Invasive species
An invasive species is an introduced species that harms its new environment.[2] Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native species that become harmful to their native environment after human alterations to its food web. Since the 20th century, invasive species have become a serious economic, social, and environmental threat 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 international trade. Notably invasive plant species include the kudzu vine, giant hogweed, Japanese knotweed, and yellow starthistle. Similarly invasive animals include European rabbits, domestic cats, and carp.
Favorable effects[edit]
The entomologist Chris D. Thomas argues that most introduced species are neutral or beneficial with respect to other species[125] but this is a minority opinion. The scientific community ubiquitously considers their effects on biodiversity to be negative.[126]
Some invasive species can provide a suitable habitat or food source for other organisms. In areas where a native has become extinct or reached a point that it cannot be restored, non-native species can fill their role. For instance, in the US, the endangered southwestern willow flycatcher mainly nests in the non-native tamarisk.[127]
The introduced mesquite is an aggressive invasive species in India, but is the preferred nesting site of native waterbirds in small cities like Udaipur in Rajasthan.[128] Similarly, Ridgway's rail has adapted to the invasive hybrid of Spartina alterniflora and Spartina foliosa, which offers better cover and nesting habitat.[129] In Australia, saltwater crocodiles, which had become endangered, have recovered by feeding on introduced feral pigs.[130]
Non-native species can act as catalysts for restoration, increasing the heterogeneity and biodiversity in an ecosystem. This can create microclimates in sparse and eroded ecosystems, promoting the growth and reestablishment of native species. For example, in Kenya, guava trees in farmland are attractive to many fruit-eating birds, which drop seeds from rainforest trees as much as 2 km (1.2 mi) away beneath the guavas, encouraging forest regeneration.[131]
Non-native species can provide ecosystem services, functioning as biocontrol agents to limit the effects of invasive agricultural pests.[127] Asian oysters, for example, filter water pollutants better than native oysters in Chesapeake Bay.[132] Some species have invaded an area so long ago that they are considered to have naturalised there. For example, the bee Lasioglossum leucozonium, shown by population genetic analysis to be an invasive species in North America,[133] has become an important pollinator of caneberry (Rubus spp.) as well as cucurbit, apple trees, and blueberry bushes.[134] In the US, the endangered Taylor's checkerspot butterfly has come to rely on invasive ribwort plantain as the food plant for its caterpillars.[135]
Some invasions offer potential commercial benefits. For instance, silver carp and common carp can be harvested for human food and exported to markets already familiar with the product, or processed into pet foods, or mink feed. Water hyacinth can be turned into fuel by methane digesters,[136] and other invasive plants can be harvested and utilized as a source of bioenergy.[137]
Predicting invasive plants[edit]
Accurately predicting the impacts of non-native plants can be an especially effective management option because most introductions of non-native plant species are intentional.[180][181][182]
Weed risk assessments attempt to predict the chances that a specific plant will have negative effects in a new environment, often using a standardized questionnaire. The resulting total score is associated with a management action such as "prevent introduction".[183][184]
Assessments commonly use information about the physiology,[183] life history,[184] native ranges,[185] and phylogenetic relationships of the species evaluated. The effectiveness of the approach is debated.[186][187]