Invasive species
An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. Since the 20th century, invasive species have become serious economic, social, and environmental threats worldwide.
Invasion of long-established ecosystems by organisms is a natural phenomenon, but human-facilitated introductions have greatly increased the rate, scale, and geographic range of invasion. For millennia, humans have served as both accidental and deliberate dispersal agents, beginning with their earliest migrations, accelerating in the Age of Discovery, and accelerating again with the spread of international trade. Notable plant species that are invasive in North America include the kudzu vine, giant hogweed, Japanese knotweed, and yellow starthistle. Notable animal species that are invasive in North America include European rabbits, domestic cats, and carp.
Terminology
Invasive species are the subset of established non-native alien or naturalized species that are a threat to native species and biodiversity. The term "invasive" is poorly defined and often very subjective. Invasive species may be plants, animals, fungi, and microbes; some include native species that have invaded human habitats such as farms and landscapes. Some broaden the term to include indigenous or "native" species that have colonized natural areas. Some sources name Homo sapiens as an invasive species, but broad appreciation of human learning capacity and their behavioral potential and plasticity may argue against any such fixed categorization. The definition of "native" can also be controversial. For example, the ancestors of modern horses evolved in North America and radiated to Eurasia before becoming extinct in North America. Upon being introduced to North America in 1493 by Spanish conquistadors, it is debatable whether the feral horses were native or exotic to the continent of their evolutionary ancestors.While invasive species can be studied within many subfields of biology, most research on invasive organisms has been in ecology and biogeography. Much of the work has been influenced by Charles Elton's 1958 book The Ecology of Invasion by Animals and Plants'' which creates a generalized picture of biological invasions. Studies remained sparse until the 1990s. This research, largely field observational studies, has disproportionately been concerned with terrestrial plants. The rapid growth of the field has driven a need to standardize the language used to describe invasive species and events. Despite this, little standard terminology exists. The field lacks any official designation but is commonly referred to as "invasion ecology" or more generally "invasion biology". This lack of standard terminology has arisen due to the interdisciplinary nature of the field, which borrows terms from disciplines such as agriculture, zoology, and pathology, as well as due to studies being performed in isolation.
| 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 |
In an attempt to avoid the ambiguous, subjective, and pejorative vocabulary that so often accompanies discussion of invasive species even in scientific papers, Colautti and MacIsaac proposed a new nomenclature system based on biogeography rather than on taxa. By discarding taxonomy, human health, and economic factors, this model focused only on ecological factors. The model evaluated individual populations rather than entire species. It classified each population based on its success in that environment. This model applied equally to indigenous and to introduced species, and did not automatically categorize successful introductions as harmful.
The USDA's National Invasive Species Information Center defines invasive species very narrowly. According to Executive Order 13112, Invasive species' means an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health."
Causes
Typically, an introduced species must persist at low population densities before it can become invasive in a new environment. At low densities, introduced species often face difficulties in reproducing and sustaining a viable population—sometimes requiring multiple introductions before establishment occurs. Patterns of repeated human movement, such as ship traffic between ports or vehicles travelling along major highways, can create recurring opportunities for arrival and establishment, a phenomenon known as high propagule pressure.Ecosystem-based mechanisms
In ecosystems, the availability of resources determines the impact of additional species on the ecosystem. Stable ecosystems have a resource equilibrium, which can be changed fundamentally by the arrival of invasive species. When changes such as a forest fire occur, normal ecological succession favors native grasses and forbs. An introduced species that can spread faster than natives can outcompete native species for food, squeezing the natives out. Nitrogen and phosphorus are often the limiting factors in these situations. Every species occupies an ecological niche in its native ecosystem; some species fill large and varied roles, while others are highly specialized. Invading species may occupy unused niches, or create new ones. For example, edge effects describe what happens when part of an ecosystem is disturbed, as in when land is cleared for agriculture. The boundary between the remaining undisturbed habitat and the newly cleared land itself forms a distinct new habitat, creating new winners and losers, and potentially hosting species that would not otherwise thrive outside the boundary habitat.In 1958, Charles S. Elton claimed that ecosystems with higher species diversity were less subject to invasive species because fewer niches remained unoccupied. Other ecologists later pointed to highly diverse, but heavily invaded ecosystems, arguing that ecosystems with high species diversity were more susceptible to invasion. This debate hinged on the spatial scale of invasion studies. Small-scale studies tended to show a negative relationship between diversity and invasion, while large-scale studies tended to show the reverse, perhaps a side-effect of invasives' ability to capitalize on increased resource availability and weaker species interactions that are more common when larger samples are considered. However, this pattern does not seem to hold true for invasive vertebrates.
File:Brown tree snake .jpg|thumb|right|The brown tree snake has had an impact on the native bird population of the island ecosystem of Guam.
Island ecosystems may be more prone to invasion because their species face few strong competitors and predators, and because their distance from colonizing species populations makes them more likely to have "open" niches. For example, native bird populations on Guam have been decimated by the invasive brown tree snake.
In New Zealand the first invasive species were the dogs and rats brought by Polynesian settlers around 1300. These and other introductions devastated endemic New Zealand species. The colonization of Madagascar brought similar harm to its ecosystems. Logging has caused harm directly by destroying habitat, and has allowed non-native species such as prickly pear and silver wattle to invade. The water hyacinth forms dense mats on water surfaces, limiting light penetration and hence harming aquatic organisms, and creating substantial management costs. The shrub lantana is now considered invasive in over 60 countries, and has invaded large geographies in several countries prompting aggressive federal efforts to control it.
Along with island ecosystems, intensively managed fenced areas are more prone to invasion. One reason is that species can enter through ways that fences cannot block. In the case of many plant species, dispersal can occur through wind, water, and birds carrying seeds either internally or externally. Small animals or insects are sometimes also able to make it through fenced areas. Another reason is that intensive management methods create opportunities for these invasive species to thrive. When fenced areas are established, they are intensively managed through clearing vegetation, mowing, and disturbing the soil. This reduces competition from the native plant species in the area and exposes the soil so that invasive species can easily populate the area. The purpose of these fenced areas is often to keep unwanted herbivores out of the areas. Because there are no threats to the invasive plants that can take over intensively managed fenced areas, they can flourish without predatory threats. Finally, in general, invasive species have traits that favor their survival. Most invasive species are extremely resilient and have traits that favor their establishment in areas where they are not native. These traits, along with the intensive management of the fenced areas, create an ideal environment in which these invasive species can thrive.
Primary geomorphological effects of invasive plants are bioconstruction and bioprotection. For example, kudzu, a vine native to Asia, was widely introduced in the southeastern United States in the early 20th century to control soil erosion. The primary geomorphological effects of invasive animals are bioturbation, bioerosion, and bioconstruction. For example, invasions of the Chinese mitten crab have resulted in higher bioturbation and bioerosion rates.
A native species can also become harmful and effectively invasive to its native environment after human alterations to its food web. This has been the case with the purple sea urchin, which has decimated kelp forests along the northern California coast due to overharvesting of its natural predator, the California sea otter.