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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Invasive species | 3/9 | https://en.wikipedia.org/wiki/Invasive_species | reference | science, encyclopedia | 2026-05-05T07:18:41.562467+00:00 | kb-cron |
An invasive species might be able to use resources previously unavailable to native species, such as deep water accessed by a long taproot, or 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. Invasive species may also alter their environment by releasing chemical compounds, modifying abiotic factors, or affecting the behavior of herbivores, all of which can impact other species. Some, like mother of thousands (Kalanchoe daigremontana), produce allelopathic compounds that inhibit competitors. Others like the toad plant (Stapelia gigantea) facilitate the growth of seedlings of other species in arid environments by providing appropriate microclimates and preventing herbivores from eating seedlings. Changes in fire regimens are another form of facilitation. Cheatgrass (Bromus tectorum), originally from Eurasia, is highly fire-adapted. It spreads rapidly after burning, and increases the frequency and intensity of fires by providing large amounts of dry detritus during the fire season in western North America. Where it is widespread, it has altered the local fire regimen so much that native plants cannot survive the frequent fires, allowing it to become dominant in its introduced range. Ecological facilitation occurs where one species, including invasive species, physically modifies a habitat in ways advantageous to other species. For example, zebra mussels (Dreissena polymorpha) 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. Introduced species may spread rapidly and unpredictably. When bottlenecks and founder effects cause a great decrease in the population size and may constrict genetic variation, individuals begin to show additive variance as opposed to epistatic variance. This conversion can lead to increased variance in the founding populations, which permits rapid evolution. Selection may then act on the capacity to disperse as well as on physiological tolerance to new stressors in the environment, such as changed temperature and different predators and prey. Rapid adaptive evolution through intraspecific phenotypic plasticity, pre-adaptation, and post-introduction evolution lead to offspring that have higher fitness. Critically, plasticity permits changes to better suit the individual to its environment. Pre-adaptations and evolution after the introduction reinforce the success of the introduced species. The enemy release hypothesis states that evolution leads to ecological balance in every ecosystem. No single species can occupy a majority of an ecosystem due to the presences of competitors, predators, and diseases. Introduced species moved to a novel habitat can become invasive, with rapid population growth, when these controls do not exist in the new ecosystem.
== Vectors == Non-native species have many vectors, but most are associated with human activity. Natural range extensions are common, but humans often carry specimens faster and over greater distances than natural forces. An early human vector occurred when prehistoric humans introduced the Pacific rat (Rattus exulans) to Polynesia. Humans have historically acted as vectors, both deliberately and accidentally. During the colonial era, there were acclimatization societies that aimed to deliberately establish animals and plants that were recognizable to Europeans. These societies felt that this would be an "improvement" to nature by adding species that were nostalgic and valuable. Another reason for deliberate introduction was that some species are thought to act as biocontrol. The cane toad was introduced in Australia in hopes of controlling the cane beetle population. Cane beetles are pests of the sugar cane crop. While only 100 toads were initially imported, there are now over 200 million present in Australia. Another example of animal introduction is the Indian mongoose, which was introduced in Hawaii to control the rat population as the rats were damaging sugar cane yields. This, however, ended up being ineffective because rats are nocturnal while the mongoose is diurnal. Their introduction ended up causing new problems for the island's ecosystems.
Vectors also include plants or seeds imported for horticulture. The pet trade moves animals across borders, where they can escape and become invasive. Organisms may also stow away on transport vehicles. Incidental human assisted transfer is the main cause of introductions – other than in polar regions. Diseases may be vectored by invasive insects: the Asian citrus psyllid (Diaphorina citri) carries the bacterial disease citrus greening. The arrival of invasive propagules to a new site is a function of the site's invasibility. Many invasive species, once they are dominant in the area, become essential to the ecosystem of that area, and their removal could be harmful. 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 === Maritime trade has rapidly affected the way marine organisms are transported within the ocean; new means of species transport include 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.