Invasive species

An invasive species is a species that is not native to a specific location (an introduced species), and that has a tendency to spread to a degree believed to cause damage to the environment, human economy or human health. The criteria for invasive species has been controversial, as widely divergent perceptions exist among researchers as well as concerns with the subjectivity of the term “invasive”. Several alternate usages of the term have been proposed. The term as most often used applies to introduced species (also called “non-indigenous” or “non-native”) that adversely affect the habitats and bioregions they invade economically, environmentally, or ecologically. Such invasive species may be either plants or animals and may disrupt by dominating a region, wilderness areas, particular habitats, or wildland–urban interface land from loss of natural controls (such as predators or herbivores). This includes non-native invasive plant species labeled as exotic pest plants and invasive exotics growing in native plant communities. It has been used in this sense by government organizations as well as conservation groups such as the International Union for Conservation of Nature (IUCN) and the California Native Plant Society. The European Union defines “Invasive Alien Species” as those that are, firstly, outside their natural distribution area, and secondly, threaten biological diversity.

The term is also used by land managers, botanists, researchers, horticulturalists, conservationists, and the public for noxious weeds. The kudzu vine (Pueraria lobata), Andean pampas grass (Cortaderia jubata), and yellow starthistle (Centaurea solstitialis) are examples. An alternate usage broadens the term to include indigenous or “native” species along with non-native species, that have colonized natural areas. Deer are an example, considered to be overpopulating their native zones and adjacent suburban gardens, by some in the Northeastern and Pacific Coast regions of the United States. Sometimes the term is used to describe a non-native or introduced species that has become widespread. However, not every introduced species has adverse effects on the environment. A nonadverse example is the common goldfish (Carassius auratus), which is found throughout the United States, but rarely achieves high densities. Notable examples of invasive species include European rabbits, grey squirrels, domestic cats, carp and ferrets.

Dispersal and subsequent proliferation of species is not solely an anthropogenic phenomenon. There are many mechanisms by which species from all Kingdoms have been able to travel across continents in short periods of time such as via floating rafts, or on wind currents. Charles Darwin performed many experiments to better understand long distance seed dispersal, and was able to germinate seeds from insect frass, faeces of waterfowl, dirt clods on the feet of birds, all of which may have traveled significant distances under their own power, or be blown off course by thousands of miles.

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

Scientists include species and ecosystem factors among the mechanisms that, when combined, establish invasiveness in a newly introduced species.

Species-based mechanisms
While all species compete to survive, invasive species appear to have specific traits or specific combinations of traits that allow them to outcompete native species. In some cases, the competition is about rates of growth and reproduction. In other cases, species interact with each other more directly.

Researchers disagree about the usefulness of traits as invasiveness markers. One study found that of a list of invasive and noninvasive species, 86% of the invasive species could be identified from the traits alone. Another study found invasive species tended to have only a small subset of the presumed traits and that many similar traits were found in noninvasive species, requiring other explanations. Common invasive species traits include the following:

Fast growth
Rapid reproduction
High dispersal ability
Phenotype plasticity (the ability to alter growth form to suit current conditions)
Tolerance of a wide range of environmental conditions (Ecological competence)
Ability to live off of a wide range of food types (generalist)
Association with humans
Prior successful invasions

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

An introduced species might become invasive if it can outcompete native species for resources such as nutrients, light, physical space, water, or food. If these species evolved under great competition or predation, then the new environment may host fewer able competitors, allowing the invader to proliferate quickly. Ecosystems in which are being used to their fullest capacity by native species can be modeled as zero-sum systems in which any gain for the invader is a loss for the native. However, such unilateral competitive superiority (and extinction of native species with increased populations of the invader) is not the rule. Invasive species often coexist with native species for an extended time, and gradually, the superior competitive ability of an invasive species becomes apparent as its population grows larger and denser and it adapts to its new location.

An invasive species might be able to use resources that were previously unavailable to native species, such as deep water sources accessed by a long taproot, or an ability to live on previously uninhabited soil types. For example, barbed goatgrass (Aegilops triuncialis) was introduced to California on serpentine soils, which have low water-retention, low nutrient levels, a high magnesium/calcium ratio, and possible heavy metal toxicity. Plant populations on these soils tend to show low density, but goatgrass can form dense stands on these soils and crowd out native species that have adapted poorly to serpentine soils.

Invasive species might alter their environment by releasing chemical compounds, modifying abiotic factors, or affecting the behaviour of herbivores, creating a positive or negative impact on other species. Some species, like Kalanchoe daigremontana, produce allelopathic compounds, that might have an inhibitory effect on competing species, and influence some soil processes like carbon and nitrogen mineralization. Other species like Stapelia gigantea facilitates the recruitment of seedlings of other species in arid environments by providing appropriate microclimatic conditions and preventing herbivory in early stages of development.

Other examples are Centaurea solstitialis (yellow starthistle) and Centaurea diffusa (diffuse knapweed). These Eastern European noxious weeds have spread through the western and West Coast states. Experiments show that 8-hydroxyquinoline, a chemical produced at the root of C. diffusa, has a negative effect only on plants that have not co-evolved with it. Such co-evolved native plants have also evolved defenses. C. diffusa and C. solstitialis do not appear in their native habitats to be overwhelmingly successful competitors. Success or lack of success in one habitat does not necessarily imply success in others. Conversely, examining habitats in which a species is less successful can reveal novel weapons to defeat invasiveness.

Changes in fire regimens are another form of facilitation. Bromus tectorum, originally from Eurasia, is highly fire-adapted. It not only spreads rapidly after burning but also increases the frequency and intensity (heat) of fires by providing large amounts of dry detritus during the fire season in western North America. In areas where it is widespread, it has altered the local fire regimen so much that native plants cannot survive the frequent fires, allowing B. tectorum to further extend and maintain dominance in its introduced range.

Facilitation also occurs where one species physically modifies a habitat in ways that are advantageous to other species. For example, zebra mussels increase habitat complexity on lake floors, providing crevices in which invertebrates live. This increase in complexity, together with the nutrition provided by the waste products of mussel filter-feeding, increases the density and diversity of benthic invertebrate communities.

Ecosystem-based mechanisms
In ecosystems, the amount of available resources and the extent to which those resources are used by organisms determines the effects of additional species on the ecosystem. In stable ecosystems, equilibrium exists in the use of available resources. These mechanisms describe a situation in which the ecosystem has suffered a disturbance, which changes the fundamental nature of the ecosystem.

When changes such as a forest fire occur, normal succession favors native grasses and forbs. An introduced species that can spread faster than natives can use resources that would have been available to native species, squeezing them out. Nitrogen and phosphorus are often the limiting factors in these situations.

Every species occupies a niche in its native ecosystem; some species fill large and varied roles, while others are highly specialized. Some invading species fill niches that are not used by native species, and they also can create new niches. An example of this type can be found within the Lampropholis delicata species of skink.

Ecosystem changes can alter species’ distributions. For example, edge effects describe what happens when part of an ecosystem is disturbed as when land is cleared for agriculture. The boundary between remaining undisturbed habitat and the newly cleared land itself forms a distinct habitat, creating new winners and losers and possibly hosting species that would not thrive outside the boundary habitat.

One interesting finding in studies of invasive species has shown that introduced populations have great potential for rapid adaptation and this is used to explain how so many introduced species are able to establish and become invasive in new environments. When bottlenecks and founder effects cause a great decrease in the population size and may constrict genetic variation, the individuals begin to show additive variance as opposed to epistatic variance. This conversion can actually lead to increased variance in the founding populations which then allows for rapid adaptive evolution. Following invasion events, selection may initially act on the capacity to disperse as well as physiological tolerance to the new stressors in the environment. Adaptation then proceeds to respond to the selective pressures of the new environment. These responses would most likely be due to temperature and climate change, or the presence of native species whether it be predator or prey. Adaptations include changes in morphology, physiology, phenology, and plasticity.

Rapid adaptive evolution in these species leads to offspring that have higher fitness and are better suited for their environment. Intraspecific phenotypic plasticity, pre-adaptation and post-introduction evolution are all major factors in adaptive evolution. Plasticity in populations allows room for changes to better suit the individual in its environment. This is key in adaptive evolution because the main goal is how to best be suited to the ecosystem that the species has been introduced. The ability to accomplish this as quickly as possible will lead to a population with a very high fitness. Pre-adaptations and evolution after the initial introduction also play a role in the success of the introduced species. If the species has adapted to a similar ecosystem or contains traits that happen to be well suited to the area that it is introduced, it is more likely to fare better in the new environment. This, in addition to evolution that takes place after introduction, all determine if the species will be able to become established in the new ecosystem and if it will reproduce and thrive.

Traits of invaded ecosystems
In 1958, Charles S. Elton claimed that ecosystems with higher species diversity were less subject to invasive species because of fewer available niches. Other ecologists later pointed to highly diverse, but heavily invaded ecosystems and argued that ecosystems with high species diversity were more susceptible to invasion.

This debate hinged on the spatial scale at which invasion studies were performed, and the issue of how diversity affects susceptibility remained unresolved as of 2011. Small-scale studies tended to show a negative relationship between diversity and invasion, while large-scale studies tended to show the reverse. The latter result may be a side-effect of invasives’ ability to capitalize on increased resource availability and weaker species interactions that are more common when larger samples are considered.

Invasion was more likely in ecosystems that were similar to the one in which the potential invader evolved. Island ecosystems may be more prone to invasion because their species faced few strong competitors and predators, or because their distance from colonizing species populations makes them more likely to have “open” niches. An example of this phenomenon was the decimation of native bird populations on Guam by the invasive brown tree snake. Conversely, invaded ecosystems may lack the natural competitors and predators that check invasives’ growth in their native ecosystems.

Invaded ecosystems may have experienced disturbance, typically human-induced. Such a disturbance may give invasive species a chance to establish themselves with less competition from natives less able to adapt to a disturbed ecosystem.

Non-native species have many vectors, including biogenic vectors, but most invasions are associated with human activity. Natural range extensions are common in many species, but the rate and magnitude of human-mediated extensions in these species tend to be much larger than natural extensions, and humans typically carry specimens greater distances than natural forces.

An early human vector occurred when prehistoric humans introduced the Pacific rat (Rattus exulans) to Polynesia.

Vectors include plants or seeds imported for horticulture. The pet trade moves animals across borders, where they can escape and become invasive. Organisms stow away on transport vehicles.

The arrival of invasive propagules to a new site is a function of the site’s invasibility.

Species have also been introduced intentionally. For example, to feel more “at home,” American colonists formed “Acclimation Societies” that repeatedly imported birds that were native to Europe to North America and other distant lands. In 2008, U.S. postal workers in Pennsylvania noticed noises coming from inside a box from Taiwan; the box contained more than two dozen live beetles. Agricultural Research Service entomologists identified them as rhinoceros beetle, hercules beetle, and king stag beetle. Because these species were not native to the U.S., they could have threatened native ecosystems. To prevent exotic species from becoming a problem in the U.S., special handling and permits are required when living materials are shipped from foreign countries. USDA programs such as Smuggling Interdiction and Trade Compliance (SITC) attempt to prevent exotic species outbreaks in America.

Many invasive species, once they are dominant in the area, are essential to the ecosystem of that area. If they are removed from the location it could be harmful to that area.

Economics plays a major role in exotic species introduction. High demand for the valuable Chinese mitten crab is one explanation for the possible intentional release of the species in foreign waters.

Within the Aquatic Environment
The development of maritime trade has rapidly affected the way marine organisms are transported within the ocean. Two ways marine organisms are transported to new environments are hull fouling and ballast water transport. In fact, Molnar et al. 2008 documented the pathways of hundreds of marine invasive species and found that shipping was the dominant mechanism for the transfer of invasive species.

Many marine organisms have the capacity to attach themselves to vessel hulls. Therefore, these organisms are easily transported from one body of water to another and are a significant risk factor for a biological invasion event. Unfortunately, controlling for vessel hull fouling is voluntary and there are no regulations currently in place to manage hull fouling. However, California and New Zealand have announced more stringent control for vessel hull fouling within their respective jurisdictions.

The other main vector for the transport of non-native aquatic species is ballast water. Ballast water taken up at sea and released in port by transoceanic vessels is the largest vector for non-native aquatic species invasions. In fact, it is estimated that 10,000 different species, many of which are non-indigenous, are transported via ballast water each day. Many of these species are considered harmful and can negatively impact their new environment. For example, freshwater zebra mussels, native to the Black, Caspian and Azov seas, most likely reached the Great Lakes via ballast water from a transoceanic vessel. Zebra mussels outcompete other native organisms for oxygen and food, such as algae. Although the zebra mussel invasion was first noted in 1988, and a mitigation plan was successfully implemented shortly thereafter, the plan had a serious flaw or loophole, whereby ships loaded with cargo when they reached the Seaway were not tested because their ballast water tanks were empty. However, even in an empty ballast tank, there remains a puddle of water filled with organisms that could be released at the next port (when the tank is filled with water after unloading the cargo, the ship takes on ballast water which mixes with the puddles and then everything including the living organisms in the puddles is discharged at the next port). Current regulations for the Great Lakes rely on ‘salinity shock’ to kill freshwater organisms left in ballast tanks.

Even though ballast water regulations are in place to protect against potentially invasive species, there exists a loophole for organisms in the 10-50 micron size class. For organisms between 10 and 50 microns, such as certain types of phytoplankton, current regulations allow less than 10 cells per milliliter be present in discharge from treatment systems. The discharge gets released when a ship takes on cargo at a port so the discharged water is not necessarily the same as the receiving body of water. Since many species of phytoplankton are less than 10 microns in size and reproduce asexually, only one cell released into the environment could exponentially grow into many thousands of cells over a short amount of time. This loophole could have detrimental effects to the environment. For example, some species in the genus Pseudo-nitzschia are smaller than 10 microns in width and contain domoic acid, a neurotoxin. If toxic Pseudo-nitzschia spp. are alive in ballast discharge and get released into their “new environment” they could cause domoic acid poisoning in shellfish, marine mammals and birds. Fortunately, human deaths related to domoic acid poisoning have been prevented because of stringent monitoring programs that arose after a domoic acid outbreak in Canada in 1987. Ballast water regulations need to be more rigorous to prevent future ramifications associated with the potential release of toxic and invasive phytoplankton.

Another important factor to consider about marine invasive species is the role of environmental changes associated with climate change, such as an increase in ocean temperature. There have been multiple studies suggesting an increase in ocean temperature will cause range shifts in organisms, which could have detrimental effects on the environment as new species interactions emerge. For example, Hua and Hwang proposed that organisms in a ballast tank of a ship traveling from the temperature zone through tropical waters can experience temperature fluctuations as much as 20 °C. To further examine the effects of temperature on organisms transported on hulls or in ballast water, Lenz et al. (2018) carried out study where they conducted a double heat stress experiment. Their results suggest that heat challenges organisms face during transport may enhance the stress tolerance of species in their non-native range by selecting for genetically adapted genotypes that will survive a second applied heat stress, such as increased ocean temperature in the founder population. Due to the complexity of climate change induced variations, it is difficult to predict the nature of temperature-based success of non-native species in-situ. Since some studies have suggested increased temperature tolerance of “hijackers” on ships’ hulls or in ballast water, it is necessary to develop more comprehensive fouling and ballast water management plans in an effort to prevent against future possible invasions as environmental conditions continue to change around the world.

Impacts of wildfire and firefighting
Invasive species often exploit disturbances to an ecosystem (wildfires, roads, foot trails) to colonize an area. Large wildfires can sterilize soils, while adding a variety of nutrients. In the resulting free-for-all, formerly entrenched species lose their advantage, leaving more room for invasives. In such circumstances plants that can regenerate from their roots have an advantage. Non-natives with this ability can benefit from a low intensity fire burns that removes surface vegetation, leaving natives that rely on seeds for propagation to find their niches occupied when their seeds finally sprout.

Wildfires often occur in remote areas, needing fire suppression crews to travel through pristine forest to reach the site. The crews can bring invasive seeds with them. If any of these stowaway seeds become established, a thriving colony of invasives can erupt in as few as six weeks, after which controlling the outbreak can need years of continued attention to prevent further spread. Also, disturbing the soil surface, such as cutting firebreaks, destroys native cover, exposes soil, and can accelerate invasions. In suburban and wildland-urban interface areas, the vegetation clearance and brush removal ordinances of municipalities for defensible space can result in excessive removal of native shrubs and perennials that exposes the soil to more light and less competition for invasive plant species.

Fire suppression vehicles are often major culprits in such outbreaks, as the vehicles are often driven on back roads overgrown with invasive plant species. The undercarriage of the vehicle becomes a prime vessel of transport. In response, on large fires, washing stations “decontaminate” vehicles before engaging in suppression activities. Large wildfires attract firefighters from remote places, further increasing the potential for seed transport.

biodiversity observatories and organizations like the botanical conservatories, water agencies, museums, NGOs and naturalists monitor more the appearance and spread of invasive species and inform the public.

The participatory science and citizens are mobilized, thanks to the smartphone that can contribute to the inventories, for example under the Interreg project RINSE via a smartphone app (“Th @ s Invasive”; free, available in French or English, and easily downloadable) allowing everyone to identify and map a large number of invasive alien species, by photographing the species in question (which will be geo-referenced by the GPS of the smartphone and sent by the software once confirmed by the eco-citizenparticipating in this general and permanent inventory that aims to limit the negative impacts of so-called “invasive” species. Better monitoring the geographical extension of these species will accelerate or even anticipate the responses, which will then be less expensive, and thus limit some of the negative effects of these phenomena of outbreaks. This is possible thanks to a preliminary work of assistance to the interactive identification by visual identification key of non-native plants. He has been in the ecozone that includes Germany, the Netherlands, Belgium and the great North-West of France and it can be used in other regions and countries, where these species would be regulated, or where they can be sought by Customs as “commercial export contaminants” (eg, “contamination with seeds in food for birds, weeds in bonsai”… Except for algae and mosses, these interactive keys are related to species information of the “Q-bank database Invasive Plants” (descriptive and informative cards, to cards area global distribution, molecular barcode when it is available, etc.

The dendrochronology has recently applied to certain grasses (perennial). It can help to retrospectively understand the dynamics of an invasive species population and refine future growth scenarios.


It is difficult to predict a priori which species can invade and impact ecosystems, nor which ecosystems are most vulnerable and sensitive to invasive species. Currently, new species of plants for gardening or farmed fish are still imported, among many others. For this reason, and in order to prevent future invasions, it is essential to increase control over the introduction routes or prohibit the importation or introduction of those species that may generate large impacts. Consequently, it is important to have an adequate legal framework. All introduced species are susceptible to escape to natural habitats and establish themselves. Therefore, the ability to rapidly detect biological invasions is essential for their eradication to be truly effective.

Early detection and rapid response
When prevention has failed, the second step from which to push the fight against biological invasions is that of early detection and rapid response. A step, whose principle is based on the intention to act before a major evil, ie before there are more individuals of the extractables or these occupy areas larger than their control advises, therefore, despite having a Preventive character, we want to denote that the main objective of this operative modality, is to prevent the establishment and / or propagation of introduced species. Rarely, introductions occur with such a number of cash and circumstances so favorable as to be able to speak from the beginning, of invasion, but rather there is a period where these species focus on survival, beyond colonization, a period where they are particularly vulnerable and where the costs of extraction are considerably lower than those of future eradication and control. However, we must not forget when giving a quick response, that this should not be precipitated, because given the complexity of interspecific relationships, we can not act until we are sure that the extraction of the invasive organism is indeed beneficial.

A tool that can facilitate this work is the application of the method of Determination of the Suitability of Action in Exotic Species by GAGO matrix, supported by already existing and easily accessible information, in a way that reduces the costs and the time involved in carrying out new specific studies. Do not forget to give a quick response, that this should not be precipitated, because given the complexity of interspecific relationships, we can not act until we are sure that the extraction of the invasive organism is indeed beneficial. A tool that can facilitate this work is the application of the method of Determination of the Suitability of Action in Exotic Species by GAGO matrix, supported by already existing and easily accessible information, in a way that reduces the costs and the time involved in carrying out new specific studies.

The complete eradication of an exotic species is sometimes possible, especially if you have a good knowledge of the species, reproduction, life cycle and if it has caused invasions in other parts of the planet to know the best way to act.

It has been possible to eradicate some potentially harmful alien species, such as, for example, the giant African snail. This plague for agriculture in many areas of Asia and the Pacific, was exterminated thanks to the campaigns held against the populations established in Florida and Australia. However, other projects have been so disastrous that they have even worsened the problem. Therefore, when an eradication process is to be carried out, a thorough study of the species and all the factors involved in the invasion must be carried out beforehand.

When the eradication of a species fails or is not possible, the populations of that species are controlled at acceptable levels so that the ecological and socioeconomic damages are as small as possible. There are three control methods that are often used, individually or in combination: the chemical, the mechanical and the biological.

Chemical control: It is probably the main method used to combat toxic pests in agriculture. For example, in United States the pesticides were able to successfully control a parasitic weed roots. But chemical controls also carry many problems, such as risks to human health and to local biodiversity. In addition, it is important to consider the possibility that many species may develop resistance to the pesticide.

Physical or mechanical control: There are certain species that can only be treated directly, by extracting them mechanically. This method is only effective when the invaded area is small. In the case of the knife grass (plant of the genus Carpobrotus), it has been tried to eliminate it with hoes to contain its expansion, since no other method was appropriate. In the successful eradication of the giant African snails of Florida and Australia, one of the crucial factors was the collection by hand of the individuals. Hunting can also be considered as a mechanical method to keep exotic animal populations under control, as in the case of hunting and trapping used to control populations of exotic small mammals.New Zealand. However, hunting alone is unlikely to be an effective control method. In addition, the difficulty of finding organisms and the expenses of equipment for their extraction or hunting make it impossible to apply this type of control in many cases.

Biological control: As we explained before, one of the causes of uncontrolled expansion of species is the fact that they come without their natural predators. Therefore, a formula to control their populations is to introduce natural enemies into the new ecosystem. This has been successful in some cases, although it must be done in a very controlled manner because the introduction of an exotic species always poses a risk to the native community. The invasion of Hypericum (Hypericum perforatum) in the United States was controlled by the introduction of a herbivorous scarab of the genus Chrysolina that feeds on this plant.

Source from Wikipedia