Everything about Invasive Species totally explained
Invasive species is a phrase with several definitions. The first definition expresses the phrase in terms of
non-indigenous species (for example plants or animals) that adversely affect the habitats they invade economically, environmentally or ecologically. It has been used in this sense by government organizations as well as conservation groups such as the
IUCN.
The second definition broadens the boundaries to include both
native and
non-native species that heavily
colonize a particular habitat. Another study found that invasive species tended to only have a small subset of the invasive traits and that many of these invasive traits were found in non-invasive species as well indicating that invasiveness involves complex interaction not easily categorized. Common invasive species traits include fast growth, rapid
reproduction, high
dispersal ability,
phenotypic plasticity (the ability to alter one’s growth form to suit current conditions), tolerance of a wide range of environmental conditions, ability to live off of a wide range of food types,
asexual reproduction, and association with humans. The single best predictor of invasiveness, however, is whether or not the species has been invasive elsewhere. Typically an introduced species must survive at low population densities before it becomes invasive in a new location. At low population densities, it's often difficult for the introduced species to reproduce and maintain its self in a new location, but often due to human actions a species might be transported to a location a number of times before it become established. Humans repeated patterns of movement from one location to another, such as ships sailing to and from ports or cars driving up and down highways, allow for species to have multiple opportunities for establishment (also known as a high “
propagule pressure”).
An introduced species might become invasive if it can out compete native species for resources such as nutrients, light, physical space, water or food. Some species when introduced into a new environment lack the competition and predation they evolved under in their native environments freeing them to proliferate quickly. Ecosystems where all available resources are being used to their full capacity by native species can be modeled as zero-sum systems, where any gain for the invader is a loss for the native. However, such unilateral competitive superiority (and instant, equivalent extinction of native species with increased populations of the invader) isn't the rule. Invasive species often coexist with native species for an extended time and gradually the superior competitive ability of an invasive species become apparent when its population grows larger and denser often after it adapts to its new location.
An invasive species might be able to use resources 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,
barb goatgrass (
Aegilops triuncialis), can be found in its introduced range in California on
serpentine soils, which have low water-holding capacity, low nutrients, high Mg/Ca 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 crowding out native species that have not adapted well to growing on serpentine soils. Invasive species are either plant or animal, from another area and over-compete other native species living in the area.
Facilitation is the mechanism by which some species can alter their environment through chemicals or manipulation of
abiotic factors, usually to make it less favorable for other species to compete against them, allowing the species to grow or reproduce. One such facilitative mechanism is allelopathy, also known as chemical competition. In allelopathy a plant or in
Interference Competition a bacterium will secrete chemicals which make the surrounding soil uninhabitable, or at least inhibitory, to other competing species.
One example of this is the
knapweed (
Centaurea diffusa). This
Eastern European weed has spread its way through the western United 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
C. diffusa. Such co-evolved native plants have also evolved defenses, and
C. diffusa doesn't appear in its native habitat to be an overwhelmingly successful competitor. This result shows how difficult it can be to predict whether a species will be invasive just from looking at its behavior in its native habitat, and demonstrates the potential for novel weapons to aid in invasiveness ).
Changes in fire regimes are another form of facilitation.
Bromus tectorum, originally from Eurasia, is highly fire-adapted. It not only spreads rapidly after burning, but actually increases the frequency and intensity (heat) of fires, by providing large amounts of dry
detritus during the dry fire season in western North America. In areas where it's widespread, it has altered the local fire regime so much that native plants can't survive the frequent fires, allowing
B. tectorum to further extend and maintain dominance in its introduced range.
Facilitation also occurs when one species physically modifies a habitat and that modification is advantageous to other species. For example,
zebra mussels increase habitat complexity on lake floors providing nooks and crannies 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 resources available and how much of those resources are utilized by organisms, determine the effects of new additions to the ecosystem. In stable ecosystems equilibrium exists in the utilization of available resources.
When changes occur in an ecosystem, like forest fires removing large stands of vegetation in an area, normal succession would favor certain native grasses and forbs, but with the introduction of a species that can multiply and spread faster on open ground than the native species, the balance is changed and the resources that would have been used by the native species are now utilized by an invader thus impacting the ecosystem and changing its composition of organisms and their use of available resources. The data shows that
nitrogen and
phosphorus are often the limiting factors for a situation such as this.
Every species has a role to play in its native ecosystem; some species fill large and varied roles while others are highly specialized. These roles are known as
niches. Some invading species are able to fill niches that are not utilized by native species, and they also can create niches that didn't exist.
When changes occur to ecosystems, conditions change that impact the dynamics of species interaction and niche development. This can cause once rare species to replace other species, because they now can utilize greater available resources that didn't exist before, an example would be the
edge effect. The changes can favor the expansion of a species that without the change wouldn't be able to colonize areas and niches that didn't exist before.
This mechanism describes a situation where the ecosystem in question has suffered a disturbance of some sort, which changes the fundamental nature of the ecosystem.
Ecology
Although an invasive species is often defined as an introduced species that has spread widely and causes harm, some species native to a particular area can, under the influence of natural events such as long-term rainfall changes or human modifications to the habitat, increase in numbers and become invasive.
All species on Earth go through periods of increasing and decreasing population numbers, in many cases accompanied by expansion and contraction of range. Human “alterations” on the landscape are especially significant.
Anthropogenic alteration of an environment may enable the expansion of a species into a geographical area where it hadn't been seen before and thus that species could be described as invasive. In essence, one must define "native" with care, as it refers to some natural geographic range of a species, and isn't coincident with human political boundaries. Whether noticed increases in population numbers and expanding geographical ranges is sufficient reason to regard a native species as "invasive" requires a broad definition of the term but some native species in disrupted
ecosystems can spread widely and cause harm and in that sense become invasive. For example the Monterey Cypress is a rare and endangered endemic naturally occurring only in two small stands in California. They are being exterminated as exotic invasive species less than from their native home.
Traits of invaded ecosystems
In 1958, Charles S. Elton argued that ecosystems with higher species diversity were less subject to invasive species because of fewer available niches. Since then, other ecologists have pointed to highly diverse, but heavily invaded ecosystems and have argued that ecosystems with high species diversity seem to be more susceptible to invasion. In the end, this debate seems largely to hinge on the spatial scale at which invasion studies are performed, and the issue of how diversity affects community susceptibility to invasion remains unresolved. Small-scale studies tend to show a negative relationship between diversity and invasion, while large-scale studies tend to show a positive relationship. The latter result may be an artifact of invasive or non-native species capitalizing on increased resource availability and weaker overall species interactions that are more common when larger samples are considered.
Invasion is more likely if an ecosystem is similar to the one in which the potential invader evolved.. Island ecosystems may be prone to invasion because their species are “naïve” and have faced few strong competitors and predators throughout their existence, or because their distance from colonizing species populations makes them more likely to have “open” niches. An example of this phenomenon is the decimation of the native bird populations on
Guam by the invasive
brown tree snake. Alternately, invaded ecosystems may lack the natural competitors and predators that keep introduced species in check in their native ecosystems, a point that's also seen in the Guam example. Lastly, invaded ecosystems have often experienced disturbance, usually human-induced. This disturbance may give invasive species, which are not otherwise co-evolved with the ecosystem, a chance to establish themselves with less competition from more adapted species
Vectors
Non-native species have many
vectors, including many natural ones, but most of the species considered "invasive" 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 the distances that species can travel to colonize are also often much greater with human agency.
One of the earliest human influenced introductions involves prehistoric humans introducing the
Pacific rat (
Rattus exulans) to Polynesia. Today, non-native species come from horticultural plants either in the form of the plants themselves or animals and seeds carried with them, from animals and plants released through the pet trade. Invasive species also come from organisms stowed away on every type of transport vehicle imaginable, to name a few unintentional vectors. For example,
ballast water taken up at sea and released in port is a major source of exotic marine life.
Species have also been introduced intentionally. For example, to feel more "at home", American colonists formed "Acclimation Societies" that repeatedly released birds that were native to Europe until they finally established along the east coast of
North America.
Economics play a major role in exotic species introduction. The scarcity and demand for the valuable
Chinese mitten crab is one explanation for the possible intentional release of the species in foreign waters.
Impact
Ecological impacts
Biological species invasions alter ecological systems in a multitude of ways. Worldwide an estimated 80% of endangered species could suffer losses due to competition with or predation by invasive species. Pimentel also reports that introduced species, such as corn, wheat, rice, and other food crops, and cattle, poultry, and other livestock, now provide more than 98% of the U.S. food system at a value of approximately $800 billion per year."
In the
Waterberg region of
South Africa, cattle grazing over the past six centuries has allowed invasive scrub and small trees to displace much of the original
grassland, resulting in a massive reduction in
forage for native bovids and other grazers. Since the 1970s large scale efforts have been underway to reduce invasive species; partial success has led to re-establishment of many species that had dwindled or left the region. Examples of these species are
giraffe,
Blue Wildebeest,
impala,
kudu and
White Rhino.
Invasive species can change the functions of ecosystems. For example invasive plants can alter the fire regime (cheatgrass,
Bromus tectorum), nutrient cycling (smooth cordgrass
Spartina alterniflora), and hydrology (
Tamarix) in native ecosystems. Invasive species that are closely related with rare native species have the potential to hybridize with native species. Harmful effects of hybridization have led to a decline and even extinction of native species. For example, hybridization with introduced cordgrass,
Spartina alterniflora, threatens the existence of California cordgrass (
Spartina foliosa) in San Francisco Bay.
Genetic pollution
species can be threatened with
extinction in a big way through the process of
genetic pollution for example uncontrolled
hybridization,
introgression and genetic swamping which leads to homogenization or replacement of local
genotypes as a result of either a numerical and/or
fitness advantage of introduced plant or animal. Nonnative species can bring about a form of extinction of native plants and animals by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones where the abundant ones can interbreed with them swamping the entire rarer gene pool creating hybrids thus driving the entire original purebred native stock to complete extinction. Attention has to be focused on the extent of this under appreciated problem that isn't always apparent from
morphological (outward appearance) observations alone. Some degree of
gene flow may be a normal, evolutionarily constructive process, and all constellations of
genes and
genotypes can't be preserved however, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.
Economic impacts
Economic costs due to invasive species can be separated into direct costs due to production loss in agriculture and forestry, and management costs of invasive species. Estimated damage and control cost of invasive species in the U.S. alone amount to more than $138 billion annually. Economic costs of invasions, when calculated as production loss and management costs, are low because they don't usually consider environmental damages. If monetary values could be assigned to the extinction of species, loss in biodiversity, and loss of ecosystem services, costs from impacts of invasive species would drastically increase. The chestnut blight fungus (
Cryphonectria parasitica) and Dutch elm disease (
Ophiostoma novo-ulmi) are two plant pathogens with serious impacts on forest health.
Tourism and recreation
Invasive species can have impacts on recreational activities such as fishing, hunting, hiking, wildlife viewing, and water-based recreation. They negatively affect a wide array of environmental attributes that are important to support recreation, including but not limited to water quality and quantity, plant and animal diversity, and species abundance. Eiswerth goes on to say that "very little research has been performed to estimate the corresponding economic losses at spatial scales such as regions, states, and watersheds."
Eurasian Watermilfoil (
Myriophyllum spicatum) in parts of the US, fill lakes with plants making fishing and boating difficult.
Health impacts
An increasing threat of exotic diseases exists due to increased transportation and encroachment of humans into previously remote ecosystems that can lead to new associations between a disease and a human host (for example,
AIDS virus in human host. Waterborne disease agents, such as Cholera bacteria (
Vibrio cholerae), and causative agents of harmful
algal blooms are often transported via ballast water. The full range of impacts of invasive species and their control goes beyond immediate effects and can have long term public health implications. For instance, pesticides applied to treat a particular pest species could pollute soil and surface water.
Historically the deliberate introduction of non-native species has been done with little or no consideration of the impact outside of having a favored animal, fish, or plant available locally, or perhaps an ill-conceived attempt to control a native pest. In areas with highly
endemic, specialized and isolated
flora and
fauna such as
Australia,
New Zealand,
Madagascar, the
Hawaiian Archipelago, and the
Galapagos Islands, introduced species that successfully establish themselves in
habitats utilized by natives compete for limited resources or prey on the native species, some of which are unable to adapt to the more competitive environment and gradually die out.
As more adaptable and generalized species are introduced to environments impacted adversely by human activities, some native species may be put at a disadvantage to survive while others thrive in the modified ecosystem. One of the primary threats to biodiversity is the spread of humanity into what were once isolated areas with land clearing and habitation putting significant pressure on local species.
Agriculture,
livestock and
fishing can also introduce changes to local populations of indigenous species which may result in a previously innocuous native species becoming a pest due to a reduction of natural predators.
Control
The control of invasive species can involve their
eradication or their
containment within a specified area. In both cases, the goal is to prevent further spread to un-invaded systems. This type of management can be implemented at several scales, from a homeowner working in his or her own backyard to large government agencies taking a national approach. The decision to eradicate a species versus contain it can depend on several factors, including, but not limited to, the type of habitat, characteristics of the organism, the spatial dimensions of the spread, time available to dedicate to control, and cost. These factors also play a role in determining which specific control technique(s) to utilize.
Mechanical control
Mechanical control involves the removal of invasive species by hand or with machines. Often, these methods are effective in controlling small populations and can be target specific, minimizing harm to non-invasive plants and animals. Mechanical control is labor intensive and requires a large time investment, as treatments must often be applied several times to ensure success.
Chemical control
Chemical compounds can be used to prevent the spread of invasive species. This method of control can be very effective in both large and small areas, but is often criticized due to the possible contamination of land and water resources and a lack of target specificity that can result in the killing of desirable plant and animal species. Also, the target species may develop a resistance to the chemicals over time, rendering this method ineffective.
Herbicides are chemicals used to control invasive plants and, depending on the target species, can be applied directly to a plant, in the soil at a plant’s base, or even to the soil before seeds develop.
Biological control
Biological control involves the release of a specific species to restrict the spread of the invasive species. With the proper research, this method of control can be both environmentally safe and successful. However, it can be ineffective if the released species don't survive or if their impact on the invasive species isn't as great as predicted. Invasive animals can be controlled with the release of predatory or
parasitic organisms (especially in the case of invasive insects) or with the transmission of diseases in a similar manner as with plants. have proposed a new nomenclature system based on biogeography rather than on taxa. Their system categorizes a species into the following stages:
| Stage |
Characteristic |
| 0 |
Propagules residing in a donor region |
| I |
Traveling |
| II |
Introduced |
| III |
Localized and numerically rare |
| IVa |
Widespread but rare |
| IVb |
Localized but dominant |
| V |
Widespread and dominant |
By removing taxonomy, human health, and economic factors from consideration, this model focuses only on ecological factors. The model evaluates individual populations, and not entire species. This model doesn't attribute badness to invasive species and goodness to native species. It merely classifies a species in a particular location based on its growth patterns in that particular microenvironment. This model could be applied equally to indigenous and to non-native species.
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