It has been suggested that this article or section be merged into Invasive species. (Discuss)

Invasive plants are plants introduced beyond their native range which demonstrate an ability to spread through their new habitat and disrupt the native ecosystem. Wikipedia does not have an article with this exact name. ... It has been suggested that Invasive plants be merged into this article or section. ... In ecology, the word ecosystem is an abbreviation of the term, ecological system. ...

Introduction

While the majority of introduced plant species pose neither economic nor ecological problems, a few species become invasive and damaging to their new habitat. It is still an open question why certain plant species become such successful invaders; several theories are currently being debated and tested, but there is no one clear, single answer, and little reliable predictive power about the nature and extent of future invasions. In biology, a species is the basic unit of biodiversity. ...

The cost of these invasive plant species can be high. Problems caused by invasive plants cost billions to the global economy every year, mainly from loss of grazing land and reduced crop productivity due to non-native weeds. The cost to the United States alone is an estimated $137 million a year in management and missed economic gain. Ecologically, they can disrupt ecosystem services and disrupt communities by being space-dominant or through impacts on keystone native species. At their worst, invasive plants have the ability to degrade whole ecosystems, both terrestrial and aquatic.

There is currently a debate about whether the benefits of introduced plant species, both in increasing local species richness and in economic advantages, is a sufficient tradeoff for the global decrease in species richness and the costs of plant species that become invasive (Brown and Sax, 2004). Whatever the ultimate moral and management decisions about invasive plants as a whole may be, the pheonomenon of invasive plants is indisputable, and has proved a fertile area of study.

Invasive plants can be spread in many ways, and introductions can be accidental or intentional. Many invasive plants have been spread through deliberate introduction because the species was perceived to have value in agriculture or ornamental gardening. However, many have also been unintentional introductions, either through planting of impure seed mixes that contain the invasive species, or by hitching a ride on a vehicle or in cargo. A gardener Gardening is the craft of growing plants with the goal of creating a beautiful environment. ...

For an introduced plant to become an invader, it has to 1) arrive, 2) survive, and 3) thrive. The plant must find a vector which will bring it to a new environment. This new habitat must be a close enough match to its native range that it is able to survive and reproduce here without human cultivation (Williams and Meffe, 1998). To actually become invasive, the introduced plant has to be able to outcompete native plants, to reproduce effectively enough to start spreading geographically through its new habitat, and to negatively impact the ecosystems in its introduced range. Sometimes, this requires a large propagule size, or repeated innoculations, or a combination of changing circumstances, before a new species will actually take hold in a new habitat.

There are hundreds if not thousands of examples of invasive plant species throughout the world. One example is Bromus tectorum (Drooping Brome, downy brome, or cheatgrass), which spreads rapidly after burning, and crowds out plants used by grazing animals in large areas of Western North America. Because it has low nutritive value to grazing animals, it does not substitute either economically or ecologically for the displaced native plants. In the southern United States, Pueraria lobata (kudzu) was originally planted to stop roadside erosion, and now covered large areas with its leafy vines. It has been known to swallow up entire fields and forests if left unchecked. In the marine realm, Caulerpa taxifolia (the “killer algae” of the Mediterranean) is able to cover vast stretches of shallow ocean floor with a fast-growing monoculture of bright-green fronds, which are inedible and uninhabitable by all but a very few animal species. Binomial name Bromus tectorum L. Drooping brome (Bromus tectorum), is a grass native to Europe, southwestern Asia and northern Africa. ... Binomial name Pueraria lobata (Willd. ... Severe soil erosion in a wheat field near Washington State University, USA. Erosion is the displacement of solids (soil, mud, rock, and other particles) by the agents of wind, water, ice, movement in response to gravity, or living organisms (in the case of bioerosion). ... Binomial name Caulerpa taxifolia (M. Vahl) C. Agardh, 1817 Caulerpa taxifolia is a species of seaweed (a type of algae) that has been commonly used as ornamentation in aquarium installations around the world. ...

Conditions that lead to invasion

Scientific literature proposes several mechanisms to explain invasiveness. These mechanisms generally fall into two different categories: one for mechanisms which focus on the invasive species, and the other which focuses on the invaded ecosystem. More likely, it is a combination of several mechanisms that cause an invasive situation to happen.

Species-based mechanisms

Species-based characteristics focus on plant competition. While all plants are able to compete in some manner in order to survive and persist, invasive species appear to have specific traits or combinations of specific traits that make them especially good competitors. In some cases it can be as simple as having the ability to grow and reproduce more rapidly than native species. Other situations are more complex, such as allelopathy, which is a common mechanism whereby the invader directly or indirectly prevents other plants from growing nearby. The suppression of growth of one plant species by another due to the release of toxic substances (Webster 1983). ...

Life history

The life history of an organism describes the different stages of life an organism will go through during its lifetime. Such traits are tempting to study because life history is a quantifiable trait that could lead to very predictive models.

Several traits have been singled out by researchers as predictors of invasive ability in plants. For example, the ability to reproduce both asexually (vegetatively) as well as sexually, rapid growth, early sexual maturity, high reproductive output, the ability to disperse young widely, tolerance of a broad range of environmental conditions, and high phenotypic plasticity are all abilities that might aid an invasive plant in establishing and proliferating in a new environment. In addition, plants that are associated with human habitats, such as crop plants (and their weeds), plants valued for ornamental purposes, or plants that are spread along roadways or by domestic animals, are more likely to find a vector to travel to a new habitat in the first place. It has been suggested that this article or section be merged with Asexual reproduction of plants. ...

Collecting data on life-history traits goes back to the beginning of invasive species study, and still forms a part of current research. The majority of studies agree in a general sense on which kind of traits mark an invasive species, but there are differences in how invasive each trait can make a species. 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 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. This is one of the great difficulties in invasive species research: while many generalities can be made about invasive species, there are always exceptions to these observations (Kolar and Lodge 2001, Thebaud et al. 1996, Reichard and Hamilton 1997).

Superior competition

A common trait of invasive species is great competitive ability, which can be stronger against plants in a new habitat than plants in their native habitat. There can be huge differences between how an invasive species interacts with its native ecosystem, and with the ecosystem it is invading. Often, the invading species has a better chance at acquiring resources, which can be light, water, space, or nutrients. Ecosystems where all available resources are being used to their full capacity by native plants 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 plants with increased populations of the invader) is not the rule (Stolgren 2003, Sax et al. 2002). Invasive plants can coexist with native plants for an extended time, and only gradually does the superior competitive ability of an invasive species become apparent, as its population grows larger and denser, and slowly increases the risk of extinction to other species.

An invasive species might be able to use resources previously unavailable to native plants, such as very 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. Few plants have adapted to grow on them, which may explain why they have had so few plant invasions. However, since goatgrass can take advantage of these soils, it can also use the light, water, and space resources that other plants are restricted from using (Huenneke et al. 1990). By using these resources, goat grass can become so dense as to exclude other species and form monospecific swards.

There are other reasons that an invader might be a superior competitor. For example, an invasive plant may be inedible to local herbivores, allowing it to flourish unmolested where the native species are constantly held in check. The herbivores would find themselves thus come into increasing competition with each other over fewer and fewer native plants, while the invader is taking the place of the native species. As the invader comes to dominate its new habitat, the local food webs are changed from the bottom up, since their foundation of native plants has been altered. (Petren and Case 1996, Gray 1986).

Facilitation

Facilitation is the mechanism by which some species can alter their environment through chemicals or manipulation of abiotic factors, usually to make it more favorable to their growth or reproduction. Sometimes, neighboring species may benefit by another’s facilitation, but often the facilitation actually benefits the target species to the detriment of its neighbors. One such facilitative mechanism is allelopathy, also known as chemical competition. In allelopathy, a plant will secrete chemicals which make the surrounding soil uninhabitable, or at least inhibitory, to other plant species. The suppression of growth of one plant species by another due to the release of toxic substances (Webster 1983). ...

One example of this is the knapweed species 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 haven't co-evolved with C. diffusa. Such co-evolved native plants have also evolved defenses, and C. diffusa does not 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 (Vivanco et al. 2004, Hierro and Callaway 2003). Categories: Plant stubs | Asteraceae ... Diffuse knapweed (Centaurea diffusa) is a member of the genus Centaurea of the family Asteraceae. ... Current division of Europe into five (or more) regions: one definition of Eastern Europe is marked in orange Eastern Europe as a region has several alternative definitions, whereby it can denote: the region lying between the variously and vaguely defined areas of Central Europe and Russia. ...

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 height of the fire season in Western North America. In areas where it is widespread, it has altered the local fire regime so much that native plants cannot survive the frequent fires, allowing B. tectorum to further extend and maintain dominance in its introduced range (Brooks et al. 2004).

Ecosystem based mechanisms

Unused resources

When examining an ecosystem, it is important to look at two things: the amount of resources available, and how much of those resources are being used. In a stable ecosystem, all of the available resources are used up, and all of the species there have enough to continue to survive.

Yet what happens if spare resources happen to appear in an ecosystem? For example, lets say a forest fire has taken out all of the grass species in a given area, leaving only hardy brush and established trees. Since these grasses are gone, and the resources used to maintain them are not being utilized, these extra resources can be used by another species, possibly one that is invasive. Sure, these most likely will be used by the native species nearby, but what if these resources require a special trait to acquire or use effectively? Perhaps long taproots are needed. Such an ecosystem is at risk to invaders, since they can move in and establish themselves with the extra resources. While this many sound like a simple swap, that is not the case. Such exotics are never the same as the species they replace, and can serve to ruin an ecosystem, simply because they have no predators or encroach onto other species. The dandelions taproot, quite apparent in this drawing, renders this plant very difficult to uproot – the plant itself gives way, but the root stays in the ground and may sprout again. ...

The data shows that nitrogen and phosphorus are often the limiting factors for a situation such as this (Davis et al 2000). However, this is not as dire of a situation as it sounds. For an invasion to occur, there have to be members of the correct invasive species ready and waiting for this flux of resources to occur in its favor. If either of these two parts are not present, the invasive species will have to find another way to establish itself. General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... This article is about the chemical element. ...

Unfilled niches

Basic ecology tells us that every species has a role to play in its native ecosystem; some are general while others are highly specialized. These roles are known as niches. This mechanism describes a situation where the invaded ecosystem in question has unfilled niches, which are promptly filled by an invader. While there are cases where the invader can stay within its niche, these species often bring about other traits in them, which cause harm for the rest of the ecosystem, such as lack of suitable predators or alleopathic traits. In ecology, a niche is a term describing the relational position of a species or population in an ecosystem. ...

While this mechanism sounds reasonable to the casual reader, the data itself is much more mixed. Various experiments have shown positive, negative and neutral correlations between ecosystems with high diversity and invasiveness. From this I would argue that while this mechanism may make sense in theory, it does not pan out that well in the literature. Perhaps its effect is simply too weak, and is over powered by other mechanisms, or perhaps the concept of a missing niche is too broad in its application, and needs to be more strictly defined, or better yet, combined into one of the other ecosystem based models (Dukes 2001).

Ecosystem instability

Imagine a situation involving a group of people who have mastered a game, and are able to beat anyone outside of their group. They know the rules inside and out, and are constantly perfecting their strategies to win. Few, if any, have a chance against this group. Then suddenly the rules suddenly change, and this group is at a loss, since their strategies no longer work the way they used to, and other players with vastly different strategies are able to come in, and win where they could not before. This is how the familiar ecosystem mechanism works.

The concept of a successful invasive species seems to be a bit odd. After all, wouldn't species, which have co-evolved together for years, be able to out compete an exotic species? There has been much time for them to evolve and become as efficient possible. 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 (Byers 2002). Examples of this type of disturbance can range from the loss of an important predator to the eutrophication of an aquatic environment. In situations such as these, the specialized evolution of the native plant species becomes wasted, and non-native species can gain a foothold. A speculative phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes. ... Eutrophication is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit. ...

Human Impact

Humans have the greatest impact on the problem of invasive species. Nearly all ecosystem disturbances come from various forms of human activity. Such activity can be organized into four categories: conservation, utilization, replacement, and removal (Bright 1998). These categories describe how we use the land, everything from wildlife preserves to parks to agriculture to urbanization. It is within these land decisions that many of the mechanisms of invasiveness come into play. Take the earlier example of the cheatgrass: when ranchers introduced cattle to an area, these heavy animals disturbed the soil allowing cheatgrass a foothold in the ecosystem. It has been suggested that Invasive plants be merged into this article or section. ... Binomial name Bos taurus Linnaeus, 1758 Cattle (called cows in vernacular usage, or kine [archaic]) are domesticated ungulates, a member of the subfamily Bovinae of the family Bovidae. ...

More general scenarios to consider:

• Horticulture and Agriculture; from seed contamination to cultivation of exotic species
• Development directly causes a keystone species to be extirpated, allowing an invader a chance at establishment
• Pollution runoff can add nutrients into the environment, opening up a niche in an ecosystem

The Latin words hortus (garden plant) and cultura (culture) together form horticulture, classically defined as the culture or growing of garden plants. ... Water pollution Environmental pollution is the release of environmental contaminants, generally resulting from human activity. ... // Nutrients and the body A nutrient is any element or compound necessary for or contributing to an organisms metabolism, growth, or other functioning. ...

See also

• Introduced species
• Invasive species
• List of invasive species

Sweet clover (), introduced and naturalized to the U.S. from Eurasia as a forage and cover crop, supports insect biodiversity. ... It has been suggested that Invasive plants be merged into this article or section. ... Purple flowers of the highly invasive Pattersons Curse infest the Warrumbungle National Park in New South Wales, Australia. ...

References and further reading

• Bright C. 1998. Life out of bounds : bioinvasion in a borderless world. 1st ed. New York: W.W. Norton.
• Brooks, M. L., C. M. D’Antonio, D. M. Richarson, J. B. Grace, J. E. Keeley, J. M. DiTomaso, R. J. Hobbs, M. Pellant, and D. Pyke. 2004. Effects of invasive alien plants on fire. BioScience 54: 677-688.
• Brown, J. H., and D. F. Sax. An Essay on Some Topics Concerning Invasive Species. Australian Ecology. 29: 530-536.
• Byers JE. 2002. Impact of non-indigenous species on natives enhanced by anthropogenic alteration of selection regimes. Oikos 97 (3): 449-458.
• Byers, James. E. and Erik G. Noonburg. 2003. Scale dependent effects of biotic resistance to biological invasion. Ecology 84: 1428-1433.
• Davis MA, Grime JP, Thompson K. 2000. Fluctuating resources in plant communities: a general theory of invasibility. Journal of Ecology 88 (3): 528-534.
• Dukes JS. 2001. Biodiversity and invasibility in grassland microcosms. Oecologia 126 (4): 563-568.
• Elton, Charles S. 1958. The Ecology of Invasions by Animals and Plants. University of Chicago Press, Chicago, Illinois, USA.
• Gray AJ. 1986. Do Invading Species have Definable Genetic-Characteristics? Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 15; 314 (1167): 655-674.
• Hierro JL, Callaway RM. 2003. Allelopathy and exotic plant invasion. Plant and Soil 256 (1): 29-39.
• Huenneke, L., S. Hamburg, R. Koide, H. Mooney, and P. Vitousek. 1990. Effects of soil resources on plant invasion and community structure in California (USA) serpentine grassland. Ecology 71:478–491.
• Hobbs, R. J., L. F. Huenneke. 1992. Disturbance, Diversity, and Invasion: Implications for Conservation. Conservation Biology6: 324-337.
• Kolar CS, Lodge DM. 2001. Progress in invasion biology: predicting invaders. Trends in Ecology & Evolution 16 (4): 199-204.
• Mooney, H. A., Hobbs, R.J. (eds). 2000. Invasive Species in a Changing World. Washington, DC: Island Press.
• Petren K, Case TJ. 1996. An experimental demonstration of exploitation competition in an ongoing invasion. Ecology 77 (1): 118-132.
• Reichard S.H., C. W. Hamilton. 1997. Predicting invasions of woody plants introduced into North America. Conservation Biology 11 (1): 193-203.
• Sakai A.K., F. W. Allendorf, J. S. Holt, D. M. Lodge, J. Molofsky, K. A. With, S. Baughman, R. J. Cabin, J. E. Cohen, N. C. Ellstrand, D. E. McCauley, P. O' Neil, I. M. Parker, J. N. Thompson, and S. G. Weller. 2001. The Population Biology of Invasive Species. Annual Review of Ecology and Systematics 32 (1): 305-332.
• Sax, D. F., S. D. Gaines, and J. H. Brown. 2002. Species Invasions Exceed Extinctions on Islands Worldwide: A Comparative Study of Plants and Birds. American Naturalist 160: 766-783.
• Stohlgren, T. J., D. T. Barnett, and J. T. Kartesz. 2003. The rich get richer: patterns of plant invasions in the United States. Frontiers in Ecology and the Environment 1-11-14.
• Thebaud C, Finzi AC, Affre L, Debussche M, Escarre J. 1996. Assessing why two introduced Conyza differ in their ability to invade Mediterranean old fields. Ecology 77 (3): 791-804.
• Theodoropoulos, David. 2003. Invasion Biology: Critique of a Pseudoscience. Avvar Books, Blythe CA.
• Townsend CR. 1991. Exotic Species Management and the Need for a Theory of Invasion Ecology. New Zealand Journal of Ecology 15(1): 1-3.
• Van Driesche J, Van Driesche R. 2000. Nature out of place: biological invasions in the global age. Washington, D.C: Island Press.
• Vivanco JM, Bais HP, Stermitz FR, Thelen GC, Callaway RM. 2004. Biogeographical variation in community response to root allelochemistry: novel weapons and exotic invasion. Ecology Letters 7 (4): 285-292.
• Williams, J.D. and G. K. Meffe. 1998. Nonindigenous Species. In: Status and Trends of the Nation’s Biological Resources. Volume 1. Reston, Virginia: United States Department of the Interior, Geological Survey.