C. solstitialis'' do not appear in their native habitats to be overwhelmingly successful competitors. Success or lack of success in one habitat does not necessarily imply success in others. Conversely, examining habitats in which a species is less successful can reveal novel weapons to defeat invasiveness. Changes in
fire regime A fire regime is the pattern, frequency, and intensity of the bushfires and wildfires that prevail in an area over long periods of time. It is an integral part of fire ecology, and renewal for certain types of ecosystems. A fire regime describes th ...
ns are another form of facilitation. ''
Bromus tectorum ''Bromus tectorum'', known as downy brome, drooping brome or cheatgrass, is a winter annual grass native to Europe, southwestern Asia, and northern Africa, but has become invasive in many other areas. It now is present in most of Europe, south ...
'', originally from Eurasia, is highly fire-adapted. It not only spreads rapidly after burning but also increases the frequency and intensity (heat) of fires by providing large amounts of dry
detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...
during the fire season in western North America. In areas where it is widespread, it has altered the local fire regimen so much that native plants cannot survive the frequent fires, allowing ''B. tectorum'' to further extend and maintain dominance in its introduced range.
Ecological facilitation Ecological facilitation or probiosis describes species interactions that benefit at least one of the participants and cause harm to neither.Stachowicz, J. J. 2001. Mutualism, facilitation, and the structure of ecological communities. BioScience 51 ...
also occurs where one species physically modifies a habitat in ways that are advantageous to other species. For example,
zebra mussel The zebra mussel (''Dreissena polymorpha'') is a small freshwater mussel. The species originates from the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in ma ...
s increase habitat complexity on lake floors, providing crevices in which
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
s 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 The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning " ...
invertebrate communities. Studies of invasive species have shown that introduced species have great potential for rapid adaptation. This explains how many introduced species are able to establish and become invasive in new environments. In addition, the rate at which an invasive species can spread can be difficult to ascertain by biologists since population growth occurs geometrically, rather than linearly. When bottlenecks and
founder effect In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, us ...
s cause a great decrease in the population size and may constrict
genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, b ...
, the individuals begin to show additive variance as opposed to epistatic variance. This conversion can actually lead to increased variance in the founding populations which then allows for rapid adaptive evolution. Following invasion events, selection may initially act on the capacity to disperse as well as physiological tolerance to the new stressors in the environment. Adaptation then proceeds to respond to the selective pressures of the new environment. These responses would most likely be due to
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
and
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
, or the presence of native species whether it be predator or prey. Adaptations include changes in morphology,
physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemic ...
,
phenology Phenology is the study of periodic events in biological life cycles and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation). Examples include the date of emergence of leav ...
, and plasticity. Rapid adaptive evolution in these species leads to offspring that have higher fitness and are better suited for their environment. Intraspecific phenotypic plasticity, pre-adaptation and post-introduction evolution are all major factors in adaptive evolution. Plasticity in populations allows room for changes to better suit the individual in its environment. This is key in adaptive evolution because the main goal is how to best be suited to the ecosystem to which the species has been introduced. The ability to accomplish this as quickly as possible will lead to a population with a very high fitness. Pre-adaptations and evolution after the initial introduction also play a role in the success of the introduced species. If the species has adapted to a similar ecosystem or contains traits that happen to be well suited to the area where it is introduced, it is more likely to fare better in the new environment. This, in addition to evolution that takes place after introduction, all determine if the species will be able to become established in the new ecosystem and if it will reproduce and thrive. The enemy-release hypothesis states that the process of evolution has led to every ecosystem having an ecological balance. Any one species cannot occupy a majority of the ecosystem due to the presences of competitors, predators, and diseases. Introduced species moved to a novel habitat can become invasive when these controlscompetitors, predators, and diseasesdo not exist in the new ecosystem. The absence of appropriate controls leads to rapid population growth.


Vectors

Non-native species have many
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
s, including biogenic vectors, but most invasions are associated with human activity. Natural range extensions are common in many species, but the rate and magnitude of human-mediated extensions in these species tend to be much larger than natural extensions, and humans typically carry specimens greater distances than natural forces. An early human vector occurred when prehistoric humans introduced the Pacific rat (''Rattus exulans'') to Polynesia. Vectors include plants or seeds imported for
horticulture Horticulture is the branch of agriculture that deals with the art, science, technology, and business of plant cultivation. It includes the cultivation of fruits, vegetables, nuts, seeds, herbs, sprouts, mushrooms, algae, flowers, seaweeds and no ...
. The
pet trade Wildlife trade refers to the of products that are derived from non-domesticated animals or plants usually extracted from their natural environment or raised under controlled conditions. It can involve the trade of living or dead individuals, ti ...
moves animals across borders, where they can escape and become invasive. Organisms stow away on transport vehicles. Among professionals in invasion biology, the overwhelming consensus is that incidental human assisted transfer is the main cause of introductionsother than for
polar regions The polar regions, also called the frigid zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North and South Poles), lying within the polar circles. These high latitudes are dominated by floa ...
. Diseases may also be vectored by invasive insects such as the
Asian citrus psyllid ''Diaphorina citri'', the Asian citrus psyllid, is a sap-sucking, hemipteran bug in the family Psyllidae. It is one of two confirmed vectors of citrus greening disease. It has a wide distribution in southern Asia and has spread to other citrus ...
and the bacterial disease
citrus greening Citrus greening disease (; or HLB) is a disease of citrus caused by a vector-transmitted pathogen. The causative agents are motile bacteria, '' Liberibacter'' spp. The disease is vectored and transmitted by the Asian citrus psyllid, '' Diaphorin ...
. The arrival of invasive propagules to a new site is a function of the site's invasibility. Species have also been introduced intentionally. For example, to feel more "at home,"
American colonists The colonial history of the United States covers the history of European colonization of North America from the early 17th century until the incorporation of the Thirteen Colonies into the United States after the Revolutionary War. In the ...
formed "Acclimation Societies" that repeatedly imported birds that were native to Europe to North America and other distant lands. In 2008, U.S. postal workers in
Pennsylvania Pennsylvania (; ( Pennsylvania Dutch: )), officially the Commonwealth of Pennsylvania, is a state spanning the Mid-Atlantic, Northeastern, Appalachian, and Great Lakes regions of the United States. It borders Delaware to its southeast, ...
noticed noises coming from inside a box from
Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the no ...
; the box contained more than two dozen live beetles.
Agricultural Research Service The Agricultural Research Service (ARS) is the principal in-house research agency of the United States Department of Agriculture (USDA). ARS is one of four agencies in USDA's Research, Education and Economics mission area. ARS is charged with ext ...
entomologists identified them as the
rhinoceros beetle Dynastinae or rhinoceros beetles are a subfamily of the scarab beetle family (Scarabaeidae). Other common names – some for particular groups of rhinoceros beetles – include Hercules beetles, unicorn beetles or horn beetles. Over 1 ...
,
Hercules beetle The Hercules beetle (''Dynastes hercules'') is a species of rhinoceros beetle native to the rainforests of Central America, South America, and the Lesser Antilles. It is the longest extant species of beetle in the world, and is also one of t ...
, and king stag beetle. Because these species were not native to the U.S., they could have threatened native ecosystems. To prevent exotic species from becoming a problem in the U.S., special handling and permits are required when living materials are shipped from foreign countries.
USDA The United States Department of Agriculture (USDA) is the federal executive department responsible for developing and executing federal laws related to farming, forestry, rural economic development, and food. It aims to meet the needs of comme ...
programs such as
Smuggling Interdiction and Trade Compliance The Smuggling Interdiction and Trade Compliance Program is a component of the United States Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS), Plant Protection and Quarantine (PPQ). The mission of the Smuggling Int ...
(SITC) attempt to prevent exotic species outbreaks in America. The intentional spread of domesticated plants to other favourable environments has been described as
biological globalization Biological globalization refers to the phenomenon where domesticated species are brought and cultivated in other favorable environments, facilitated by and for the benefit of humans. It has been defined as "the spread of plants domesticated in one ...
. Many invasive species, once they are dominant in the area, are essential to the ecosystem of that area. If they are removed from the location it could be harmful to that area. Economics plays a major role in exotic species introduction. High demand for the valuable
Chinese mitten crab The Chinese mitten crab ('; ,  "big sluice crab"), also known as the Shanghai hairy crab (, pinyin, p ''Shànghǎi máoxiè''), is a medium-sized burrowing crab that is named for its furry claws, which resemble mittens. It is native t ...
is one explanation for the possible intentional release of the species in foreign waters.


Within the aquatic environment

The development of maritime trade has rapidly affected the way marine organisms are transported within the ocean. Two ways marine organisms are transported to new environments are hull fouling and ballast water transport. In fact, Molnar et al. 2008 documented the pathways of hundreds of marine invasive species and found that shipping was the dominant mechanism for the transfer of invasive species. Many marine organisms have the capacity to attach themselves to vessel hulls. Therefore, these organisms are easily transported from one body of water to another and are a significant risk factor for a biological invasion event. Controlling for vessel hull fouling is voluntary and there are no regulations currently in place to manage hull fouling. However, the governments of
California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
and
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...
have announced more stringent control for vessel hull fouling within their respective jurisdictions. The other main vector for the transport of non-native aquatic species is ballast water.
Ballast water Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to voy ...
taken up at sea and released in port by transoceanic vessels is the largest vector for non-native aquatic species invasions. In fact, it is estimated that 10,000 different species, many of which are non-indigenous, are transported via ballast water each day. Many of these species are considered harmful and can negatively affect their new environment. For example, freshwater
zebra mussel The zebra mussel (''Dreissena polymorpha'') is a small freshwater mussel. The species originates from the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in ma ...
s, native to the
Black Black is a color which results from the absence or complete absorption of visible light. It is an achromatic color, without hue, like white and grey. It is often used symbolically or figuratively to represent darkness. Black and white ha ...
, Caspian and Azov seas, most likely reached the
Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
via ballast water from a transoceanic vessel. Zebra mussels outcompete other native organisms for oxygen and food, such as
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
. 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 A loophole is an ambiguity or inadequacy in a system, such as a law or security, which can be used to circumvent or otherwise avoid the purpose, implied or explicitly stated, of the system. Originally, the word meant an arrowslit, a narrow ver ...
, 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). Current regulations for the
Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
rely on ‘
salinity Salinity () is the saltiness or amount of salt (chemistry), salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensio ...
shock’ to kill
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
organisms left in ballast tanks. 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 Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
, current regulations allow less than 10 cells per milliliter be present in discharge from treatment systems. The discharge gets released when a ship takes on cargo at a port so the discharged water is not necessarily the same as the receiving body of water. Since many species of phytoplankton are less than 10 microns in size and
reproduce asexually Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the ...
, only one cell released into the environment could exponentially grow into many thousands of cells over a short amount of time. This loophole could have detrimental effects to the environment. For example, some species in the genus ''
Pseudo-nitzschia ''Pseudo-nitzschia'' is a marine planktonic diatom genus that accounts for 4.4% of pennate diatoms found worldwide. Some species are capable of producing the neurotoxin domoic acid (DA), which is responsible for the neurological disorder in huma ...
'' are smaller than 10 microns in width and contain
domoic acid Domoic acid (DA) is a kainic acid-type neurotoxin that causes amnesic shellfish poisoning (ASP). It is produced by algae and accumulates in shellfish, sardines, and anchovies. When sea lions, otters, cetaceans, humans, and other predators eat c ...
, a
neurotoxin Neurotoxins are toxins that are destructive to nerve tissue (causing neurotoxicity). Neurotoxins are an extensive class of exogenous chemical neurological insultsSpencer 2000 that can adversely affect function in both developing and mature nerv ...
. If toxic ''Pseudo-nitzschia'' spp. are alive in ballast discharge and get released into their “new environment” they could cause domoic acid poisoning in
shellfish Shellfish is a colloquial and fisheries term for exoskeleton-bearing aquatic invertebrates used as food, including various species of molluscs, crustaceans, and echinoderms. Although most kinds of shellfish are harvested from saltwater environ ...
,
marine mammal Marine mammals are aquatic mammals that rely on the ocean and other marine ecosystems for their existence. They include animals such as seals, whales, manatees, sea otters and polar bears. They are an informal group, unified only by their ...
s and
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...
s. 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 Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...
in 1987. Ballast water regulations need to be more rigorous to prevent future ramifications associated with the potential release of toxic and invasive phytoplankton. Another important factor to consider about marine invasive species is the role of environmental changes associated with
climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...
, such as an increase in
ocean temperature The ocean temperature varies by depth, geographical location and season. Both the temperature and salinity of ocean water differs. Warm surface water is generally saltier than the cooler deep or polar waters; in polar regions, the upper layers of ...
. There have been multiple studies suggesting an increase in ocean temperature will cause range shifts in organisms, which could have detrimental effects on the environment as new species interactions emerge. For example, Hua and Hwang proposed that organisms in a ballast tank of a ship traveling from the temperate zone through tropical waters can experience temperature fluctuations as much as 20 °C. To further examine the effects of temperature on organisms transported on hulls or in ballast water, Lenz et al. (2018) carried out study where they conducted a double heat stress experiment. 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
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
s that will survive a second applied heat stress, such as increased ocean temperature in the founder population. Due to the complexity of climate-change-induced variations, it is difficult to predict the nature of temperature-based success of non-native species ''in-situ''. 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.


Effects of wildfire and firefighting

Invasive species often exploit disturbances to an ecosystem (
wildfire A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
s,
road A road is a linear way for the conveyance of traffic that mostly has an improved surface for use by vehicles (motorized and non-motorized) and pedestrians. Unlike streets, the main function of roads is transportation. There are many types o ...
s,
foot trail A trail, also known as a path or track, is an unpaved lane or small road usually passing through a natural area. In the United Kingdom and the Republic of Ireland, a path or footpath is the preferred term for a pedestrian or hiking trail. Th ...
s) to colonize an area. Large wildfires can sterilize soils, while adding a variety of
nutrient A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excre ...
s. In the resulting free-for-all, formerly entrenched species lose their advantage, leaving more room for invasives. In such circumstances, plants that can regenerate from their roots have an advantage. Non-natives with this ability can benefit from a low intensity fire burn that removes surface
vegetation Vegetation is an assemblage of plant species and the ground cover they provide. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic charac ...
, leaving natives that rely on seeds for propagation to find their niches occupied when their seeds finally sprout. Wildfires often occur in remote areas, needing fire suppression crews to travel through pristine forest to reach the site. The crews can bring invasive seeds with them. If any of these stowaway seeds become established, a thriving colony of invasives can erupt in as few as six weeks, after which controlling the outbreak can need years of continued attention to prevent further spread. Also, disturbing the soil surface, such as cutting firebreaks, destroys native cover, exposes soil, and can accelerate invasions. In suburban and wildland-urban interface areas, the vegetation clearance and brush removal ordinances of municipalities for defensible space can result in excessive removal of native shrubs and
perennials A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also widel ...
that exposes the soil to more light and less competition for invasive plant species. Fire suppression vehicles are often major culprits in such outbreaks, as the vehicles are often driven on back roads overgrown with invasive plant species. The undercarriage of the vehicle becomes a prime vessel of transport. In response, on large fires, washing stations "decontaminate" vehicles before engaging in suppression activities. Large wildfires attract firefighters from remote places, further increasing the potential for seed transport.


Adverse effects

Invasive species can affect the invaded habitats and bioregions adversely, causing ecological, environmental, or economic damage.


Ecological

The European Union defines "Invasive Alien Species" as those that are, firstly, outside their natural distribution area, and secondly, threaten
biological diversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...
. Biotic invasion is considered one of the five top drivers for global
biodiversity loss Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...
and is increasing because of tourism and
globalization Globalization, or globalisation (Commonwealth English; see spelling differences), is the process of interaction and integration among people, companies, and governments worldwide. The term ''globalization'' first appeared in the early 20t ...
. This may be particularly true in inadequately regulated
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does incl ...
systems, though
quarantine A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been ...
s and
ballast water Ballast is used in ships to provide moment to resist the lateral forces on the hull. Insufficiently ballasted boats tend to tip or heel excessively in high winds. Too much heel may result in the vessel capsizing. If a sailing vessel needs to voy ...
rules have improved the situation. Invasive species may drive local native species to extinction via
competitive Competition is a rivalry where two or more parties strive for a common goal which cannot be shared: where one's gain is the other's loss (an example of which is a zero-sum game). Competition can arise between entities such as organisms, indivi ...
exclusion,
niche Niche may refer to: Science *Developmental niche, a concept for understanding the cultural context of child development * Ecological niche, a term describing the relational position of an organism's species *Niche differentiation, in ecology, the ...
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 at sites of introduction, leading ultimately to the homogenisation of the world's fauna and flora and the
loss of biodiversity Biodiversity loss includes the worldwide extinction of different species, as well as the local reduction or loss of species in a certain habitat, resulting in a loss of biological diversity. The latter phenomenon can be temporary or permanent, de ...
. It is difficult to unequivocally attribute extinctions to a species invasion. Although evidence is strong that the recent extinction of about 90 amphibian species can be traced to the chytrid fungus spread by international trade, most scientific research has focused on animal invaders. Concern over the impacts of invasive species on biodiversity typically weighs the actual evidence (either ecological or economic) in relation to the potential risk. 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 aeae in
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
(the indigenous ''
Bacopa monnieri ''Bacopa monnieri'' is a perennial, creeping herb native to the wetlands of southern and Eastern India, Australia, Europe, Africa, Asia, and North and South America. It is known by the common names water hyssop, waterhyssop, brahmi, thyme-leafed ...
'') is regarded as a pest species in artificially manipulated water bird refuges because it quickly covers shallow
mudflat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...
s 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 The amethyst gem clam (''Gemma gemma'') is species of very small saltwater clam, a marine bivalve mollusk in the family Veneridae The Veneridae or venerids, common name: Venus clams, are a very large family of minute to large, saltwater cla ...
(''Gemma gemma'') and the European green crab (''
Carcinus maenas ''Carcinus maenas'' is a common littoral crab. It is known by different names around the world. In the British Isles, it is generally referred to as the shore crab, or green shore crab. In North America and South Africa, it bears the name europ ...
''). The gem clam was introduced into California's
Bodega Harbor Bodega Harbor is a small, shallow, natural harbor on the Pacific coast of northern California in the United States, approximately northwest of San Francisco. The harbor is approximately in area. The harbor is in Sonoma County at , on the easter ...
from the
East Coast of the United States The East Coast of the United States, also known as the Eastern Seaboard, the Atlantic Coast, and the Atlantic Seaboard, is the coastline along which the Eastern United States meets the North Atlantic Ocean. The eastern seaboard contains the coa ...
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. Invasive species can change the functions of ecosystems. For example, invasive plants can alter the
fire regime A fire regime is the pattern, frequency, and intensity of the bushfires and wildfires that prevail in an area over long periods of time. It is an integral part of fire ecology, and renewal for certain types of ecosystems. A fire regime describes th ...
(cheatgrass, ''
Bromus tectorum ''Bromus tectorum'', known as downy brome, drooping brome or cheatgrass, is a winter annual grass native to Europe, southwestern Asia, and northern Africa, but has become invasive in many other areas. It now is present in most of Europe, south ...
''),
nutrient cycling A nutrient cycle (or ecological recycling) is the movement and exchange of inorganic and organic matter back into the production of matter. Energy flow is a unidirectional and noncyclic pathway, whereas the movement of mineral nutrients is cycli ...
(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 Hybridization (or hybridisation) may refer to: *Hybridization (biology), the process of combining different varieties of organisms to create a hybrid *Orbital hybridization, in chemistry, the mixing of atomic orbitals into new hybrid orbitals *Nu ...
with introduced cordgrass, ''Spartina alterniflora'', threatens the existence of California cordgrass (''
Spartina foliosa ''Sporobolus foliosus'' is a species of grass known by the common name California cordgrass. It was reclassified from ''Spartina foliosa'' after a taxonomic revision in 2014. It is native to the salt marshes and mudflats of coastal California an ...
'') in
San Francisco Bay San Francisco Bay is a large tidal estuary in the U.S. state of California, and gives its name to the San Francisco Bay Area. It is dominated by the big cities of San Francisco, San Jose, and Oakland. San Francisco Bay drains water f ...
. Invasive species cause competition for native species and because of this 400 of the 958 endangered species under the
Endangered Species Act The Endangered Species Act of 1973 (ESA or "The Act"; 16 U.S.C. § 1531 et seq.) is the primary law in the United States for protecting imperiled species. Designed to protect critically imperiled species from extinction as a "consequence of ec ...
are at risk. The unintentional introduction of forest pest species and plant pathogens can change
forest ecology Forest ecology is the scientific study of the interrelated patterns, processes, flora, fauna and ecosystems in forests. The management of forests is known as forestry, silviculture, and forest management. A forest ecosystem is a natural wood ...
and damage the
timber industry Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, w ...
. Overall,
forest ecosystem Forest ecology is the scientific study of the interrelated patterns, processes, flora, fauna and ecosystems in forests. The management of forests is known as forestry, silviculture, and forest management. A forest ecosystem is a natural woodland u ...
s in the U.S. are widely invaded by exotic pests, plants, and pathogens. The Asian long-horned beetle (''
Anoplophora glabripennis The Asian long-horned beetle (''Anoplophora glabripennis''), also known as the starry sky, sky beetle, or ALB, is native to eastern China, and Korea. This species has now been accidentally introduced into the United States, where it was first dis ...
'') 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 The hemlock woolly adelgid (; ''Adelges tsugae''), or HWA, is an insect of the order Hemiptera (true bugs) native to East Asia. It feeds by sucking sap from hemlock and spruce trees (''Tsuga'' spp.; ''Picea'' spp.). In its native range, HWA ...
has inflicted damage on old-growth spruce, fir and hemlock forests and damages the
Christmas tree A Christmas tree is a decorated tree, usually an evergreen conifer, such as a spruce, pine or fir, or an artificial tree of similar appearance, associated with the celebration of Christmas. The custom was further developed in early modern ...
industry. And the chestnut blight fungus (''
Cryphonectria parasitica The pathogenic fungus ''Cryphonectria parasitica'' (formerly ''Endothia parasitica'') is a member of the Ascomycota (sac fungi). This necrotrophic fungus is native to East Asia and South East Asia and was introduced into Europe and North America ...
'') and Dutch elm disease (''
Ophiostoma novo-ulmi ''Ophiostoma'' is a genus of fungi within the family Ophiostomataceae. It was circumscribed in 1919 by mycologists Hans Sydow and Paul Sydow. Species *'' Ophiostoma adjuncti'' *'' Ophiostoma ainoae'' *'' Ophiostoma allantosporum'' *'' Ophiost ...
'') are two plant pathogens with serious impacts on these two species and on forest health.
Alternative link and additional publication citation information: Tree Search, US Forest Service, USDA
http://www.treesearch.fs.fed.us/pubs/745
Garlic mustard, ''
Alliaria petiolata ''Alliaria petiolata'', or garlic mustard, is a biennial flowering plant in the mustard family (Brassicaceae). It is native to Europe, western and central Asia, north-western Africa, Morocco, Iberia and the British Isles, north to northern S ...
'', 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 In forestry and ecology, understory (American English), or understorey (Commonwealth English), also known as underbrush or undergrowth, includes plant life growing beneath the forest canopy without penetrating it to any great extent, but abo ...
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. Native
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
can be threatened with
extinction Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...
through the process of
genetic pollution Genetic pollution is a controversial term for uncontrolled gene flow into wild populations. It is defined as "the dispersal of contaminated altered genes from genetically engineered organisms to natural organisms, esp. by cross-pollination", but ...
. Genetic pollution is unintentional
hybridization Hybridization (or hybridisation) may refer to: *Hybridization (biology), the process of combining different varieties of organisms to create a hybrid *Orbital hybridization, in chemistry, the mixing of atomic orbitals into new hybrid orbitals *Nu ...
and
introgression Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Intr ...
, which leads to homogenization or replacement of local
genotypes The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
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 Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Intr ...
. 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 ''Sporobolus foliosus'' is a species of grass known by the common name California cordgrass. It was reclassified from ''Spartina foliosa'' after a taxonomic revision in 2014. It is native to the salt marshes and mudflats of coastal California an ...
.'' The higher pollen count and male fitness of the invading species resulted in
introgression Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Intr ...
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 In population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent a ...
is normal, and preserves constellations of
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
s and genotypes. An example of this is the interbreeding of migrating
coyote The coyote (''Canis latrans'') is a species of canine native to North America. It is smaller than its close relative, the wolf, and slightly smaller than the closely related eastern wolf and red wolf. It fills much of the same ecological nich ...
s with the
red wolf The red wolf (''Canis rufus'') is a canine native to the southeastern United States. Its size is intermediate between the coyote (''Canis latrans'') and gray wolf (''Canis lupus''). The red wolf's taxonomic classification as being a separate s ...
, in areas of eastern
North Carolina North Carolina () is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. The state is the List of U.S. states and territories by area, 28th largest and List of states and territories of the United ...
where the
red wolf The red wolf (''Canis rufus'') is a canine native to the southeastern United States. Its size is intermediate between the coyote (''Canis latrans'') and gray wolf (''Canis lupus''). The red wolf's taxonomic classification as being a separate s ...
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.


Environmental

Invasive species and accompanying control efforts can have long term
public health Public health is "the science and art of preventing disease, prolonging life and promoting health through the organized efforts and informed choices of society, organizations, public and private, communities and individuals". Analyzing the det ...
implications. For instance,
pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and ...
s applied to treat a particular pest species could pollute soil and surface water. Encroachment of humans into previously remote ecosystems has exposed exotic diseases such as HIV to the wider population. Introduced birds (e.g.
pigeons Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...
), rodents and insects (e.g.
mosquito Mosquitoes (or mosquitos) are members of a group of almost 3,600 species of small flies within the family Culicidae (from the Latin ''culex'' meaning " gnat"). The word "mosquito" (formed by ''mosca'' and diminutive ''-ito'') is Spanish for "li ...
,
flea Flea, the common name for the order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds. Fleas live by ingesting the blood of their hosts. Adult fleas grow to about long, ...
,
louse Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been recognized as an order, infraorder, or a parvorder, as a resul ...
and
tsetse fly Tsetse ( , or ) (sometimes spelled tzetze; also known as tik-tik flies), are large, biting flies that inhabit much of tropical Africa. Tsetse flies include all the species in the genus ''Glossina'', which are placed in their own family, Glos ...
pests) can serve as vectors and reservoirs of human afflictions. Throughout recorded history, epidemics of human diseases, such as
malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. ...
,
yellow fever Yellow fever is a viral disease of typically short duration. In most cases, symptoms include fever, chills, loss of appetite, nausea, muscle pains – particularly in the back – and headaches. Symptoms typically improve within five days. ...
,
typhus Typhus, also known as typhus fever, is a group of infectious diseases that include epidemic typhus, scrub typhus, and murine typhus. Common symptoms include fever, headache, and a rash. Typically these begin one to two weeks after exposure. ...
, and
bubonic plague Bubonic plague is one of three types of plague caused by the plague bacterium ('' Yersinia pestis''). One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting, as wel ...
, spread via these vectors. A recent example of an introduced disease is the spread of the
West Nile virus West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family '' Flaviviridae'', from the genus '' Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The v ...
, which killed humans, birds, mammals, and reptiles. The introduced
Chinese mitten crab The Chinese mitten crab ('; ,  "big sluice crab"), also known as the Shanghai hairy crab (, pinyin, p ''Shànghǎi máoxiè''), is a medium-sized burrowing crab that is named for its furry claws, which resemble mittens. It is native t ...
s are carriers of Asian lung fluke. Waterborne disease agents, such as
cholera Cholera is an infection of the small intestine by some strains of the bacterium '' Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting an ...
bacteria (''
Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and oth ...
''), and causative agents of
harmful algal bloom A harmful algal bloom (HAB) (or excessive algae growth) is an algal bloom that causes negative impacts to other organisms by production of natural phycotoxin, algae-produced toxins, mechanical damage to other organisms, or by other means. HABs are ...
s are often transported via ballast water. In South Africa's Cape Town region, analysis demonstrated that the restoration of priority source water sub-catchments through the removal of thirsty alien plant invasions (i.e. Australian acacias, pines and eucalyptus, Australian black wattle, ...) would generate expected annual water gains of 50 billion liters within 5 years compared to the business-as-usual scenario (which is important as Cape Town experiences significant
water scarcity Water scarcity (closely related to water stress or water crisis) is the lack of fresh water resources to meet the standard water demand. There are two types of water scarcity: physical or economic water scarcity. Physical water scarcity is whe ...
). This is the equivalent to 1/6th of the city's current supply needs. These annual gains will double within 30 years. The catchment restoration is significantly more cost-effective then other water augmentation solutions (1/10 the unit cost of alternative options). A water fund has been established, and these exotic species are being eradicated.


Economic

Globally, 1.4 trillion dollars are spent every year in managing and controlling invasive species. Some invaders can negatively affect the economy of the local area.


United States

For example, in the
Great Lakes Region The Great Lakes region of North America is a binational Canadian–American region that includes portions of the eight U.S. states of Illinois, Indiana, Michigan, Minnesota, New York, Ohio, Pennsylvania and Wisconsin along with the Canadian p ...
the
sea lamprey The sea lamprey (''Petromyzon marinus'') is a parasitic lamprey native to the Northern Hemisphere. It is sometimes referred to as the "vampire fish". Description The sea lamprey has an eel-like body without paired fins. Its mouth is jawless, ...
is an invasive species that acts as a predator. In its original habitat, the sea lamprey used co-evolution to act as a
parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...
without killing the
host organism In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist ''guest'' (symbiont). The guest is typically provided with nourishment and shelter. Examples include a ...
. However, in the Great Lakes Region, this co-evolutionary link is absent, 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 The lake trout (''Salvelinus namaycush'') is a freshwater char living mainly in lakes in northern North America. Other names for it include mackinaw, namaycush, lake char (or charr), touladi, togue, and grey trout. In Lake Superior, it can also ...
and
salmon Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus '' Salmo'') and North Pacific (genus '' Onco ...
. The sea lampreys' destructive effects on large fish negatively affect the fishing industry and have helped cause the collapse of the population of some species. 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
recreation Recreation is an activity of leisure, leisure being discretionary time. The "need to do something for recreation" is an essential element of human biology and psychology. Recreational activities are often done for enjoyment, amusement, or plea ...
al and
tourism Tourism is travel for pleasure or business; also the theory and practice of touring (disambiguation), touring, the business of attracting, accommodating, and entertaining tourists, and the business of operating tour (disambiguation), tours. Th ...
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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the Endling, last individual of the species, although the Functional ext ...
of species, loss in biodiversity, and loss of
ecosystem services Ecosystem services are the many and varied benefits to humans provided by the natural environment and healthy ecosystems. Such ecosystems include, for example, agroecosystems, forest ecosystem, grassland ecosystems, and aquatic ecosystems. ...
, 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 than 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.
Weed A weed is a plant considered undesirable in a particular situation, "a plant in the wrong place", or a plant growing where it is not wanted.Harlan, J. R., & deWet, J. M. (1965). Some thoughts about weeds. ''Economic botany'', ''19''(1), 16-24. ...
s reduce yield in agriculture, though they may provide essential nutrients. Some deep-rooted weeds can "mine" nutrients (see dynamic accumulator) from the
subsoil Subsoil is the layer of soil under the topsoil on the surface of the ground. Like topsoil, it is composed of a variable mixture of small particles such as sand, silt and clay, but with a much lower percentage of organic matter and humus, and ...
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 Cattle (''Bos taurus'') are large, domesticated, cloven-hooved, herbivores. They are a prominent modern member of the subfamily Bovinae and the most widespread species of the genus '' Bos''. Adult females are referred to as cows and adult ...
(e.g., leafy spurge, '' Euphorbia virgata'') or are unpalatable because of thorns and spines (e.g.,
yellow starthistle ''Centaurea solstitialis'', the yellow star-thistle, is a species of thorny plant in the genus ''Centaurea'', which is part of the family Asteraceae. A winter annual, it is native to the Mediterranean Basin region and invasive in many other p ...
). 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 A honey bee (also spelled honeybee) is a eusocial flying insect within the genus ''Apis'' of the bee clade, all native to Afro-Eurasia. After bees spread naturally throughout Africa and Eurasia, humans became responsible for the current cosm ...
infected by the invasive varroa mite. Introduced rats (''
Rattus rattus The black rat (''Rattus rattus''), also known as the roof rat, ship rat, or house rat, is a common long-tailed rodent of the stereotypical rat genus ''Rattus'', in the subfamily Murinae. It likely originated in the Indian subcontinent, but is ...
'' and '' R. norvegicus'') have become serious pests on farms, destroying stored grains. The introduction of leaf miner flies (
Agromyzidae The Agromyzidae are a family commonly referred to as the leaf-miner flies, for the feeding habits of their larvae, most of which are leaf miners on various plants. A worldwide family of roughly 2,500 species, they are small, some with wing l ...
), including the American serpentine leaf miner (''
Liriomyza trifolii ''Liriomyza trifolii'', known generally as the American serpentine leafminer or celery leafminer, is a species of leaf miner fly in the family Agromyzidae. ''L. trifolii'' is a damaging pest, as it consumes and destroys produce and other plant p ...
''), to California has also caused losses in California's
floriculture Floriculture, or flower farming, is a branch of horticulture concerned with the cultivation of flowering and ornamental plants for gardens and for floristry, comprising the floral industry. The development of new varieties by plant breeding is ...
industry, as the larvae of these invasive species feed on ornamental plants. Invasive plant pathogens and insect vectors for plant diseases can also suppress agricultural yields and nursery stock. Citrus greening is a
bacterial disease Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and are often beneficial but others can cause infectious diseases. The number ...
vectored by the invasive
Asian citrus psyllid ''Diaphorina citri'', the Asian citrus psyllid, is a sap-sucking, hemipteran bug in the family Psyllidae. It is one of two confirmed vectors of citrus greening disease. It has a wide distribution in southern Asia and has spread to other citrus ...
(ACP). Because of the impacts of this disease on
citrus ''Citrus'' is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, lemons, grapefruits, pomelos, and limes. The genus ''Citrus'' is native to ...
crops, citrus is under quarantine and highly regulated in areas where ACP has been found. Invasive species can impact outdoor recreation, such as fishing,
hunting Hunting is the human activity, human practice of seeking, pursuing, capturing, or killing wildlife or feral animals. The most common reasons for humans to hunt are to harvest food (i.e. meat) and useful animal products (fur/hide (skin), hide, ...
,
hiking Hiking is a long, vigorous walk, usually on trails or footpaths in the countryside. Walking for pleasure developed in Europe during the eighteenth century.AMATO, JOSEPH A. "Mind over Foot: Romantic Walking and Rambling." In ''On Foot: A Histor ...
,
wildlife viewing Wildlife observation is the practice of noting the occurrence or abundance of animal species at a specific location and time, either for research purposes or recreation. Common examples of this type of activity are bird watching and whale watchin ...
, 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 Water quality refers to the chemical, physical, and biological characteristics of water based on the standards of its usage. It is most frequently used by reference to a set of standards against which compliance, generally achieved through tr ...
and
quantity Quantity or amount is a property that can exist as a multitude or magnitude, which illustrate discontinuity and continuity. Quantities can be compared in terms of "more", "less", or "equal", or by assigning a numerical value multiple of a u ...
, 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 Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
. The orb-weaving spider ''
Zygiella x-notata ''Zygiella x-notata'', sometimes known as the missing sector orb weaver or the silver-sided sector spider,Factsheet 6: Missing-sector Orbweaver (Zygiella x-notata)', British Arachnological Society, 2016. Retrieved 27 November 2016 is a spider sp ...
'', which is invasive to California, disrupts garden work with their large webs.


Europe

The overall economic cost of invasive alien species in Europe between 1960 and 2020 has been estimated at around US$140 billion (including potential costs that may or may not have actually materialised) or US$78 billion (only including observed costs known to have materialised). These estimates are very conservative. Models based on these data suggest a true ''annual'' cost of around US$140 billion in 2020. is one of the most invaded countries in
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, with an estimate of more than 3,000 alien species. The impacts of invasive alien species on the economy has been wide-ranging, from management costs, to loss of crops, to infrastructure damage. The overall economic cost of invasions to Italy between 1990 and 2020 was estimated at US$819.76 million (EUR€704.78 million). However, only 15 recorded species have more reliably estimated costs, hence the actual cost may be much larger than the aforementioned sum. has an estimated minimum of 2,750 introduced and invasive alien species. Renault et al. (2021) obtained 1,583 cost records for 98 invasive alien species and found that they caused a conservative total cost between US$1.2 billion and 11.5 billion over the period 1993–2018. This study also extrapolated costs for species invading France, but for which costs were reported only in other countries but not in France, which yielded an additional cost ranging from US$151 million to $3.03 billion. Damage costs were nearly eight times higher than management expenditure. Insects, and in particular the Asian tiger mosquito ''
Aedes albopictus ''Aedes albopictus'' (''Stegomyia albopicta''), from the mosquito (Culicidae) family, also known as the (Asian) tiger mosquito or forest mosquito, is a mosquito native to the tropical and subtropical areas of Southeast Asia. In the past few cen ...
'' and the yellow fever mosquito '' Ae. aegypti'', totalled very high economic costs, followed by non-graminoid terrestrial flowering and aquatic plants (''
Ambrosia artemisiifolia ''Ambrosia artemisiifolia'', with the common names common ragweed, annual ragweed, and low ragweed, is a species of the genus ''Ambrosia'' native to regions of the Americas. Taxonomy The species name, ''artemisiifolia'', is given because the lea ...
'', '' Ludwigia'' sp. and ''
Lagarosiphon major ''Lagarosiphon major'' is a monocotic aquatic plant native to Southern Africa. Common names include African elodea, curly waterweed, oxygen weed and South African oxygen weed. It is used as freshwater aquarium plant. It is an invasive plant in ...
''). Over 90% of alien species currently recorded in France had no costs reported in the literature, resulting in high biases in taxonomic, regional and activity sector coverages. However, no reports does not mean that there are no negative consequences and thus no costs.


Favorable effects

Invasive species have the potential to 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. Examples of this are: * The
Tamarisk The genus ''Tamarix'' (tamarisk, salt cedar, taray) is composed of about 50–60 species of flowering plants in the family Tamaricaceae, native to drier areas of Eurasia and Africa. The generic name originated in Latin and may refer to the Ta ...
, a non-native woody plant, and the Southwestern Willow Flycatcher, an endangered bird. 75% of Southwestern Willow Flycatchers were found to nest in these plants and their success was the same as the flycatchers that had
nest A nest is a structure built for certain animals to hold eggs or young. Although nests are most closely associated with birds, members of all classes of vertebrates and some invertebrates construct nests. They may be composed of organic materi ...
ed 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 California clapper rail ( ''Rallus longirostris obsoletus''), had grown partial to the new hybrid grass of '' Spartina alterniflora'' and ''
Spartina foliosa ''Sporobolus foliosus'' is a species of grass known by the common name California cordgrass. It was reclassified from ''Spartina foliosa'' after a taxonomic revision in 2014. It is native to the salt marshes and mudflats of coastal California an ...
'' (invasive). The new grass grew more densely than the local version and did not die back during the winter, providing better cover and nesting habitat for the secretive bird. During the 1990s, as the hybrid spread, the rail population had soared. *Since
zebra mussels The zebra mussel (''Dreissena polymorpha'') is a small freshwater mussel. The species originates from the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in ma ...
became established, the clarity of the once-murky water in
Lake Erie Lake Erie ( "eerie") is the fourth largest lake by surface area of the five Great Lakes in North America and the eleventh-largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and therefore also ha ...
has increased substantially, increasing visibility to in some areas, compared to less than at the middle of the 20th century. This has encouraged growth of some aquatic plants, which in turn have become nurseries for fish such as the
yellow perch The yellow perch (''Perca flavescens''), commonly referred to as perch, striped perch, American perch, American river perch or preacher is a freshwater perciform fish native to much of North America. The yellow perch was described in 1814 by Sam ...
. The zebra mussel also constitutes a food source for fish species such as the smallmouth bass and the previously endangered
lake sturgeon The lake sturgeon (''Acipenser fulvescens''), also known as the rock sturgeon, is a North American temperate freshwater fish, one of about 25 species of sturgeon. Like other sturgeons, this species is a bottom feeder with evolutionarily basal tr ...
, with demonstrable effects on population sizes. Lake Erie is now reportedly the world's premier smallmouth bass fishery. Migrating ducks have also started to make use of the mussels as a food source. *Saltwater crocodiles have lived in
Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...
for millions of years, but by the 1960's and 1970's were in danger of extinction. Wild pigs, introduced by European settlers, became a staple in their diet. The native estuarine crocodile population has since recovered in large part thanks to these invasive feral swine. 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 Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
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. In Kenya,
guava Guava () is a common tropical fruit cultivated in many tropical and subtropical regions. The common guava ''Psidium guajava'' (lemon guava, apple guava) is a small tree in the myrtle family ( Myrtaceae), native to Mexico, Central America, t ...
has real potential as a tool in the restoration of tropical forest. Studies of isolated guava trees in farmland showed that they were extremely attractive to a wide range of fruit-eating birds. In the course of visiting them, birds dropped seeds beneath the guavas, many of them from trees in nearby fragments of rainforest, and many of these seeds germinated and grew into young trees. Surprisingly, distance to the nearest forest did not seem to matter at alltrees up to away (the longest distance studied) were just as good as trees much nearer to forest fragments. Guavas establish easily on degraded land, and each tree is potentially the nucleus of a patch of regenerating rainforest. Of course, most seedlings that grow beneath guavas are just more guavas, but guava is an early-successional tree that soon dies out when overtopped by bigger trees, nor does it actively invade primary forest. Invasive alien trees can also be useful for restoring native forest. In Puerto Rico, native pioneer trees could cope with natural disturbances such as drought, hurricanes, floods and landslides, but are mostly unable to colonise land that has undergone deforestation, extended agricultural use and eventual abandonment. In these sites, low-diversity pioneer communities of invasive trees develop, but over time native trees invade. Alien pioneers may dominate for 30 to 40 years but the eventual outcome, after 60 to 80 years, is a diverse mixture of native and alien species, but with a majority of native species. In the absence of the initial alien colonists, abandoned agricultural land tends to become pasture and remain that way almost indefinitely. The last benefit of non-native species is that they provided ecosystem services. Furthermore, non-native species can function as
biocontrol Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also in ...
agents to limit the effects of invasive species, such as the use of non-native species to control agricultural pests. Asian oysters, for example, filter water pollutants better than native oysters to
Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the Eastern Shore of Maryland / ...
. A 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 Naturalization (or naturalisation) is the legal act or process by which a non-citizen of a country may acquire citizenship or nationality of that country. It may be done automatically by a statute, i.e., without any effort on the part of the in ...
. For example, the bee ''
Lasioglossum leucozonium ''Lasioglossum leucozonium'' (Schrank, 1781), also known as ''Lasioglossum similis'', is a widespread solitary sweat bee found in North America, Europe, Asia, and parts of northern Africa. While now a common bee in North America, population gen ...
'', shown by population genetic analysis to be an invasive species in North America, has become an important pollinator of caneberry (''
Rubus ''Rubus'' is a large and diverse genus of flowering plants in the rose family, Rosaceae, subfamily Rosoideae, with over 1,350 species. Raspberries, blackberries, and dewberries are common, widely distributed members of the genus. Most of the ...
'' spp.) as well as cucurbit,
apple trees An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, where its wild ancestor, ' ...
, and
blueberry Blueberries are a widely distributed and widespread group of perennial flowering plants with blue or purple berries. They are classified in the section ''Cyanococcus'' within the genus '' Vaccinium''. ''Vaccinium'' also includes cranberries ...
bushes. The checkerspot butterfly had an advantage to any female that laid her eggs on
ribwort plantain ''Plantago lanceolata'' is a species of flowering plant in the plantain family Plantaginaceae. It is known by the common names ribwort plantain, narrowleaf plantain, English plantain, ribleaf, lamb's tongue, and buckhorn. It is a common weed on c ...
an invasive plant. The plantain leaves remained green long enough for the caterpillars to survive during dry summers, which seemed to be getting a little drier with the first signs of climate change. In contrast, the native plants they used to eat shriveled up and most of the caterpillars starved or desiccated. With this difference in survival, the butterflies started to evolve a liking for laying their eggs on plantains: the proportion of female butterflies content to lay their eggs on this plant rose from under a third in 1984 to three-quarters in 1987. A few years later, the switch was complete. The federally endangered Taylor's checkerspot ('' Euphydryas editha taylori'', a subspecies of Edith's checkerspot, whose historical habitats have been lost) is so reliant on it that conservationists are actively planting plantains out into the wild. To provide a supply of butterflies, prisoners at the Mission Creek Corrections Center for Women in Washington state breed checkerspots in a greenhouse so that they can be released into these new habitats. Odd as it might seem, actively encouraging an alien plant (increasing gains) is helping to conserve a much-loved native insect (reducing losses). Some invasions offer potential commercial benefits. For instance,
silver carp The silver carp (''Hypophthalmichthys molitrix'') is a species of freshwater cyprinid fish, a variety of Asian carp native to China and eastern Siberia, from the Amur River drainage in the north to the Xi Jiang River drainage in the south. Alt ...
and
common carp The Eurasian carp or European carp (''Cyprinus carpio''), widely known as the common carp, is a widespread freshwater fish of eutrophic waters in lakes and large rivers in Europe and Asia.Fishbase''Cyprinus carpio'' Linnaeus, 1758/ref>Arkive The ...
can be harvested for human food and exported to markets already familiar with the product, or processed into
pet food Pet food is animal feed intended for consumption by pets. Typically sold in pet stores and supermarkets, it is usually specific to the type of animal, such as dog food or cat food. Most meat used for animals is a byproduct of the human food ind ...
s, or
mink Mink are dark-colored, semiaquatic, carnivorous mammals of the genera '' Neogale'' and '' Mustela'' and part of the family Mustelidae, which also includes weasels, otters, and ferrets. There are two extant species referred to as "mink": ...
feed.
Water hyacinth ''Pontederia crassipes'' (formerly ''Eichhornia crassipes''), commonly known as common water hyacinth is an aquatic plant native to South America, naturalized throughout the world, and often invasive outside its native range.methane digesters,. Cited in Duke, J. (1983
''Handbook of Energy Crops''
. Purdue University, Center for New Crops & Plants Products
and other invasive plants can also be harvested and utilized as a source of
bioenergy Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms biom ...
. But elsewhere, most of the time, the tens of thousands of introduced species usually either swiftly die out or settle down and become model eco-citizens, pollinating crops, spreading seeds, controlling predators, and providing food and habitat for native species. They rarely eliminate natives. Rather than reducing biodiversity, the novel new worlds that result are usually richer in species than what went before.


Control, eradication, and study

Human behavioural potential and plasticity in species-environment interactions create possibilities for remediating adverse effects of species invasions. The public is interested in learning more about invasive species, and is most motivated by invasive species that are impacting their local area/community.


Cargo inspection and quarantine

The original motivation was to protect against
agricultural pest A pest is any animal or plant harmful to humans or human concerns. The term is particularly used for creatures that damage crops, livestock, and forestry or cause a nuisance to people, especially in their homes. Humans have modified the environ ...
s while still allowing the export of agricultural products. In 1994 the first set of global standards were agreed to, including the
Agreement on the Application of Sanitary and Phytosanitary Measures The Agreement on the Application of Sanitary and Phytosanitary Measures, also known as the SPS Agreement or just SPS, is an international treaty of the World Trade Organization (WTO). It was negotiated during the Uruguay Round of the General Agree ...
(SPS Agreement). These are overseen by the
World Trade Organization The World Trade Organization (WTO) is an intergovernmental organization that regulates and facilitates international trade. With effective cooperation in the United Nations System, governments use the organization to establish, revise, and ...
. The
International Maritime Organization The International Maritime Organization (IMO, French: ''Organisation maritime internationale'') is a specialised agency of the United Nations responsible for regulating shipping. The IMO was established following agreement at a UN conference ...
oversees the International Convention for the Control and Management of Ships' Ballast Water and Sediments (the Ballast Water Management Convention). Although primarily targeted at other, more general environmental concerns, the
Convention on Biological Diversity The Convention on Biological Diversity (CBD), known informally as the Biodiversity Convention, is a multilateral treaty. The Convention has three main goals: the conservation of biological diversity (or biodiversity); the sustainable use of its ...
does specify some steps that its members should take to control invasive species. The CBD is the most significant international agreement on the environmental consequences of invasive species because most such measures are voluntary and unspecific.


Slowing spread

Firefighter A firefighter is a first responder and rescuer extensively trained in firefighting, primarily to extinguish hazardous fires that threaten life, property, and the environment as well as to rescue people and in some cases or jurisdictions als ...
s are increasingly becoming responsible for decontamination of their own equipment, public water equipment, and private water equipment, due to the risk of aquatic invasive species transfer. In the United States this is especially a concern for wildland firefighters because quagga and
zebra Zebras (, ) (subgenus ''Hippotigris'') are African equines with distinctive black-and-white striped coats. There are three living species: the Grévy's zebra (''Equus grevyi''), plains zebra (''E. quagga''), and the mountain zebra (''E. zebr ...
mussel invasion and wildfires happen to be co-occurring in the American West.


Reestablishing species

The field of
island restoration The ecological restoration of islands, or island restoration, is the application of the principles of ecological restoration to islands and island groups. Islands, due to their isolation, are home to many of the world's endemic species, as well as ...
has developed as a field of
conservation biology Conservation biology is the study of the conservation of nature and of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. It is an in ...
and
ecological restoration Restoration ecology is the scientific study supporting the practice of ecological restoration, which is the practice of renewing and restoring degraded, damaged, or destroyed ecosystems and habitats in the environment by active human interrupt ...
, a large part of which deals with the eradication of invasive species. A 2019 study suggests that if eradications of invasive animals were conducted on just 169 islands the survival prospects of 9.4% of the Earth's most highly threatened terrestrial insular vertebrates would be improved. Invasive vertebrate eradication on islands was found to align with the majority of United Nations
Sustainable Development Goals The Sustainable Development Goals (SDGs) or Global Goals are a collection of 17 interlinked objectives designed to serve as a "shared blueprint for peace and prosperity for people and the planet, now and into the future".United Nations (2017) R ...
(specifically Goal 15) and numerous associated targets such as marine and terrestrial biodiversity conservation, promotion of local and global partnerships, economic development, climate change mitigation, human health and sanitation and sustainable production and consumption. Rodents were carried to
South Georgia South Georgia ( es, Isla San Pedro) is an island in the South Atlantic Ocean that is part of the British Overseas Territory of South Georgia and the South Sandwich Islands. It lies around east of the Falkland Islands. Stretching in the east� ...
, an island in the southern Atlantic Ocean with no permanent inhabitants, in the 18th century by sealing and whaling ships. They soon wrought havoc on the island's bird population, eating eggs and attacking chicks. In 2018, the South Georgia Island was declared free of invasive
rodent Rodents (from Latin , 'to gnaw') are mammals of the order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are rodents. They are n ...
s after a multi-year
extermination Extermination or exterminate may refer to: * Pest control, elimination of insects or vermin * Genocide, extermination—in whole or in part—of an ethnic, racial, religious, or national group * Homicide or murder in general * "Exterminate!", the ...
effort. Post-extermination, bird populations have rebounded, including populations of the
South Georgia pipit The South Georgia pipit (''Anthus antarcticus'') is a sparrow-sized bird only found on the South Georgia archipelago off the Antarctic Peninsula. It is the only songbird in Antarctica, South Georgia's only passerine, and one of the few non-seabir ...
and
South Georgia pintail The South Georgia pintail (''Anas georgica georgica''), also misleadingly known as the South Georgian teal, is the nominate subspecies of the yellow-billed pintail (''Anas georgica''), a duck in the dabbling duck subfamily Anatinae. It is endemic ...
, two species found only on the island. Problematic exotic disease introductions in the past century or so include the
chestnut blight The pathogenic fungus ''Cryphonectria parasitica'' (formerly ''Endothia parasitica'') is a member of the Ascomycota (sac fungi). This necrotrophic fungus is native to East Asia and South East Asia and was introduced into Europe and North America ...
which has almost eliminated the
American chestnut The American chestnut (''Castanea dentata'') is a large, fast-growing deciduous tree of the beech family native to eastern North America. As is true of all species in genus Castanea, the American chestnut produces burred fruit with edible nuts. ...
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 negative and the positive aspects of introduced species. Problems can also arise like in the case of the tangled ecology of San Francisco Bay who also tripped as ecological restorers. In the mid-twentieth century, engineers drained many of the bay's marshes and mud banks for building projects. But attitudes changed. Conservationists became concerned about the loss of natural habitat, and from the 1970s, engineers spent more millions of dollars on plugging up their drains to restore lost mudflats, salt marshes, and other wetlands. As part of this program, the Army Corps of Engineers began planting rewetted marshes with a cordgrass native to the eastern United States Spartina alterniflora. This new grass began to interbreed with its close relative, the local
California cordgrass ''Sporobolus foliosus'' is a species of grass known by the common name California cordgrass. It was reclassified from ''Spartina foliosa'' after a taxonomic revision in 2014. It is native to the salt marshes and mudflats of coastal California an ...
(Spartina foliosa). The result was a new hybrid grass that colonized much more aggressively than either of its forebears. It spread to areas no one had intended, blanketing previously open mudflats, clogging channels, getting in the way of oyster farmers, and—worst of all, for many—spoiling million-dollar views and damaging the value of upscale waterfront properties. So a decade ago, authorities launched a multimillion-dollar project to rid the bay of both the alien from the east and the hybrid. But that went wrong too. It turned out that one of the bay's most totemic and endangered birds, the chicken-sized and largely flightless California clapper rail (''
Rallus longirostris The mangrove rail (''Rallus longirostris'') is a species of bird in subfamily Rallinae of family Rallidae, the rails, gallinules, and coots. It is found in Central and South America.HBW and BirdLife International (2021) Handbook of the Birds o ...
obsoletus''), had grown partial to the new hybrid grass. The grass grew more densely than the local version and did not die back during the winter, providing better cover and nesting habitat for the secretive bird. During the 1990s, as the hybrid spread, the rail population had soared. But after 2004, as the eradication got underway, the bird's numbers crashed. There was no mistaking the cause. In time and space, the bird population declined following the eradication of the alien grass.


Taxon substitution

Non-native species can be introduced to fill an ecological engineering role that previously was performed by a native species now extinct. The procedure is known as taxon substitution. On many islands, tortoise extinction has resulted in dysfunctional ecosystems with respect to seed dispersal and herbivory. On the offshore islets of
Mauritius Mauritius ( ; french: Maurice, link=no ; mfe, label= Mauritian Creole, Moris ), officially the Republic of Mauritius, is an island nation in the Indian Ocean about off the southeast coast of the African continent, east of Madagascar. It ...
, tortoises now extinct had served as the keystone herbivores. Introduction of the non-indigenous
Aldabra giant tortoise The Aldabra giant tortoise (''Aldabrachelys gigantea'') is a species of tortoise in the family Testudinidae. The species is endemic to the islands of the Aldabra Atoll in the Seychelles. It is one of the largest tortoises in the world.Pritch ...
s on two islets in 2000 and 2007 has begun to restore ecological equilibrium. The introduced tortoises are dispersing seeds of several native plants and are selectively grazing invasive plant species. Grazing and browsing are expected to replace ongoing intensive manual weeding, and the introduced tortoises are already breeding.


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. In 2005 Chef
Bun Lai Bun Lai (born 1973) is a Hong Kong-born American chef. He is a leader in the sustainable food movement. His family restaurant, Miya's in New Haven, Connecticut, is the first sustainable sushi restaurant in the world. His mother, who received a ...
of
Miya's Miya's was a restaurant in New Haven, Connecticut, United States, credited as the first sustainable sushi restaurant in the world. The restaurant was founded by Yoshiko Lai, a Japanese nutritionist. As of 2021, they are permanently closed. Cu ...
Sushi in
New Haven, Connecticut New Haven is a city in the U.S. state of Connecticut. It is located on New Haven Harbor on the northern shore of Long Island Sound in New Haven County, Connecticut and is part of the New York City metropolitan area. With a population of 134 ...
created the first menu dedicated to the idea of using invasive species, during which time half the menus invasive species offerings were conceptual because invasive species were not yet commercially available. Today, Miya's offers a plethora of invasive species such as Chesapeake
blue catfish The blue catfish (''Ictalurus furcatus'') is the largest species of North American catfish, reaching a length of and a weight of . The typical length is about 25–46 in (64–117 cm). The fish can live to 20 years. The native distribution o ...
, Florida lionfish, Kentucky
silver carp The silver carp (''Hypophthalmichthys molitrix'') is a species of freshwater cyprinid fish, a variety of Asian carp native to China and eastern Siberia, from the Amur River drainage in the north to the Xi Jiang River drainage in the south. Alt ...
, Georgia
cannonball jellyfish The cannonball jellyfish (''Stomolophus meleagris''), also known as the cabbagehead jellyfish, is a species of jellyfish in the family Stomolophidae. Its common name derives from its similarity to a cannonball in shape and size. Its dome-shaped ...
, and invasive edible plants such as Japanese knotweed and
Autumn olive ''Elaeagnus umbellata'' is known as Japanese silverberry, umbellata oleaster, autumn olive, autumn elaeagnus, or spreading oleaster. The species is indigenous to eastern Asia and ranges from the Himalayas eastwards to Japan. It is a hardy, aggres ...
.
Joe Roman Joe Roman is a conservation biologist, academic, and author of the books ''Whale'' and '' Listed: Dispatches from America's Endangered Species Act''. His conservation research includes studies of the historical population size of whales, the ...
, a
Harvard Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...
and
University of Vermont The University of Vermont (UVM), officially the University of Vermont and State Agricultural College, is a public land-grant research university in Burlington, Vermont. It was founded in 1791 and is among the oldest universities in the Unite ...
conservation biologist who is the recipient of the
Rachel Carson Rachel Louise Carson (May 27, 1907 – April 14, 1964) was an American marine biologist, writer, and conservationist whose influential book '' Silent Spring'' (1962) and other writings are credited with advancing the global environmental ...
Environmental award, is the editor and chief of Eat The Invaders, a website dedicated to encouraging people to eat invasive species as part of a solution to the problem. Skeptics point out that once a foreign species has entrenched itself in a new place—such as the
Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
lionfish that has now virtually taken over the waters of the Western
Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...
,
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean ...
and
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...
—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 Caribbean monk seal, and the
passenger pigeon The passenger pigeon or wild pigeon (''Ectopistes migratorius'') is an extinct species of pigeon that was endemic to North America. Its common name is derived from the French word ''passager'', meaning "passing by", due to the migratory habits ...
. Proponents of invasivorism also point to the success that
Jamaica Jamaica (; ) is an island country situated in the Caribbean Sea. Spanning in area, it is the third-largest island of the Greater Antilles and the Caribbean (after Cuba and Hispaniola). Jamaica lies about south of Cuba, and west of Hispa ...
has had in significantly decreasing the population of lionfish by encouraging the consumption of the fish. In recent years, organizations including Reef Environmental Educational Foundation and the Institute for Applied Ecology, among others, have published cookbooks and recipes that include invasive species as ingredients.


Pesticides

Pesticide Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and ...
s are commonly used to control and eradicate invasives. Herbicides used against invasive plants include
fungal herbicide Plant pathology (also phytopathology) is the scientific study of diseases in plants caused by pathogens (infectious organisms) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, o ...
s. Although the
effective population size The effective population size (''N'e'') is a number that, in some simplified scenarios, corresponds to the number of breeding individuals in the population. More generally, ''N'e'' is the number of individuals that an idealised population w ...
of an introduced population is bottlenecked, some
genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources of genetic variation, b ...
has been known to provide invasive plants with resistance against these fungal bioherbicides. Meyer ''et al.'' 2010 finds invasive populations of ''
Bromus tectorum ''Bromus tectorum'', known as downy brome, drooping brome or cheatgrass, is a winter annual grass native to Europe, southwestern Asia, and northern Africa, but has become invasive in many other areas. It now is present in most of Europe, south ...
'' with resistance to ''
Ustilago ''Ustilago'' is a genus of approximately 200 smut fungi parasitic on grasses. Uses ''Ustilago maydis'' is eaten as a traditional Mexican food in many parts of the country, and is even available canned. Farmers have even been known to spread th ...
bullata'' used as a biocontrol, and Bruckart ''et al.'' 2017 find the same in ''
Microstegium vimineum ''Microstegium vimineum'', commonly known as Japanese stiltgrass, packing grass, or Nepalese browntop, is an annual grass that is common in a wide variety of habitats and is well adapted to low light levels. Despite being non-native in the Unite ...
'' subject to ''
Bipolaris ''Bipolaris'' is a genus of fungi belonging to the family Pleosporaceae. It was circumscribed by mycologist Robert A. Shoemaker in 1959. Species *'' Bipolaris australis'' *'' Bipolaris brizae'' *'' Bipolaris buchloës'' *'' Bipolaris cactivo ...
microstegii'' and '' B. drechsleri''. This is not solely a character of invasive plant genetics Burdon ''et al.'' 1995 show this is normal for wild plants in their study of the native weed ''
Linum marginale ''Linum marginale'', commonly known as native flax or wild flax, is a species of flowering plant in the family Linaceae and is endemic to Australia. It is a perennial herb with few branches, linear leaves, and blue flowers with five usually bl ...
'' and its fungal pathogen ''
Melampsora lini ''Melampsora lini'' is a species of fungus and plant pathogen found in Ireland and commonly known as flax rust. Life cycle The pathogen is an obligate biotroph meaning that each stage of its cycle is dependent on having a living host. The dir ...
''. They further show that some of this resistance is due to irregular
germination Germination is the process by which an organism grows from a seed or spore. The term is applied to the sprouting of a seedling from a seed of an angiosperm or gymnosperm, the growth of a sporeling from a spore, such as the spores of fungi, fe ...
leading to persistence in the
seed bank A seed bank (also seed banks or seeds bank) stores seeds to preserve genetic diversity; hence it is a type of gene bank. There are many reasons to store seeds. One is to preserve the genes that plant breeders need to increase yield, disease res ...
. MacDonald ''et al.'' 2011 and Fowler ''et al.'' 2013 find that
crop A crop is a plant that can be grown and harvested extensively for profit or subsistence. When the plants of the same kind are cultivated at one place on a large scale, it is called a crop. Most crops are cultivated in agriculture or hydropon ...
s also have another disadvantage over any uncontrolled plant wild native or invasive namely their greater uptake of nutrients. Crops are deliberately bred to increase nutrient intake to enable their increased product output. Any pathogen will find a less attractive environment in or on the invasive plant than a crop, and MacDonald finds
fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
is often irrelevant to that invasive ⇔ crop nutrient differential.


Predicting future invasive species

Predicting future invasive species is a topic of priority for many researchers. This trend has been motivated by the extreme adverse effects and persistent nature of invasive species. In other words, because invaders cause significant damage (see "Adverse effects" section above) and are nearly impossible to completely eradicate once they have established in a new environment, there is a consensus that invasions are best managed by predicting which species have a high likelihood of causing adverse effects and preventing the introduction of those species into new environments.


Predicting invasive plants

Plants are frequently the subjects of research aiming to predict future invasive species. Accurately predicting invasive plants could be especially effective in reducing negative impacts from invaders because a significant proportion of plants that became invasive were introduced to new environments on purpose. In the United States, it has been estimated that 60% of invasive plants were introduced intentionally. Therefore, because humans have control over the majority of these introductions, accurate knowledge about which species are likely to cause adverse effects could be used to outlaw the import and export of problematic plants. It is important to note that the vast majority (estimates range between 66-90%) of non-native plants do not have negative impacts when relocated to new environments. Furthermore, these harmless non-native plants make up a significant component of the horticulture industry (an industry that did $13.8 billion in sales in the United States in 2019). Therefore, most scientists agree that banning the relocation of all non-native plants would be unnecessarily harsh, in addition to being unrealistic. This further emphasizes the need to predict which species will become invasive and only regulate those plants.


Weed risk assessments

Weed risk assessments are commonly-used tools to predict the chances of a specific plant species having negative effects in a specific new environment (sometimes called "Pest risk analyses", especially when used for more taxa than only plants). Many weed risk assessments take the form of a standardized questionnaire. In these cases, a phytosanitary expert will assign the focal species a score for each question based on the risk posed by the plant in the new environment. At the end of the weed risk assessment, the scores from all the questions are summed, and the total score is associated with a management action (often "allow introduction" for low risk species, "evaluate further" for species that are between low risk and high risk, and "prevent introduction" for high risk species).


See also

*
Lists of invasive species These are lists of invasive species by country or region. A species is regarded as invasive if it has been introduced by human action to a location, area, or region where it did not previously occur naturally (i.e., is not a native species), beco ...
*
Aquatic invasive species regulations in Michigan The state of Michigan defines an aquatic invasive species as "an aquatic species that is nonnative to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health".Natur ...
*
Climate change and invasive species Climate change and invasive species refers to the process of the environmental destabilization caused by climate change. This environmental change facilitates the spread of invasive species — species that are not historically found in a certain ...
*
Genetic modification Genetic engineering, also called genetic modification or genetic manipulation, is the modification and manipulation of an organism's genes using technology. It is a set of technologies used to change the genetic makeup of cells, including ...
of native predator species *
Agricultural robot An agricultural robot is a robot deployed for agricultural purposes. The main area of application of robots in agriculture today is at the harvesting stage. Emerging applications of robots or drones in agriculture include weed control, cloud s ...
*
Ecologically based invasive plant management Ecologically based invasive plant management (EBIPM) is a decision-making framework to improve the management of invasive plant species. When land managers are faced with infestations of invasive plants, a step by step framework to develop integrate ...
*
Garden refugee Escaped plants are cultivated plants, usually garden plants, that are not originally native to an area, and due to their dispersal strategies, have escaped from cultivation and have settled in the wild and bred there, whether intentionally or ...
* Hemerochory * Invasion genetics *
Naturalisation (biology) Naturalisation (or naturalization) is the ecological phenomenon through which a species, taxon, or population of exotic (as opposed to native) origin integrates into a given ecosystem, becoming capable of reproducing and growing in it, and proce ...
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Colonisation (biology) Colonisation or colonization is the process in biology by which a species spreads to new areas. Colonisation often refers to ''successful'' immigration where a population becomes integrated into an ecological community, having resisted initia ...


References


Attribution

This article incorporates CC-BY-3.0 text from the reference


Citations


Sources

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Further reading


Removing Threat from Invasive Species with Genetic Engineering

Regulation of GM Organisms for Invasive Species Control

Should We Fight Invasive Species With Genetic Engineering


External links


Invasive Plant Terminology

North American Invasive Species Network
a consortium that uses a coordinated network to advance science-based understanding and enhance management of non-native, invasive species.
Great Britain Non-native Species Secretariat (NNNS) website

Invasive Species Compendium
an encyclopaedic resource that draws together scientific information on all aspects of invasive species
Invasive Species
National Invasive Species Information Center,
United States National Agricultural Library The United States National Agricultural Library (NAL) is one of the world's largest agricultural research libraries, and serves as a national library of the United States and as the library of the United States Department of Agriculture. Located ...

Invasive Species Specialist Group
– Global Invasive Species Database * Pacific Island Ecosystems at Risk project
PIER

invadingspecies.com
of the
Ontario Ministry of Natural Resources The Northern Development, Mines, Natural Resources and Forestry is a government ministry of the Canadian province of Ontario that is responsible for Ontario's provincial parks, forests, fisheries, wildlife, mineral aggregates and the Crown lands ...
and
Ontario Federation of Anglers and Hunters The Ontario Federation of Anglers and Hunters (OFAH) was established in 1928, and is Canada's leading conservation organization, as well as a non-profit registered charity. The Federation lobbies for the protection of wildlife and Canadian outdoor ...

Aquatic invasive species in Ireland
Inland Fisheries Ireland
Invasive alien species in Belgium
Belgian Forum on Invasive Species (BFIS)
"Invasive species" from the Global Legal Information Network Subject Term Index
{{DEFAULTSORT:Invasive Species Invasive species Environmental conservation Environmental terminology Habitat Pest control Forest pathology