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Cattle Bos primigenius taurus introduced but not naturalized worldwide
Sweet clover (Melilotus sp.), introduced and naturalized in the Americas from Europe as a forage and cover crop

An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there by human activity, directly or indirectly, and either deliberately or accidentally. Non-native species can have various effects on the local ecosystem. Introduced species that become established and spread beyond the place of introduction are considered naturalized. The process of human-caused introduction is distinguished from biological colonization, in which species spread to new areas through "natural" (non-human) means such as storms and rafting. The Latin expression neobiota captures the characteristic that these species are new biota to their environment in terms of established biological network (e.g. food web) relationships. Neobiota can further be divided into neozoa (also: neozoons, sing. neozoon, i.e. animals) and neophyta (plants).

The impact of introduced species is highly variable. Some have a substantial negative effect on a local ecosystem (in which case they are also classified more specifically as an invasive species), while other introduced species may have little or no negative impact (no invasiveness), and integrate well into the ecosystem they have been introduced to. Some species have been introduced intentionally to combat pests. They are called biocontrols and may be regarded as beneficial as an alternative to pesticides in agriculture for example. In some instances the potential for being beneficial or detrimental in the long run remains unknown.[1][2][3] The effects of introduced species on natural environments have gained much scrutiny from scientists, governments, farmers and others.

Terminology

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The formal definition of an introduced species from the United States Environmental Protection Agency is "A species that has been intentionally or inadvertently brought into a region or area. Also called an exotic or non-native species".[4][5]

In the broadest and most widely used sense, an introduced species is synonymous with "non-native" and therefore applies as well to most garden and farm organisms; these adequately fit the basic definition given above. However, some sources add to that basic definition "and are now reproducing in the wild",[6] which means that species growing in a garden, farm, or house may not meet the criteria unless they escape and persist.

Subset descriptions

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See also: Glossary of invasion biology terms

There are many terms associated with introduced species that represent subsets of introduced species, and the terminology associated with introduced species is now in flux for various reasons. Examples of these terms are "invasive", "acclimatized", "adventive", "naturalized", and "immigrant" species.[citation needed]

The term "invasive" is used to describe introduced species that cause ecological, economic, or other damage to the area in which they were introduced.[citation needed]

Acclimatized species are introduced species that have changed physically and/or behaviorally in order to adjust to their new environment. Acclimatized species are not necessarily optimally adjusted to their new environment and may just be physically/behaviorally sufficient for the new environment.[citation needed]

Adventive species are often considered synonymous with "introduced species", but this term is sometimes applied exclusively to introduced species that are not permanently established.[7]

Naturalized species are often introduced species that do not need human help to reproduce and maintain their population in an area outside their native range (no longer adventive), but that also applies to populations migrating and establishing in a novel environment (e.g.: in Europe, house sparrows are well established since early Iron Age though they originated from Asia).[citation needed]

Immigrant species are species that travel, sometimes by themselves, but often with human help, between two habitats. Invasiveness is not a requirement.[8]

Invasive species

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Introduction of a species outside its native range is all that is required to be qualified as an "introduced species". Such species might be termed naturalized, "established", or "wild non-native species". If they further spread beyond the place of introduction and cause damage to nearby species, they are called "invasive species". The transition from introduction, to establishment and to invasion has been described in the context of plants.[9] Introduced species are essentially "non-native" species. Invasive species are those introduced species that spread widely or quickly and cause harm, be that to the environment,[10] human health, other valued resources, or the economy. There have been calls from scientists to consider a species "invasive" only in terms of their spread and reproduction rather than the harm they may cause.[11]

According to a practical definition, an invasive species is one that has been introduced and become a pest in its new location, spreading (invading) by natural means. The term is used to imply both a sense of urgency and actual or potential harm. For example, U.S. Executive Order 13112 (1999) defines "invasive species" as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health".[12] The biological definition of invasive species, on the other hand, makes no reference to the harm they may cause, only to the fact that they spread beyond the area of original introduction.[citation needed]

Some argue that "invasive" is a loaded word and harm is difficult to define.[6]

From a regulatory perspective, it is neither desirable nor practical to list as undesirable or outright ban all non-native species (although the State of Hawaii has adopted an approach that comes close to this). Regulations require a definitional distinction between non-natives that are deemed especially onerous and all others. Introduced "pest" species, that are officially listed as invasive, best fit the definition of an invasive species. Early detection and rapid response is the most effective strategy for regulating a pest species and reducing economic and environmental impacts of an introduction.[13] Management of invasion pathways are on the forefront of eliminating unwanted invasive species this would include preliminary steps; educating the public, cooperation from industries and government resources.[14]

In Great Britain, the Wildlife and Countryside Act 1981 prevents the introduction of any animal not naturally occurring in the wild or any of a list of both animals or plants introduced previously and proved to be invasive.

Nature of introductions

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By definition, a species is considered "introduced" when its transport into an area outside of its native range is human mediated. Introductions by humans can be described as either intentional or accidental. Intentional introductions have been motivated by individuals or groups who either (1) believe that the newly introduced species will be in some way beneficial to humans in its new location or, (2) species are introduced intentionally but with no regard to the potential impact. Unintentional or accidental introductions are most often a byproduct of human movements and are thus unbound to human motivations. Subsequent range expansion of introduced species may or may not involve human activity.

Wheat Triticum introduced worldwide from its place of origin (Mesopotamia)

Intentional introductions

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Species that humans intentionally transport to new regions can subsequently become successfully established in two ways. In the first case, organisms are purposely released for establishment in the wild. It is sometimes difficult to predict whether a species will become established upon release, and if not initially successful, humans have made repeated introductions to improve the probability that the species will survive and eventually reproduce in the wild. In these cases, it is clear that the introduction is directly facilitated by human desires.

Male silver pheasant

In the second case, species intentionally transported into a new region may escape from captive or cultivated populations and subsequently establish independent breeding populations. Escaped organisms are included in this category because their initial transport to a new region is human motivated.

The widespread phenomena of intentional introduction has also been described as biological globalization.

Positive Introductions

Although most introduced species have negative impacts on the ecosystems they enter into, there are still some species that have affected the ecosystem in a positive way. For example, in New Hampshire invasive plants can provide some benefits to some species. Invasive species such as autumn olive, oriental bittersweet, and honeysuckle produce fruit that is used by a handful of fruit-eating bird species.[15] The invasive plants can also be a source of pollen and nectar for many insects, such as bees. These invasive plants were able to help their ecosystem thriving, and increase the native animal's chances of survival. Several introduced exotic trees served as nest sites for resident waterbird species in Udaipur city, India.[16]

Motivations for intentional introductions

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Economic
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Perhaps the most common motivation for introducing a species into a new place is that of economic gain. Non-native species can become such a common part of an environment, culture, and even diet that little thought is given to their geographic origin. For example, soybeans, kiwi fruit, wheat, honey bees, and all livestock except the American bison and the turkey are non-native species to North America. Collectively, non-native crops and livestock account for 98% of US food.[17] These and other benefits from non-natives are so vast that, according to the Congressional Research Service, they probably exceed the costs.[18]

Other examples of species introduced for the purposes of benefiting agriculture, aquaculture or other economic activities are widespread.[19] Eurasian carp was first introduced to the United States as a potential food source. The apple snail was released in Southeast Asia with the intent that it be used as a protein source, and subsequently to places like Hawaii to establish a food industry. In Alaska, foxes were introduced to many islands to create new populations for the fur trade. About twenty species of African and European dung beetles have established themselves in Australia after deliberate introduction by the Australian Dung Beetle Project in an effort to reduce the impact of livestock manure. The timber industry promoted the introduction of Monterey pine (Pinus radiata) from California to Australia and New Zealand as a commercial timber crop. These examples represent only a small subsample of species that have been moved by humans for economic interests.

The rise in the use of genetically modified organisms has added another potential economic advantage to introducing new/modified species into different environments. Companies such as Monsanto that earn much of their profit through the selling of genetically modified seeds has added to the controversy surrounding introduced species. The effect of genetically modified organisms varies from organism to organism and is still being researched today, however, the rise of genetically modified organisms has added complexity to the conversations surrounding introduced species.

Human enjoyment
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Introductions have also been important in supporting recreation activities or otherwise increasing human enjoyment. Numerous fish and game animals have been introduced for the purposes of sport fishing and hunting. The introduced amphibian (Ambystoma tigrinum) that threatens the endemic California salamander (A. californiense) was introduced to California as a source of bait for fishermen.[20] Pet animals have also been frequently transported into new areas by humans, and their escapes have resulted in several introductions, such as feral cats,[21] parrots,[22] and pond slider.[23]

Lophura nycthemera (silver pheasant), a native of East Asia, has been introduced into parts of Europe for ornamental reasons.

Many plants have been introduced with the intent of aesthetically improving public recreation areas or private properties. The introduced Norway maple for example occupies a prominent status in many of Canada's parks.[24] The transport of ornamental plants for landscaping use has and continues to be a source of many introductions. Some of these species have escaped horticultural control and become invasive. Notable examples include water hyacinth, salt cedar, and purple loosestrife.

In other cases, species have been translocated for reasons of "cultural nostalgia", which refers to instances in which humans who have migrated to new regions have intentionally brought with them familiar organisms. Famous examples include the introduction of common starlings to North America by the American Eugene Schieffelin, a lover of the works of Shakespeare and the chairman of the American Acclimatization Society, who, it is rumoured, wanted to introduce all of the birds mentioned in Shakespeare's plays into the United States. He deliberately released eighty starlings into Central Park in New York City in 1890, and another forty in 1891.

Yet another prominent example of an introduced species that became invasive is the European rabbit in Australia. Thomas Austin, a British landowner, had rabbits released on his estate in Victoria because he missed hunting them. A more recent example is the introduction of the common wall lizard (Podarcis muralis) to North America by a Cincinnati boy, George Rau, around 1950 after a family vacation to Italy.[25]

Addressing environmental problems
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Intentional introductions have also been undertaken with the aim of ameliorating environmental problems. A number of fast spreading plants such as kudzu have been introduced as a means of erosion control. Other species have been introduced as biological control agents to control invasive species. This involves the purposeful introduction of a natural enemy of the target species with the intention of reducing its numbers or controlling its spread.

A special case of introduction is the reintroduction of a species that has become locally endangered or extinct, done in the interests of conservation.[26] Examples of successful reintroductions include wolves to Yellowstone National Park in the U.S., and the red kite to parts of England and Scotland. Introductions or translocations of species have also been proposed in the interest of genetic conservation, which advocates the introduction of new individuals into genetically depauperate populations of endangered or threatened species.[27]

Unintentional introductions

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Unintentional introductions occur when species are transported by human vectors. Increasing rates of human travel are providing accelerating opportunities for species to be accidentally transported into areas in which they are not considered native. For example, three species of rat (the black, Norway and Polynesian) have spread to most of the world as hitchhikers on ships, and arachnids such as scorpions and exotic spiders are sometimes transported to areas far beyond their native range by riding in shipments of tropical fruit. This was seen during the introduction of Steatoda nobilis (Noble false widow) worldwide through banana shipments.[28]

Further there are numerous examples of marine organisms being transported in ballast water, among them the comb jelly Mnemiopsis leidyi, the bacterium Vibrio cholerae, or the zebra mussel. The Mediterranean and Black Seas, with their high volume shipping from exotic sources, are most impacted by this problem.[29] Busy harbors are all potential hotspots as well: over 200 species have been introduced to the San Francisco Bay in this manner making it the most heavily invaded estuary in the world.[30]

There is also the accidental release of the Africanized honey bees (AHB), known colloquially as "killer bees") or Africanized bee to Brazil in 1957 and the Asian carp to the United States. The insect commonly known as the brown marmorated stink bug (Halyomorpha halys) was introduced accidentally in Pennsylvania. Another form of unintentional introductions is when an intentionally introduced plant carries a parasite or herbivore with it. Some become invasive, for example, the oleander aphid, accidentally introduced with the ornamental plant, oleander.

Yet another unintentional pathway of introduction is during the delivery of humanitarian aid in the aftermath of natural disasters.[31][32] This occurred during relief efforts for Hurricane Maria in Dominica, it was found that the common green iguana, the Cuban tree frog, and potentially the Venezuela snouted tree frog were introduced with the former two becoming established.[32]

Most accidentally or intentionally introduced species do not become invasive as the ones mentioned above. For instance, Some 179 coccinellid species have been introduced to the U.S. and Canada; about 27 of these non-native species have become established, and only a handful can be considered invasive, including the intentionally introduced Harmonia axyridis, multicolored Asian lady beetle.[33] However the small percentage of introduced species that become invasive can produce profound ecological changes. In North America, Harmonia axyridis has become the most abundant lady beetle and probably accounts for more observations than all the native lady beetles put together.[34]

Introduced plants

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Main article: Adventive plant
Aesculus hippocastanum, native to Greece and the Balkan peninsula, has been introduced across most of Europe and parts of North America as an ornamental plant.[35]

Many non-native plants have been introduced into new territories, initially as either ornamental plants or for erosion control, stock feed, or forestry. Whether an exotic will become an invasive species is seldom understood in the beginning.[36]

A very troublesome marine species in southern Europe is the seaweed Caulerpa taxifolia. Caulerpa was first observed in the Mediterranean Sea in 1984, off the coast of Monaco. By 1997, it had covered some 50 km2. It has a strong potential to overgrow natural biotopes, and represents a major risk for sublittoral ecosystems. The origin of the alga in the Mediterranean was thought to be either as a migration through the Suez Canal from the Red Sea, or as an accidental introduction from an aquarium.[37]

This species has become invasive in Australia, where it threatens native rare plants and causes erosion and soil slumping around river banks.[38] It has also become invasive in France where it has been listed as an invasive plant species of concern in the Mediterranean region, where it can form monocultures that threaten critical conservation habitats.[39]

Introduced animals

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Chickens Gallus gallus domesticus, from Asia, introduced in the rest of the world

Most introduced species do not become invasive. Examples of introduced animals that have become invasive include the gypsy moth in eastern North America, the zebra mussel and alewife in the Great Lakes, the Canada goose and gray squirrel in Europe, the beaver in Tierra del Fuego, the muskrat in Europe and Asia, the cane toad and red fox in Australia, nutria in North America, Eurasia, and Africa, and the common brushtail possum in New Zealand. In Taiwan, the success of introduced bird species was related to their native range size and body size; larger species with larger native range sizes were found to have larger introduced range sizes.[40]

One notoriously devastating introduced species is the small Indian mongoose (Urva auropunctata). Originating in a region encompassing Iran and India, it was introduced to the West Indies and Hawaii in the late 1800s for pest control. Since then, it has thrived on prey unequipped to deal with its speed, nearly leading to the local extinction of a variety of species.[41]

In some cases, introduced animals may unintentionally promote the cause of rewilding.[42] For example, escaped horses and donkeys that have gone feral in the Americas may play ecological roles similar to those of the equids that became extinct there at the end of the Pleistocene.[43]

The exotic pet trade has also been a large source of introduced species. The species favored as pets have more general habitat requirements and larger distributions.[44] Therefore, as these pets escape or are released, unintentionally or intentionally, they are more likely to survive and establish non-native populations in the wild. Among the popular exotic pets that have become alien or invasive species are parrots, frogs, terrapins, and iguanas.[citation needed]

Most commonly introduced species

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Some species, such as the Western honey bee, brown rat, house sparrow, ring-necked pheasant, and European starling, have been introduced very widely. In addition there are some agricultural and pet species that frequently become feral; these include rabbits, dogs, ducks, snakes, goats, fish, pigs, and cats. Many water fleas such as Daphnia, Bosmina and Bythotrephes have introduced around the world, causing dramatic changes in native freshwater ecosystems.[45]

Genetics

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When a new species is introduced, the species could potentially breed with members of native species, producing hybrids. The effect of the creating of hybrids can range from having little effect, a negative effect, to having devastating effects on native species. Potential negative effects include hybrids that are less fit for their environment resulting in a population decrease. This was seen in the Atlantic Salmon population when high levels of escape from Atlantic Salmon farms into the wild populations resulted in hybrids that had reduced survival.[46] Potential positive effects include adding to the genetic diversity of the population which can increase the adaptation ability of the population and increase the number of healthy individuals within a population. This was seen in the introduction of guppies in Trinidad to encourage population growth and introduce new alleles into the population. The results of this introduction included increased levels of heterozygosity and a larger population size.[47] Wide-spread introductions of non-native iguanas are causing devastating effects on native Iguana populations in the Caribbean Lesser Antilles, as hybrids appear to have higher fitness than native iguanas, leading to competitive outcompetition and replacement.[48][49] Numerous populations have already become extinct and hybridization continues to reduce the number of native iguanas on multiple islands.

In plants, introduced species have been observed to undergo rapid evolutionary change to adapt to their new environments, with changes in plant height, size, leaf shape, dispersal ability, reproductive output, vegetative reproduction ability, level of dependence on the mycorrhizal network, and level of phenotype plasticity appearing on timescales of decades to centuries.[50]

On a planetary body

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Main article: Interplanetary contamination

It has been hypothesized that invasive species of microbial life could contaminate a planetary body after the former is introduced by a space probe or spacecraft, either deliberately or unintentionally.[51] It has also been hypothesized that the origin of life on earth is due to introductions of life from other planets billions of years ago, possibly by a sentient race. Projects have been proposed to introduce life to other lifeless but habitable planets in other star systems some time in the future. In preparation for this, projects have been proposed to see if anything is still alive from any of the feces left behind during the six Moon landings from 1969 to 1972.[52]

See also

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References

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Further reading

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Introduced species

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An introduced species is an organism transported by human activity to a region beyond its native or natural distribution range, either intentionally for purposes such as agriculture, ornamentation, or biological control, or unintentionally through vectors like shipping ballast water, contaminated cargo, or pet trade escapes. While many introduced species remain benign or fail to establish self-sustaining populations, a subset establishes, spreads, and exerts ecological, economic, or health effects, with outcomes ranging from beneficial enhancements to severe disruptions. Human-mediated introductions date back millennia, coinciding with agriculture and colonization, as early societies domesticated and relocated plants like wheat and animals for food production, inadvertently or deliberately altering landscapes to favor productivity over pristine native compositions. Beneficial introduced species include staple crops such as , which sustain global food systems, and pollinators like the European honeybee (Apis mellifera), which support agricultural yields despite originating outside many regions where they now thrive. These examples illustrate how introductions can fill ecological niches, boost in human-modified environments, or provide direct utility, challenging blanket narratives of harm by demonstrating causal contributions to stability and resource provision in altered ecosystems. Conversely, problematic introductions, often termed invasive when they proliferate unchecked, impose costs through mechanisms like resource competition, predation on natives, alteration, and disease transmission, leading to declines, altered nutrient cycles, and annual global economic damages estimated in hundreds of billions. Empirical studies quantify these impacts, such as invasive plants reducing native species richness by up to 50% in affected s or aquatic invasives exacerbating via accelerated decomposition. Defining characteristics include release from natural enemies, high propagule pressure from repeated human vectors, and adaptability to novel conditions, which enable persistence but also spark debates over management strategies like eradication versus tolerance, informed by site-specific evidence rather than ideological aversion to novelty.

Terminology and Definitions

Core Concepts

An introduced species is defined as a taxon that occurs outside its native range due to direct or indirect human actions, such as intentional transport for agriculture, ornamental purposes, or accidental release via shipping and trade. This human-mediated translocation distinguishes introduced species from those expanding naturally through dispersal mechanisms like migration or wind-borne seeds, as the latter occur without anthropogenic intervention. Establishment requires the species to form a self-sustaining population in the recipient ecosystem, often facilitated by factors including propagule pressure (the number and frequency of individuals introduced), environmental suitability (e.g., matching climate and soil conditions), and reduced biotic resistance from native competitors, predators, or pathogens. Not all introduced species become problematic; many fail to establish, while others naturalize—reproducing consistently without human aid but remaining localized and non-dominant. Invasion occurs when an introduced spreads aggressively, alters structure, or exploits resources in ways that displace natives, often due to evolutionary adaptations post-introduction, such as increased reproductive output or tolerance to conditions. Empirical studies indicate that invasive introduced contribute to by hybridization, predation, or resource competition; for instance, over 40% of listed as threatened are impacted by introduced taxa. Causal mechanisms emphasize that human accelerates these processes, with volumes correlating directly to introduction rates—e.g., ballast water from ships has facilitated thousands of marine transfers since the . Key ecological principles underscore that introduced species can create novel interactions absent in native assemblages, potentially enhancing or destabilizing functions like cycling or , though evidence favors net negative outcomes in most documented cases due to lack of co-evolved checks. Source evaluations reveal that definitions from governmental bodies like the USGS prioritize verifiable human causation over emotive labels, contrasting with some advocacy-driven narratives that conflate all non-natives as inherently harmful without impact assessments. Quantifying impacts requires site-specific data, as generalizing harms ignores contexts where introduced species fill vacant niches post-native extinctions or support human-dependent economies, such as crop plants derived from ancient introductions. Introduced species are defined as organisms whose transport beyond their native range has been mediated by human activities, either intentionally (e.g., for or ornamentation) or unintentionally (e.g., via shipping ballast ), distinguishing them from species that colonize new areas through natural dispersal mechanisms such as migration, wind, or currents without human intervention. This human-mediated aspect is critical, as it excludes "immigrant" or "colonizer" species that arrive independently, even if they establish populations, emphasizing causal agency in ecological terminology. The terms "introduced," "alien," "exotic," and "non-native" are often used interchangeably to describe outside their indigenous range due to influence, but precise distinctions arise in specialized contexts: "alien" and "non-native" broadly encompass any human-transported regardless of establishment success, while "introduced" frequently implies at least initial viability in the new environment, and "exotic" may connote perceived as novel or ornamental without requiring of human causation. In frameworks like the Global Register of Introduced and Invasive Species, "introduced" specifically denotes established alien —those that have formed self-sustaining populations—contrasting with transient aliens that fail to reproduce independently. Introduced species differ from ", a characterized not merely by their non-native status but by demonstrable negative impacts, such as displacing natives, altering ecosystems, or causing economic losses; empirical data indicate that only a small fraction—estimated at less than 10% in many assessments—of introduced exhibit invasive traits, with most remaining benign or neutral post-introduction. "" species represent a further refinement: introduced that have achieved reproductive and persistence without ongoing propagation, bridging the gap between mere presence and potential invasiveness, though naturalization alone does not imply harm. These distinctions underscore that introduction is a neutral descriptor of translocation , whereas terms like invasive incorporate evaluative judgments based on observed ecological consequences.

Historical Context

Ancient and Pre-Industrial Introductions

Human introductions of non-native species trace back to prehistoric migrations associated with the , where early farmers disseminated domesticated plants and animals from their centers of origin. In the , (Triticum spp.) and (Hordeum vulgare) were domesticated around 9000 BCE and subsequently spread to through Anatolian migrants by approximately 7000 BCE, marking one of the earliest documented cases of intentional species translocation for . Domesticated animals such as sheep (Ovis aries) and (Capra hircus), originating in the between 8000 and 7000 BCE, followed similar dispersal patterns via overland migration routes, enabling pastoral economies in new environments. Oceanic colonization provided further examples of prehistoric introductions, particularly by Austronesian voyagers in the Pacific. Polynesians carried the Pacific rat ( exulans), pigs (Sus scrofa), dogs ( familiaris), and chickens (Gallus gallus domesticus) to islands like those in starting around 3000 years , integrating these into island ecosystems as part of a portable subsistence package. Archaeological evidence, including chicken bones dated to 3000 in and , confirms these translocations accompanied and often led to rapid population establishment in predator-naive habitats. Such introductions frequently resulted in commensal or domestic roles, with rats exploiting human-modified landscapes. In , expansive empires accelerated species movements through trade and conquest. The disseminated commensal rodents like the (Rattus rattus) and (Mus musculus) across its territories, including , as evidenced by zooarchaeological remains in sites like . Romans also intentionally introduced game animals such as (Dama dama) to provinces for hunting and ornamental purposes, altering local faunal assemblages. Plant introductions included Mediterranean species to , with at least 50 new edible plants incorporated into and agriculture via military campaigns and commerce, though specific impacts on wild ecosystems varied. Pre-industrial trade networks in the medieval and early modern periods continued this pattern, as seen in Viking-mediated of the (Mya arenaria) from to European waters between the 13th and 14th centuries, likely as hull fouling on ships. These early introductions, often tied to subsistence, , or , preceded the scale of industrial-era translocations but established precedents for ecological modification through human agency.

Modern and Globalized Introductions

The advent of the in the late initiated a profound escalation in introductions, driven by mechanized transportation, colonial expansion, and burgeoning international commerce that connected distant ecosystems on an unprecedented scale. Steam-powered ships and railroads enabled the rapid transcontinental shipment of goods, passengers, and live organisms, vastly increasing both intentional translocations—such as ornamental and —and unintentional vectors like hull fouling and discarded . This period marked the first wave of modern globalization's biological footprint, with trade volumes surging and facilitating the dispersal of thousands of non-native worldwide, many of which established populations in novel environments. In the , the global trade boom in specimens, horticultural imports, and agricultural commodities amplified introductions, particularly through European empires exchanging and across hemispheres. For instance, the ornamental plant trade introduced species like the horse-chestnut () widely beyond its Balkan origins, while agricultural experiments disseminated forage crops such as white sweetclover () to and for soil improvement and livestock feed. Concurrently, colonial acclimatization societies promoted "beneficial" introductions, such as European game birds and mammals to hunting estates in settler colonies, often disregarding ecological risks; by mid-century, over 100 such societies operated globally, prioritizing economic utility over native preservation. These efforts, coupled with the era's , laid groundwork for many persistent invasives, as evidenced by the correlation between rising trade metrics and establishment rates in recipient regions. The 20th century's second wave, propelled by , containerized shipping, and post-World War II , further intensified introductions, with annual alien establishment rates climbing amid exponential trade growth—global merchandise trade volume expanded from $58 billion in 1948 to over $18 trillion by 2019. Unintentional pathways dominated, including water discharging aquatic invertebrates like the (Dreissena polymorpha) into the North American in the 1980s, and harboring such as the (Anoplophora glabripennis) in urban ports. Intentional releases persisted via biocontrol programs, exemplified by the (Rhinella marina) imported to , , in 1935 to prey on sugarcane pests, and aquaculture escapes like into U.S. waterways from the 1970s onward. The pet trade surge, particularly from the 1970s, flooded markets with exotic vertebrates; in alone, over 150 non-native and were introduced via releases or escapes, establishing the state as a hotspot for invasive herpetofauna due to its subtropical and lax regulations. These dynamics underscore how modern connectivity, while boosting human prosperity, has causally amplified biotic homogenization, with now implicated in 60% of documented extinctions since 1500.

Mechanisms of Introduction

Intentional Mechanisms

Intentional mechanisms of species introduction involve deliberate human transport and release of non-native organisms to achieve specific utilitarian or aesthetic goals, such as improving agricultural yields, enhancing landscapes, controlling pests, or supporting recreational activities. These pathways account for a significant portion of global species translocations, with historical records tracing back to ancient trade routes but accelerating during colonial expansions and modern globalization. For example, the Department of Agriculture's plant introduction efforts in the systematically imported varieties to diversify production, exemplifying structured governmental programs. Agricultural introductions represent one of the oldest and most widespread intentional mechanisms, driven by the need to expand food production in new regions. Europeans transported staple crops like (Triticum aestivum) to the following Columbus's voyages in 1492, enabling large-scale cultivation in temperate zones previously lacking such grains. Similarly, species were deliberately introduced to ; by 1882, the 'Washington' navel orange had proliferated to over 500,000 trees through USDA-coordinated efforts, transforming arid valleys into major export hubs. These actions prioritized economic gains but often overlooked long-term compatibility with local ecosystems. Horticultural and ornamental introductions occur via the global trade in plants for , , and , with nurseries and botanists selecting species for visual appeal or utility. Plants such as Japanese barberry () and purple loosestrife () were imported to in the for decorative hedges and wetland aesthetics, respectively, but escaped cultivation to invade native habitats. (), introduced from in the for forage and in the , exemplifies how initial motives fueled rapid dissemination through deliberate planting programs. Horticultural markets continue to propagate risks, as evidenced by surveys indicating that many invasive plants remain commercially available despite known escape tendencies. Biological control mechanisms entail the targeted release of predators, parasites, or pathogens to suppress established pests or invasives, often coordinated by agricultural agencies. In , three South American flea beetles and moths were intentionally introduced starting in the 1970s to curb alligator weed (Alternanthera philoxeroides), achieving substantial reductions in infested waterways without chemical interventions. However, failures like the 1935 release of cane toads (Rhinella marina) in to control sugarcane beetles illustrate how incomplete ecological assessments can amplify problems, as the toads lacked efficacy against targets but toxicified broader food webs. Such programs underscore the causal chain from pest pressure to species importation, tempered by variable success rates documented in peer-reviewed evaluations. Recreational and faunal enrichment introductions include stocking game animals and birds for hunting or aesthetic reasons, frequently initiated by acclimatization societies in the 19th and early 20th centuries. European starlings (Sturnus vulgaris) were released in New York City's Central Park in 1890–1891 to replicate the bird species mentioned in Shakespeare's works, leading to continent-wide expansion numbering billions by the mid-20th century. Fish stocking for , such as non-native species in U.S. lakes, follows similar logics, with state agencies annually releasing millions to sustain fisheries, though this can disrupt native invertebrate communities. These mechanisms highlight human prioritization of cultural or sporting values over precautionary safeguards.

Unintentional Mechanisms

Unintentional mechanisms facilitate the transport and release of without deliberate human intent to introduce them into new ecosystems, often as byproducts of global , shipping, and . These pathways include contaminants in , stowaways on vessels or , and hitchhikers on or personal effects. Such introductions have accelerated with increased , contributing to the establishment of numerous non-native that can disrupt local . Maritime transport via ballast water discharge is a prominent vector for aquatic species. Ships uptake water for stability during voyages, inadvertently entraining , fish larvae, and benthic organisms, which are then released in foreign ports. This pathway accounted for approximately 40% of aquatic introductions in the United States. A notable example is the (Dreissena polymorpha), first detected in the in 1988 after transport in ballast water from , where it has since proliferated, clogging water intake pipes and outcompeting native mussels. Hull fouling complements ballast water as a maritime vector, with sessile organisms such as , mussels, , and hydroids attaching to submerged ship surfaces during stays. These biofouling communities can survive transoceanic journeys and detach or spawn in new locations upon arrival. Surveys of vessel hulls have identified diverse non-native species, including the hydrozoan Moerisia lyonsi in and the amphipod tigrinus in coastal areas, both linked to fouling-mediated spread. Biofouling affects not only commercial ships but also recreational boats, amplifying risks in regional waters. Trade-related contaminants, particularly in , packing materials, and nursery stock, enable terrestrial and -associated introductions. adhere to imported machinery, crates, or balls, surviving transit to establish in recipient . Unintentional contaminants of materials like in nursery rank among high-risk pathways for and invasions. For example, fungal pathogens and nematodes have been introduced via infected or debris in global , contributing to widespread declines in native trees and crops. Air and overland transport vectors involve small mobile species hitching rides in , vehicles, or passenger luggage. , such as mosquitoes, can be carried in holds or wheel wells, with interceptions revealing diverse taxa. Yellow fever mosquitoes () have been detected at airports like Schiphol, , via airline transport from tropical regions. Analysis of U.S. inspections documented 232 non-native across 394 records, 12% of which established populations, underscoring the pathway's potency for . Overland, anglers unintentionally spread aquatic through live bait releases or contaminated gear. These mechanisms often intersect with intentional , amplifying risks; for instance, stowaways in machinery or equipment transported for . Regulatory efforts, such as the International Maritime Organization's (ratified 2013), aim to mitigate maritime vectors, though enforcement gaps persist. Overall, unintentional pathways have introduced thousands of globally, with ongoing monitoring essential to curb further invasions.

Categories of Introduced Species

Introduced Plants

Introduced plants refer to non-native species dispersed beyond their indigenous ranges primarily through human agency, including deliberate translocation for food production, , mitigation, or timber, and inadvertent conveyance via commerce, shipping, or contaminated materials. Unlike many introduced animals, plants often propagate efficiently via seeds, rhizomes, or vegetative fragments, facilitating establishment in novel habitats. Globally, human agriculture depends heavily on such introductions; for instance, (Triticum aestivum), domesticated in the approximately 10,000 years ago, was transported to the by Spanish colonizers in the 1700s and English settlers in during the 1800s, enabling large-scale cultivation that now supports diverse food systems. Similarly, (Zea mays), originating in around 9,000 years ago, has been disseminated worldwide, contributing to caloric intake for billions through hybrid varieties optimized for yield. Intentional introductions dominate plant translocations, with agricultural and horticultural motives accounting for the majority. In the United States, 82% of invasive woody trace origins to horticultural releases by nurseries, botanical gardens, or private collectors, often prized for ornamental value or utility. examples include blue gum eucalyptus (), first planted in in 1853 and scaled commercially from the 1870s to address timber shortages, where rapid growth rates—up to 10 feet per year—promised quick harvests despite ultimate failures in pulp production due to wood quality issues. Forage and soil stabilization efforts introduced species like (), showcased at the 1876 as an ornamental vine from and later promoted by the U.S. Department of Agriculture from 1935 for along roadsides and degraded lands during recovery programs. These frequently integrate into managed landscapes without issue, providing economic benefits such as enhanced livestock feed or windbreaks, yet escape and proliferation occur when lacking natural predators or compatible soil constraints. ![Blue gum eucalyptus plantation in California][float-right] Unintentional pathways, though less prevalent, contribute significantly to invasive subsets, comprising at least 12% of U.S. invasive plants and 21% of noxious weeds via mechanisms like seed contamination in imported , ballast soils, or hitchhiking on vehicles and equipment. Problematic cases include yellow starthistle (Centaurea solstitialis), accidentally introduced in the 19th century with contaminated seed and now infesting over 15 million acres in rangelands, where it reduces forage quality and alters fire cycles by increasing fuel loads. While many introduced plants yield net positives—such as stabilizing soils in eroded agroecosystems or bolstering resources in fragmented habitats through abundant —others impose ecological costs by outcompeting natives, hybridizing with locals, or reshaping dynamics, necessitating site-specific evaluations of proliferation risks before deployment. Empirical assessments, prioritizing long-term monitoring over anecdotal successes, reveal that most introductions remain benign or beneficial in cultivation but underscore the causal role of human facilitation in enabling escapes that disrupt recipient biota.

Introduced Animals

Introduced animals refer to non-native vertebrates and invertebrates transported by human activities to regions outside their historical ranges, often establishing self-sustaining populations. These include mammals, birds, reptiles, amphibians, , and such as mollusks and , introduced either intentionally for purposes like , , , or pets, or unintentionally through vectors like water, cargo, or accidental releases. Unlike plants, animals frequently exhibit high mobility and reproductive rates post-introduction, amplifying their potential to become invasive by preying on natives, competing for resources, or transmitting diseases. Intentional introductions of animals have historically aimed at economic or recreational benefits but often yielded unintended ecological disruptions. In , European rabbits (Oryctolagus cuniculus) were deliberately released in 1859 by landowner Thomas Austin near , Victoria, for sport hunting; within decades, their population exploded to over 600 million, causing widespread , vegetation loss, and declines in native herbivores and predators due to . Similarly, European starlings (Sturnus vulgaris) were released in New York City's in 1890–1891 by a group seeking to introduce all birds mentioned in Shakespeare's works; from an initial 100 individuals, they spread across , numbering over 200 million by the mid-20th century, aggressively competing with native cavity-nesting birds like bluebirds and woodpeckers for nest sites while inflicting approximately $800 million in annual agricultural damage through crop consumption. Amphibians exemplify failed biocontrol efforts among intentional introductions. Cane toads (Rhinella marina) were imported from to northeastern , , in 1935 to prey on sugarcane beetles; however, the toads ignored the target pests, instead proliferating unchecked due to lacking natural predators and toxic skin secretions that killed native predators like quolls and goannas upon ingestion, leading to localized population crashes in frog-eating species and broader community shifts in wetlands. Reptiles such as Burmese pythons, released or escaped from the pet trade in since the 1990s, have similarly invaded the , preying on mammals and reducing small mammal biomass by up to 90% in some areas. Unintentional introductions often involve stowaways in global trade. Ship rats (Rattus rattus), Norway rats (R. norvegicus), and Pacific rats (R. exulans) arrived on islands via maritime vessels as early as the , decimating seabird colonies by consuming eggs and chicks; on rat-infested tropical islands, up to 50% of native bird species have faced risks, with rats altering nutrient cycling by preventing guano deposition that fertilizes reefs and forests. Aquatic invasives like zebra mussels (Dreissena polymorpha), discharged from ballast water into the around 1988, have clogged water intake pipes and outcompeted native bivalves, reshaping food webs by filtering and promoting algal blooms. Ecological impacts of introduced animals predominantly involve negative alterations to and function, with predation and as primary mechanisms; for instance, invasive predators have contributed to at least 40% of documented extinctions on islands globally. Disease transmission, such as carried by introduced mosquitoes, further exacerbates declines in naive hosts. While some introduced animals provide socioeconomic benefits like fisheries (e.g., certain salmonids), purely ecological positives are limited and context-dependent, such as occasional substitution for extinct dispersers in seed or networks, though these rarely offset broader harms without management. Assessments indicate that net effects hinge on establishment success and native resilience, with eradication efforts on islands restoring populations and reef health post-rat removal.

Introduced Microorganisms and Fungi

Introduced microorganisms, encompassing , viruses, and , along with fungi, are frequently transported unintentionally via global trade, travel, and contaminated materials such as or infected hosts. These agents, due to their and high reproductive rates, can establish rapidly in new environments, often evading early detection and leading to widespread pathological effects on native biota. Unlike macroscopic , their introductions typically manifest as emergent diseases rather than direct , disrupting host populations and associated ecosystems through dynamics rather than resource displacement. Fungal pathogens exemplify severe ecological consequences. Cryphonectria parasitica, the causal agent of , originated in and was introduced to the around 1900 through imported Japanese chestnut () nursery stock, first detected in in 1904. This fungus girdles stems and trunks via cankers, functionally eliminating the (Castanea dentata) from eastern forests within decades, with losses estimated in the billions of mature trees by the mid-20th century, altering forest composition, wildlife habitats, and timber economies. Similarly, Dutch elm disease, driven by Ophiostoma novo-ulmi and related species vectored by elm bark beetles (Scolytus spp.), entered in the 1930s via contaminated European elm stock, spreading rapidly and killing tens of millions of American elms () in urban and riparian settings by the 1970s, reducing canopy diversity and increasing susceptibility to other stressors. The chytrid fungus (Bd), a zoosporic pathogen, has been disseminated globally since the late , often carried asymptomatically by introduced species like the North American bullfrog (Lithobates catesbeianus), which serves as a . Bd infects keratinized skin, disrupting osmoregulation and causing , which has precipitated population declines or extinctions in over 500 species across six continents, particularly in montane , by impairing and electrolyte balance. This pathogen's introduction highlights how microbial agents can cascade through food webs, reducing invertebrate and vertebrate diversity in affected wetlands. Bacterial introductions, such as certain soil-associated strains, can alter native microbial assemblages and biogeochemical cycles, though impacts are often subtler and harder to attribute solely to single taxa. For instance, anthropogenic transport of bacteria via agricultural imports may shift or decomposition rates, potentially favoring invasive or reducing native over time. Viral introductions, including those affecting or , similarly propagate via vectors or infected propagules, exacerbating disease in novel hosts; examples include vectored plant viruses like those in the Potyvirus genus, which have spread via trade and diminished crop yields while indirectly influencing pollinator-dependent ecosystems. Overall, these introductions underscore the disproportionate role of microbial pathogens in , with indicating net negative effects through host-specific mortality rather than balanced ecological integration.

Genetic and Evolutionary Aspects

Hybridization and Gene Flow

Hybridization between introduced and native species involves the interbreeding of non-native individuals with closely related indigenous populations, often resulting in fertile that enable —the transfer of genetic material across species boundaries via . This process disrupts native gene pools by introducing alleles from the introduced lineage, which can homogenize and erode locally adapted traits shaped by over millennia. In , prezygotic barriers like behavioral isolation may limit hybridization, but human-mediated introductions frequently overcome these through proximity in disturbed habitats; in , weaker reproductive barriers and mechanisms such as facilitate higher rates of successful crossing. Empirical studies document that such gene flow occurs in approximately 10-25% of threatened species, underscoring its prevalence in conservation contexts. A prominent example is the hybridization between introduced (Oncorhynchus mykiss), originating from coastal Pacific stocks, and native subspecies (O. clarki) in western North American rivers since the late . Genetic analyses reveal extensive , with non-native haplotypes comprising up to 40-80% in some cutthroat populations by the , leading to the of pure native lineages in over 40% of historical ranges. This has caused , reducing hybrid fitness through mismatched coadapted gene complexes, and exacerbated declines amid . Climate-driven range shifts further accelerate this process, as warming waters expand overlap zones, projecting near-complete genomic replacement in vulnerable drainages by 2050 under moderate emissions scenarios. In coastal ecosystems, introduced smooth cordgrass (Spartina alterniflora) from the U.S. East Coast has hybridized with native cordgrass (S. foliosa) in since the 1970s, producing aggressive hybrids with heightened salinity tolerance and clonal spread. These hybrids exhibit bidirectional gene flow, but dominant introgression from the invasive parent has swamped native nuclear genomes, correlating with a 30-50% reduction in native genetic purity and altered marsh dynamics by 2000. While hybrid vigor initially boosted invasiveness—covering 100-200 hectares annually—long-term effects include diminished native and in tidal wetlands. Similar patterns occur in like beachgrasses (Ammophila spp.), where European introductions hybridize with North American natives, yielding novel genotypes that intensify dune stabilization but dilute endemic adaptations to local erosion regimes. Conservation genetics emphasizes that hybridization-driven rarely confers adaptive benefits to natives in introduced scenarios, as foreign alleles often lack selection in environments, instead promoting maladaptive swamping—evidenced by meta-analyses showing 85% of cases yield reduced viability. interventions, such as targeted removal of invaders before breeding seasons, have preserved native in isolated systems, but widespread demands genomic monitoring to distinguish viable hybrids from those warranting eradication. Peer-reviewed assessments caution against underestimating these dynamics, as undetected can silently precipitate extinctions over decades, independent of demographic declines.

Adaptation and Evolutionary Trajectories

Introduced species frequently encounter novel abiotic conditions, such as altered climates, soils, or predator regimes, imposing strong selective pressures that drive rapid adaptive . Contemporary genetic analyses reveal that many invasives harness standing from source populations to evolve traits enhancing survival and reproduction, often within decades or fewer than 50 generations. For instance, resurrection experiments with the invasive plant (St. John's wort), introduced to around 1900, demonstrated evolved reductions in plant height and shifts to earlier flowering times between 5 and 50 years post-introduction, facilitating in new grasslands. These changes reflect heritable adaptations rather than mere plasticity, as evidenced by common garden trials comparing ancestral and derived genotypes. Phenotypic plasticity often bridges initial establishment, allowing quick phenotypic responses before genetic fixation, but sustained invasions correlate with genetic assimilation of adaptive traits. In the cane toad (Rhinella marina), introduced to in 1935 for biocontrol, populations at the invasion front evolved significantly longer hind legs—up to 15% longer than in source populations—within approximately 80 years, accelerating dispersal rates by over 10% via stronger selection on mobility genes. Similarly, introduced house mice (Mus musculus) on exhibited rapid nonadaptive alongside localized adaptive differentiation in morphology and , diverging from mainland populations in under 100 generations due to island-specific selection. Such trajectories underscore how founding events, despite reducing neutral by 20-50% on average, preserve adaptive loci sufficient for when selection is intense. Long-term evolutionary paths vary: successful invasives may achieve local , forming ecotypes tailored to regional conditions, while others face evolutionary traps from shifting environments or interventions. Reviews of invasive and animals indicate that rapid not only boosts spread but can generate "invasive traits" like increased or herbicide resistance, as seen in Pinus radiata plantations where fire-selected variants diverged adaptively within a century of introduction. However, persistent from ongoing introductions can homogenize populations, constraining divergence and promoting maladaptive hybrids in some cases. Empirical data from over 100 studied invasives affirm that evolutionary potential, often underestimated in early models, underpins many establishment successes, with genomic tools increasingly quantifying shifts under novel pressures.

Ecological Impacts

Positive Contributions to Ecosystems

Introduced species have demonstrated positive ecological roles through classical biological control, where targeted introductions suppress pest populations that threaten native and stability. For instance, the vedalia beetle (Rodolia cardinalis), imported from to in 1888–1889, rapidly established and reduced populations of the cottony cushion scale (), an invasive that had devastated trees and associated vegetation; this intervention preserved native and communities by averting widespread defoliation and loss without notable non-target effects on local arthropods. Similarly, the introduction of the Cotesia glomerata in has controlled the cabbage white butterfly (), mitigating damage to brassicaceous plants and indirectly supporting pollinator-dependent native flora by reducing herbivory pressure. These cases illustrate how deliberate introductions can restore trophic balances in invaded systems, with success rates for establishment around 33% and effective control in approximately 10% of programs, often yielding long-term reductions in pest densities. Certain introduced plants contribute positively by facilitating native species establishment in degraded or early-successional ecosystems. In old-field sites across central , nonnative species such as common mullein () and other invasives have accelerated succession by stabilizing soils, providing microhabitats, and increasing overall plant diversity, including natives, through niche filling in nutrient-poor or disturbed areas abandoned from . White sweet clover (), introduced to for and soil improvement, fixes atmospheric via , elevating soil levels in grasslands and prairies, which supports subsequent growth of native perennials in otherwise infertile post-agricultural lands. Such facilitation effects are particularly evident in anthropogenically altered landscapes, where introduced species act as "ecosystem engineers" to enhance habitat heterogeneity and resource availability, potentially aiding recovery without requiring active restoration. Introduced animals can augment food webs by providing novel resources that sustain native predators or herbivores in resource-limited environments. In some Pacific island ecosystems, introduced fruit- and nectar-feeding birds have bolstered and services, compensating for declines in native avian mutualists and maintaining reproductive success amid . These contributions, while context-dependent and often outweighed by broader negative impacts in intact systems, underscore the potential for introduced species to fulfill vacant ecological functions, particularly following human-induced disturbances like or . Empirical assessments emphasize evaluating such benefits alongside risks, as positive outcomes are more pronounced in novel or simplified ecosystems rather than diverse, equilibrium communities.

Negative Consequences for Biodiversity

Introduced species, particularly those that become invasive, frequently impose severe reductions in native through direct mechanisms such as predation, for resources, and transmission, as evidenced by meta-analyses of 253 empirical studies documenting consistent declines in following invasions. Predatory introduced animals can decimate populations of native species lacking co-evolved defenses; for instance, the (Boiga irregularis), accidentally introduced to post-World War II, caused the extirpation of 10 of 12 native forest bird species by the late 20th century, with predation rates exceeding natural levels due to the absence of the snake's predators and Guam's historically high prey availability. This has cascaded to impair forest regeneration, as surviving trees rely on extinct birds for , further eroding structure. Competition represents another primary pathway, where introduced species exploit similar niches more efficiently, displacing natives; invasive plants, for example, often outcompete indigenous flora for light, water, and nutrients, leading to homogenized plant communities and reduced habitat suitability for dependent . In , European starlings (Sturnus vulgaris), introduced in the 1890s, aggressively usurp nesting cavities from native cavity-nesting birds like bluebirds and woodpeckers, contributing to localized declines in their populations through eviction and interference competition. Similarly, zebra mussels (Dreissena polymorpha), detected in the in 1988, filter vast quantities of —up to 1 liter per mussel daily—depleting food resources for native grazers and fish, while encrusting and suffocating endemic unionid mussels, which have experienced near-total extirpation in affected waters. Pathogen transmission by introduced microorganisms or vectors exacerbates these effects, often triggering rapid population crashes; the chytrid fungus (), spread globally via trade since the mid-20th century, has driven declines in at least 501 and confirmed or presumed extinctions of 90 since the , primarily through infections disrupting in susceptible hosts. These impacts compound via altered trophic dynamics, such as invasive plants facilitating secondary invaders or predators reducing herbivory on natives, yielding net losses documented across ecosystems from terrestrial forests to aquatic habitats. Empirical syntheses confirm that such invasions rank among the top drivers of contemporary extinctions, surpassing habitat loss in isolated cases.

Assessing Net Ecological Effects

Assessing the net ecological effects of introduced species requires evaluating both positive and negative outcomes across , ecosystem functions, and services, often revealing context-dependent results rather than uniform harm. Empirical studies indicate that while some introduced species drive native declines or alter processes like nutrient cycling, others enhance productivity or restore degraded functions, with net effects varying by ecosystem type, stage, and co-occurring stressors such as habitat loss or . For instance, a global review found that only about 16% of the roughly 23,000 documented introduced species exhibit significant ecological impacts, suggesting most are neutral or benign. Similarly, the Intergovernmental Science-Policy Platform on and Services (IPBES) classified 15% of assessed alien species impacts as positive for , including habitat provision or increased in human-modified landscapes. Methods for net assessment include standardized risk protocols that score impacts on native biota and functions, experimental manipulations to isolate effects, and modeling to predict long-term dynamics, though these often overlook synergies with or evolutionary adaptations. Long-term monitoring data from sites like European grasslands show introduced plants sometimes boosting overall and without proportional native losses, yielding net functional gains. However, challenges persist: impacts are scale-dependent, with small-plot experiments frequently exaggerating negatives that dissipate at landscape levels, and temporal lags can mask initial benefits like in eroded areas. Bias toward documenting harms—stemming from selective publication and policy focus—underrepresents positives, such as introduced earthworms improving North American and post-glacial extirpation of natives. Critiques of traditional paradigms highlight that equating "introduced" with "invasive" ignores cases where net effects favor resilience, as in salt marshes where non-natives like Spartina alterniflora hybrids accelerate sediment accretion and support higher bird diversity despite native displacement. Quantitative meta-analyses reveal that while predators like introduced reduce endemic fish in isolated lakes, ecosystem-wide metrics (e.g., total productivity) often remain stable or improve due to trophic efficiencies. In degraded or novel ecosystems, introduced species may yield net positives by filling vacant niches, as evidenced by increased multifunctionality in invaded U.S. estuaries compared to uninvaded controls. Overall, rigorous assessment demands integrating instrumental values—like by alien bees—and avoiding native-centric biases, prioritizing empirical baselines over preconceived notions of ecological purity.

Socioeconomic and Human Impacts

Economic Advantages

Introduced species form the foundation of global , with roughly 99% of cultivated crops and comprising non-native taxa domesticated in one region and disseminated worldwide. This translocation has vastly expanded arable production, enabling sustenance for populations and generating an estimated $30 in annual economic value from inclusive of these beneficial introductions. In pollination-dependent sectors, the European (Apis mellifera), introduced to in the early 17th century, sustains key , nut, and crops through cross- services. derives approximately $15 billion yearly from honey bee alone, underscoring the ' role in maintaining yields for commodities like almonds and apples that would otherwise suffer reduced output. Forestry economies benefit from deliberately introduced fast-growing trees, such as Eucalyptus native to but planted across , , and for timber and pulp production. In , Eucalyptus plantations cover over 500,000 hectares, delivering high yields that enhance farmer incomes, fuelwood supply, and industrial outputs while supporting multiple-use economic models. Similarly, Pinus radiata introductions in have revolutionized timber exports through efficient plantation management. Livestock-associated introductions, including dung beetles in , yield ancillary gains by recycling , aerating , curbing bush populations, and boosting pasture regrowth. Since the program establishing 23 , these beetles have amplified agricultural turnover by more than $1 billion annually through improved nutrient cycling and reduced veterinary costs. Aquaculture parallels this with like , fostering commercial fisheries and recreational revenues in non-native ranges.

Economic Drawbacks

Invasive introduced species generate substantial economic costs worldwide, primarily through damages to , reduced in key industries, and expenditures on prevention, control, and eradication efforts. Globally, biological invasions incurred at least $1.288 USD in costs from to , with annual expenses averaging $26.8 billion USD and rising to $162.7 billion USD by , driven largely by direct damages rather than management alone. Updated assessments from the Intergovernmental Science-Policy Platform on and Services (IPBES) place current annual global costs at over $423 billion USD, reflecting underreported indirect effects such as lost revenue from diminished services. These figures underscore a trend of accelerating expenses, with costs in escalating from $2 billion USD per year in the to over $26 billion USD annually since 2010. In the United States, highly reliable observed costs from totaled $1.22 trillion USD (in 2017 values) between 1960 and 2020, encompassing both tangible damages and response measures. experienced $140.2 billion USD in cumulative costs over the same period, with approximately 60% attributed to damages like crop losses and infrastructure repairs, and the remainder to management activities. sectors—agriculture, , and fisheries—account for a disproportionate share, with global losses in these areas exceeding hundreds of billions USD over the past five decades due to yield reductions, increased use, and harvesting inefficiencies. For instance, invasive pests and weeds in erode crop productivity; in the US, such impacts contribute to billions in annual forgone output, while invasive trees alone have generated $19.2 billion USD in global costs from 1960 to 2020 through timber value losses and suppression of native regeneration. Infrastructure and water management face acute burdens from species like the (Dreissena polymorpha), introduced to n in the , which fouls , boats, and power plants, imposing annual maintenance costs estimated at $500 million USD in the alone as of the early 2000s, with cumulative effects amplifying regional economic strain. In fisheries, introduced predators such as (Petromyzon marinus) in the upper have decimated native populations like , leading to commercial losses exceeding $7 million USD annually in the before control programs, and ongoing management expenses in the hundreds of millions USD per year across affected systems. Forestry sectors suffer from pathogens carried by introduced insects, such as the (Agrilus planipennis), which has killed tens of millions of ash trees in since its detection in 2002, resulting in urban tree replacement costs surpassing $10 billion USD and lost timber value. Management and mitigation further compound drawbacks, as control expenditures often lag behind damages; for example, global spending on invasion prevention remains far below the $1.131 trillion USD in documented damages from 1970 to 2020, perpetuating a cycle of reactive, high-cost interventions like chemical treatments and mechanical removals. Indirect economic ripple effects, including tourism declines from degraded habitats (e.g., algal blooms fueled by invasive aquatic plants reducing recreational access) and health-related costs from vectors like the Asian tiger mosquito (Aedes albopictus), add unquantified billions, though peer-reviewed syntheses emphasize that reported totals likely underestimate true burdens due to data gaps in non-market losses. These patterns highlight how introduced species disrupt supply chains and inflate operational expenses across economies, with costs disproportionately affecting developing regions through agricultural vulnerabilities.

Broader Societal and Health Effects

Introduced species can shape societal dynamics through cultural integration and recreational opportunities, though such influences often generate conflicts. For instance, non-native species like certain game birds or have bolstered and traditions in regions such as and , fostering community bonds and local identities tied to these activities. Similarly, introduced plants and animals contribute to culinary practices and festivals, enhancing social cohesion and mental well-being in diverse communities. However, invasive subsets can provoke societal tensions, as differing perceptions—ranging from viewing them as nuisances to valued resources—hinder unified management efforts and exacerbate divides between stakeholders like conservationists, indigenous groups, and rural economies. On the health front, introduced species predominantly pose risks via disease transmission and allergen exposure. Vectors such as the Asian tiger mosquito (Aedes albopictus), introduced to the Americas and Europe, facilitate the spread of pathogens causing dengue, Zika, and West Nile virus, with outbreaks linked to thousands of human cases annually in affected areas. Other invasives induce allergies, skin irritations, or poisoning; for example, certain non-native plants like giant hogweed (Heracleum mantegazzianum) cause severe photodermatitis upon contact, leading to documented medical treatments in introduced regions. Ecosystem disruptions from invasives, such as tree loss from emerald ash borer (Agrilus planipennis), correlate with reduced urban air quality and elevated respiratory issues, indirectly contributing to higher mortality rates in vulnerable populations. While some introduced species offer health benefits—such as non-native honeybees aiding pollination for nutrient-rich crops—net effects lean negative, particularly where invasives amplify zoonotic risks or psychological stressors like phobias from encounters with aggressive species.

Management and Policy Approaches

Prevention and Biosecurity Measures

Prevention of introduced species establishment prioritizes blocking pathways of unintentional and intentional introductions, as post-establishment control is often more costly and less effective. Empirical analyses indicate that proactive measures, such as pre-border screening, yield net economic benefits by averting exceeding implementation costs, with one framework estimating benefits from prescreening aquatic introductions at ratios up to 100:1 in high-risk scenarios. International agreements form the backbone of coordinated prevention efforts. The International Convention for the Control and Management of Ships' Ballast Water and Sediments (BWM Convention), adopted in 2004 and entering into force on September 8, 2017, mandates ballast water treatment to standards that neutralize harmful aquatic organisms and pathogens, targeting a primary vector responsible for transoceanic species transfers. Complementing this, the (CBD) Guiding Principles, established in 2002, emphasize risk assessment and minimization of introductions through trade regulations and public awareness, influencing over 190 signatory nations' policies. Nationally, frameworks like the U.S. Department of the Interior's Strategic Plan (2021–2025) direct federal agencies to implement early detection and rapid response protocols, including pathway disruption for shipping, , and horticultural imports. Quarantine and inspection protocols enforce biosecurity at borders. In the United States, the USDA Animal and Plant Health Inspection Service (APHIS) requires import permits and phytosanitary certificates for plants and plant products, coupled with mandatory quarantines to detect latent pests, preventing an estimated 500,000 potential introductions annually through inspections at ports of entry. For animals, the Centers for Disease Control and Prevention (CDC) mandates quarantine periods—up to 30 days for certain mammals—if exposure to zoonotic pathogens is suspected, as updated in regulations effective August 22, 2025. These measures draw on standardized risk analysis frameworks that evaluate introduction likelihood, establishment probability, spread potential, and impact severity, categorizing species into low-, medium-, or high-risk tiers to inform permit denials. Biosecurity practices extend prevention to on-ground activities and public behavior. Protocols recommend decontaminating equipment, such as cleaning hiking gear with hot water and disinfectants before interstate transport, to interrupt soil- and hitchhiker-mediated spread, as evidenced by field trials reducing propagule transfer by over 90%. Restricting firewood movement—prohibited beyond 10–50 miles in many U.S. states since the early 2000s—curbs insect vectors like the emerald ash borer, which has infested over 25 million trees despite such rules. Voluntary initiatives, including "Check, Clean, Dry" campaigns for boating and angling gear, have documented compliance rates correlating with 70–80% reductions in aquatic species dispersal in monitored watersheds. Despite these tools, enforcement gaps persist, with non-compliance in high-volume trade pathways underscoring the need for adaptive, evidence-based refinements over rigid prohibitions.

Control, Eradication, and Mitigation

Control of introduced species typically involves integrated approaches combining mechanical, chemical, biological, and physical methods to suppress populations and limit spread. Mechanical control includes manual removal or mowing, effective for small-scale infestations of like , while chemical methods deploy herbicides or rodenticides, such as for rats, achieving high efficacy in contained areas but risking non-target impacts. Biological control introduces host-specific natural enemies, such as or pathogens, which has suppressed invasive in approximately 50% of evaluated cases globally, though failures occur due to poor establishment or inadequate host specificity. Eradication, the complete elimination of a population, succeeds in 88% of documented efforts for invasive vertebrates on islands, based on 1,550 attempts across 998 sites over the past century, often via aerial broadcasting of anticoagulants for rats or hunting/trapping for goats. Notable cases include the 2008 eradication of rats from Island using brodifacoum bait, restoring populations, and the 2019 removal of black and Norway rats from North Seymour Island in the Galápagos, confirmed via monitoring. Goats were eradicated from Santiago Island in the Galápagos by 2006 through techniques, where sterilized radio-collared individuals led hunters to herds, preventing reinvasion via fencing and patrols. These efforts yield recoveries, with abundance increasing post-eradication in over 80% of monitored sites. Mitigation strategies apply when eradication proves infeasible due to vast ranges or high costs, focusing on suppression to functional levels where ecological or economic harms are minimized. For instance, ongoing aerial baiting and maintain low densities of rats and possums in , targeting eradication by 2050 despite annual costs exceeding $100 million USD. Challenges include reinvasion risks from untreated areas, with deficiencies like insufficient funding leading to 30-40% of programs failing long-term, and unintended effects such as non-target mortality in biological controls, as seen in some fungal releases harming native . Economic analyses indicate early intervention reduces costs by up to 90% compared to delayed responses, underscoring prevention's primacy, though data gaps persist in quantifying net benefits amid variable effectiveness. Peer-reviewed syntheses emphasize , integrating monitoring to adjust tactics, as static approaches often overlook ecological feedbacks.

Policy Debates and Regulatory Frameworks

The (CBD), adopted in 1992 and ratified by 196 parties, mandates prevention of introductions of alien species that threaten , habitats, or species, with Article 8(h) requiring control or eradication of such species. The CBD's Guiding Principles on Invasive Alien Species (2002) emphasize , precaution, and approaches, influencing national policies worldwide, though implementation varies due to capacity constraints in developing nations. The Global Biodiversity Framework (2022) updates this via Target 6, aiming to reduce invasive alien species impacts by 50% by 2030 through prevention, early detection, and management, with 83% of countries lacking specific legislation as of recent assessments. In the United States, the Lacey Act (1900, amended multiple times) prohibits interstate transport and import of injurious wildlife, listing over 20 species across taxa like the (Cygnus olor) and (Boiga irregularis) based on demonstrated harm to native ecosystems or human health. Executive Order 13112 (1999) established the Invasive Species Council to coordinate federal efforts, focusing on prevention via pathways like and , though critics note inconsistent enforcement and reliance on reactive measures rather than predictive risk models. State-level regulations, surveyed across 21 eastern states, reveal 706 chapters emphasizing prohibitions on high-risk species, but with gaps in funding and harmonization, leading to fragmented responses. Australia's Biosecurity Act 2015 imposes stringent border controls and internal quarantines to prevent exotic species establishment, prioritizing pests like feral deer and rabbits through the National Priority List of Exotic Environmental Pests. This framework, informed by historical invasions such as the European rabbit (Oryctolagus cuniculus) in 1859, mandates risk-based assessments for imports, with penalties up to AUD 420,000 for breaches, reflecting a precautionary stance that has limited new vertebrate introductions but raised trade compliance costs. Policy debates center on the versus evidence-based flexibility, with proponents of strict regulations arguing that unquantified ecological risks justify bans on deliberate introductions, as seen in calls for mandatory predictive risk assessments under frameworks like the CBD. Critics, including economists, contend that aggregated cost estimates—such as the IPBES's $423 billion annual global figure—overstate harms by aggregating rare high-impact cases while ignoring benefits from species like crop pollinators or forage plants, where 16% of assessed introductions yield net positives. Polarization arises from differing valuations: environmental advocates prioritize preservation, potentially amplifying threats via institutional biases in academia toward negative framing, whereas trade-focused analyses highlight regulatory overreach stifling and , as evidenced by U.S. state surveys showing policy proliferation without proportional efficacy gains. Empirical gaps in long-term impact data fuel disputes over "" versus "whitelisting" systems, with the latter—approving only vetted species—proposed to balance innovation but rarely adopted due to administrative burdens.

Scientific Controversies

Critiques of Invasive Species Alarmism

Critics within argue that the discourse surrounding often employs alarmist rhetoric that overemphasizes potential harms while downplaying the fact that the vast majority of introduced species cause no significant ecological disruption. Empirical assessments, such as the "tens rule" proposed by ecologist Mark Williamson, indicate that approximately 10% of introduced plant species establish self-sustaining populations, and roughly 10% of those established species become invasive, resulting in fewer than 1% of all introductions posing major threats. This low incidence rate suggests that blanket condemnations of non-native species foster unnecessary fear, diverting resources from more pressing like habitat loss. Ecologist Mark A. Davis has been a prominent voice in challenging the field's foundational assumptions, contending in a 2011 commentary co-authored with 18 colleagues that conservation efforts should prioritize a ' actual environmental impacts over its geographic origin. Davis argues that ecosystems are inherently dynamic and resilient, capable of incorporating non- without collapse, and that can exhibit invasive-like behaviors, such as overabundant deer suppressing plants in North American forests. In his 2009 book Invasion Biology, Davis critiques the discipline for conflating rarity of severe invasions with inevitability, noting that many purported "invasions" reflect natural range expansions accelerated by human activity rather than novel existential threats. He further posits that alarmism stems partly from anthropocentric biases favoring "pristine" baselines that ignore pre-human biogeographic flux, leading to policies that eradicate benign or beneficial introductions at high cost. Accusations of " denialism" leveled against such critics have been contested as misrepresentations that stifle debate, with detractors like Davis acknowledging documented harms—such as the zebra mussel's disruption of fisheries—but insisting that these do not justify demonizing all non-natives. A analysis describes this labeling as a rhetorical tactic that equates skepticism of overgeneralizations with outright rejection of evidence, potentially hindering interdisciplinary progress in . For instance, studies on introduced rats in island ecosystems have questioned claims of universal devastation, finding that while predation occurs, compensatory mechanisms in prey populations and variable long-term effects often mitigate purported extinctions, challenging narratives of irreversible damage. These critiques highlight methodological flaws in impact assessments, including reliance on and failure to account for context-dependent outcomes, where a deemed invasive in one integrates harmlessly elsewhere. Proponents argue that such , amplified by media and policy incentives, promotes inefficient management strategies—like widespread chemical controls—that risk collateral damage to non-target and ignore opportunities for leveraging introduced taxa in restoration, such as nitrogen-fixing plants aiding degraded soils. Overall, this perspective urges a shift toward evidence-based of net effects, cautioning against ideologically driven nativism that could impede adaptive conservation in a changing .

Challenges to Native-Non-Native Dichotomies

The of as native or non-native relies on historical benchmarks that are often anthropocentric and temporally arbitrary, such as defining nativity relative to periods before European in the or . This approach overlooks paleoecological evidence of past natural migrations during glacial-interglacial cycles, where distributions shifted dynamically without human mediation, rendering strict dichotomies inconsistent with long-term biogeographical flux. In a 2011 open letter in Nature, 19 ecologists led by Mark A. Davis argued that the native-alien framework has diminishing utility in conservation, as it predisposes negative judgments against non-native regardless of their ecological roles; they advocated assessing based on current environmental impacts rather than origins, noting that non-natives frequently enhance local without causing harm. Empirical support includes the observation that introduced constitute a substantial portion of regional biota in many ecosystems—up to 20-30% in some studies—yet the majority integrate functionally without disrupting natives. Climate change exacerbates these challenges by driving poleward and upslope range expansions in tens of thousands of , mimicking patterns traditionally labeled as invasions but attributable to abiotic factors like warming temperatures rather than human transport. For example, barred owls (Strix varia) have expanded westward across since the mid-20th century, prompting costly eradication efforts (e.g., over $5 million in targeted removals) despite evidence of climate facilitation in their dispersal, which could aid adaptation. Similarly, such as and have entered Mediterranean forests via warming waters and openings, where they perform herbivory roles akin to extinct native equivalents, challenging origin-based exclusion. The "tens rule"—an empirical indicating that roughly 10% of introduced establish populations and only a subset of those (about 1% overall) cause significant ecological or economic damage—underscores that origin correlates weakly with adverse effects, as can exhibit invasive behaviors under altered conditions (e.g., post-disturbance booms). Critics like Franz Essl propose intermediate categories such as "neonatives" for shifting over 100 km due to anthropogenic , emphasizing functional equivalence over historical pedigree to avoid policies that impede resilience. This perspective aligns with first-principles evaluation of causal mechanisms, prioritizing verifiable interactions like or facilitation irrespective of biogeographic labels.

Empirical Gaps in Impact Assessment

Assessing the ecological and economic impacts of introduced species often suffers from methodological biases that limit comprehensive understanding. Experimental studies, spanning from to 2014 and encompassing over 400 manipulations, disproportionately focus on aquatic systems (71% of cases) and macroorganisms like and vertebrates, while underrepresenting terrestrial habitats, microbes, and biota. This skew restricts insights into broader dynamics and may overestimate impacts in underrepresented contexts. Causal attribution remains challenging due to reliance on observational or short-term data, which fail to isolate introduced species effects from confounders such as habitat alteration or native species shifts. For invasive alien plants, assessments lack standardized quantification methods, leading to inconsistent evaluations across regions and ecosystems; impacts are frequently inferred from well-studied species, biasing perceptions toward harm while overlooking neutral or context-dependent outcomes. In Europe, field studies on invasive plants report significant effects in only 43% of cases, with decreases in native responses outnumbering increases but many interactions unquantified. Economic evaluations exacerbate gaps by incorporating ancillary costs (e.g., research and monitoring, comprising 80-90% of totals) and ineffective management expenditures while excluding benefits like enhanced services. Global cost estimates, often cited in trillions of dollars, overlook counterfactual scenarios and positive contributions, such as invasive mussels improving and boosting values by approximately $14 billion in affected U.S. regions. is further muddled, as introduced species are sometimes misidentified as primary drivers rather than secondary factors in declines, with only 6.2% of listings directly attributing threats to them. Taxon-specific evidence reveals additional voids; for invasive birds in , risk assessments draw heavily on anecdotal reports rather than rigorous data, yielding inflated environmental and economic scores that drop significantly upon re-evaluation (e.g., from 1-15 to 0-10 for environmental impacts). Overall, while a small proportion of introduced species—estimated at less than 10% that establish and fewer causing verifiable harm—drive notable changes, the absence of systematic long-term monitoring for the majority perpetuates in impact .

Notable Examples and Case Studies

Successful and Beneficial Introductions

The introduction of the European honey bee (Apis mellifera) to in 1622 by European colonists has proven highly successful, establishing widespread feral and managed populations that provide essential services for . These bees pollinate approximately one-third of U.S. food crops, including fruits, nuts, and , contributing an estimated $11.68 billion to the as of 2009, with total pollination value exceeding $15 billion annually in recent assessments. Without this introduced species, many commercial orchards and field crops would require alternative, costlier methods, underscoring its role in sustaining food production systems. In agricultural settings, non-native earthworms, primarily European species such as , introduced via and transport since the , have enhanced soil aeration, nutrient cycling, and water infiltration, leading to improved crop yields in tilled lands. These earthworms decompose more efficiently than many native , releasing nutrients like and faster, which supports higher productivity in farmlands across the Midwest and . Empirical studies confirm reduced and increased fertility in managed fields, though benefits are context-dependent and less pronounced in undisturbed forests. White sweet clover (Melilotus albus), introduced to from in the mid-18th century, has succeeded as a and conditioner, fixing atmospheric through to enrich poor or depleted soils. Widely planted for hay, pasture improvement, and , it supports and has been recommended for conservation plantings in the and Midwest, where it establishes readily and outcompetes weeds in disturbed areas without requiring fertilizers. U.S. Department of Agriculture guides highlight its value in rehabilitating marginal lands, with production enabling rotations that boost subsequent yields by 20-30% in some trials. Deliberate introductions of dung beetles (subfamily Scarabaeinae), primarily African and European species, to from 1964 to 1991 addressed the accumulation of undecomposed dung, which covered up to 90% of pastures and harbored pests like bushflies. Over 40 were released, with at least 23 establishing populations that reduced dung pad persistence from months to days, decreasing fly breeding sites by up to 80% and loads in , thereby increasing and meat production by millions of kilograms annually. This program, coordinated by the Commonwealth Scientific and Industrial Research Organisation, demonstrates targeted introductions yielding measurable ecological and economic gains in pastoral systems.

Problematic and High-Impact Cases

Introduced in to , , as a biological control for sugarcane pests, the (Rhinella marina) has spread across much of the continent, causing significant declines in native predator populations through toxic ingestion. Studies indicate that lethal poisoning of frog-eating species, such as the (Dasyurus hallucatus) and several snake taxa, represents the primary impact pathway, with local extirpations observed in invaded areas. Broader analyses attribute part of 's annual A$24.5 billion costs to such amphibians, though direct economic quantification for cane toads remains integrated within pest aggregates. Accidentally introduced to the via ballast water in the late 1980s, zebra mussels (Dreissena polymorpha) have proliferated, fouling infrastructure and altering ecosystems with annual management costs exceeding $500 million in the region. Their has imposed $3.1 billion in damages on the power industry alone from 1993 to 1999, including pipe cloggings and water intake disruptions costing up to $60 million yearly in maintenance. Ecologically, dense mussel beds filter vast water volumes, reducing availability and shifting food webs, though some studies note compensatory effects in certain fish populations. The brown treesnake (Boiga irregularis), likely transported to Guam via military cargo post-World War II, has driven the extirpation of 10 of 12 native forest bird species by the through predation, with population crashes exceeding 90% for survivors like the Mariana fruit dove. Indirect effects include reduced , impairing forest regeneration and contributing to canopy tree declines observed in surveys. Lizard populations have also plummeted, altering arthropod abundances and vegetation dynamics, while human safety costs from snakebites number in the thousands annually. Released intentionally in 1859 near , Victoria, European rabbits (Oryctolagus cuniculus) exploded to hundreds of millions, devastating arid ecosystems through and burrowing that exacerbates . They threaten at least 304 native plant and animal species via and habitat degradation, contributing to biodiversity losses in grasslands where native perennial grasses have been replaced by less palatable exotics. Agricultural damages include crop destruction and reduced pasture productivity, with control efforts like in 1950 temporarily halving populations but failing to eradicate the pest.

Extraterrestrial and Emerging Contexts

Introductions on Other Planetary Bodies

Human missions and robotic probes have inadvertently introduced terrestrial microorganisms and multicellular organisms to the Moon, raising concerns about forward contamination under guidelines established by the (COSPAR). The most notable recent incident occurred on April 11, 2019, when Israel's crashed near the Sea of Serenity, dispersing approximately 100 million dehydrated tardigrades (Hypsibius exemplaris) included in the Arch Mission Foundation's Lunar Library payload. These microscopic animals, known for entering a cryptobiotic tun state that confers resistance to vacuum, radiation, and desiccation, were not sterilized prior to launch, as the mission was privately funded and not subject to full standards. Survival of these tardigrades on the lunar surface remains improbable due to unrelenting ultraviolet radiation, extreme temperature fluctuations from -173°C to 127°C, and absence of liquid , though models suggest dormant viability in shadowed craters could persist for years if shielded from solar exposure. Earlier evidence of microbial endurance came from NASA's mission in November 1969, which retrieved a camera from the 1967 lander; culturing revealed viable Streptococcus mitis bacteria that had withstood 31 months in lunar conditions, likely originating from pre-launch assembly rather than post-landing growth. Such findings underscore the resilience of select terrestrial microbes, with recent simulations indicating that endospore-forming bacteria like Bacillus subtilis could remain viable in the Moon's permanently shadowed regions for up to 140,000 years, protected from UV and thermal extremes. No confirmed introductions of Earth organisms have occurred on other planetary bodies like Mars or Venus, owing to rigorous sterilization protocols for missions such as NASA's Perseverance rover, which limit bioburden to fewer than 500,000 spores per spacecraft. Unintentional contamination risks persist from failed probes, but COSPAR Category IV requirements for Mars mandate bioburden reduction to prevent interference with indigenous life detection; for instance, the 1999 Mars Polar Lander crash posed theoretical risks, though no viable organisms were later evidenced. These events highlight tensions between exploration imperatives and the need to preserve extraterrestrial environments for scientific integrity, with tardigrades and bacteria representing de facto introduced species whose long-term ecological impact, if any, is negligible absent reproduction-enabling conditions.

Interactions with Climate Change and Biotechnology

Climate change influences the establishment, spread, and ecological impacts of introduced species by altering temperature regimes, precipitation patterns, and habitat suitability, often favoring non-native taxa with broad physiological tolerances. Warmer conditions and extended growing seasons have enabled range expansions of invasive plants and animals; for instance, in the , climate-driven stream warming has facilitated the upstream invasion of non-native ( trutta), contributing to a 24-48% decline in native ( clarkii) populations in affected streams from 1980 to 2010. Similarly, elevated atmospheric CO2 levels enhance the growth and competitiveness of certain invasive plants, such as cheatgrass (), which has expanded across North American rangelands partly due to CO2 fertilization effects documented in controlled experiments showing up to 40% increases under doubled CO2 concentrations. These shifts create synergistic stressors, where introduced species exacerbate climate vulnerabilities, such as by altering fire regimes or reducing native resilience. Conversely, some introduced species exhibit heightened sensitivity to extremes compared to natives; meta-analyses of experiments indicate that non-native experience 25.9% greater reductions under conditions than co-occurring natives, potentially limiting their dominance in aridifying regions. Introduced species may also assist in efforts, such as non-native trees used in for , though their long-term ecological costs remain debated. Overall, predictive models project that by 2050, up to 80% of tracked in could expand ranges due to warming, necessitating integrated management that accounts for these dynamics rather than static native-non-native classifications. Biotechnology offers targeted interventions for managing problematic introduced species, particularly through genetic modifications that suppress populations without broad environmental disruption. CRISPR-based gene drives, which bias inheritance to spread engineered traits rapidly through populations, have been modeled to eradicate invasive rodents on islands; simulations for mice (Mus musculus) on Island predict near-total suppression within 5-10 years of release, potentially restoring native breeding sites affected since the species' 19th-century introduction. For aquatic invasives, synthetic incompatibility systems—genetic engineering rendering matings between modified and wild individuals infertile—target species like ( spp.) in the , with lab trials demonstrating over 90% fertility reduction in hybrid offspring. These tools intersect with climate change by addressing invasives whose ranges expand under warming; for example, gene drives could counter mosquito (Aedes aegypti) proliferation in tropicalizing regions, where temperature increases have boosted competence, as evidenced by field data linking 1°C warming to 10-20% higher dengue transmission rates. However, deployment requires rigorous containment to prevent unintended spread, with regulatory frameworks emphasizing site-specific risk assessments; peer-reviewed evaluations highlight low escape probabilities under modeled scenarios but underscore ecological unknowns, such as off-target effects in non-target . Complementary approaches, like sterile insect releases enhanced by genetic markers for tracking, have reduced invasive fruit fly () populations by 80-95% in Pacific island trials since 2015, offering scalable precedents adaptable to climate-altered invasion fronts.

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