Invasive species
An invasive or alien species is an introduced species to an environment that becomes overpopulated and harms its new environment. 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 - for example, the purple sea urchin (Strongylocentrotus purpuratus) which has decimated kelp forests along the northern California coast due to overharvesting of its natural predator, the California sea otter (Enhydra lutris). 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. Notable examples of invasive plant species include the kudzu vine, Andean pampas grass, English ivy, Japanese knotweed, and yellow starthistle. Examples of invasive animals include the New Zealand mud snail, some water fleas (such as Daphnia), feral pig, European rabbit, grey squirrel, domestic cat, carp, and ferret. Glossary of invasion biology terms Alien or naturalized species are those species which are not native to an area but established, and those that are a threat to native species and biodiversity are often called invasive species. The term "invasive" is poorly defined and often very subjective. Invasive species may be plants, animals, fungi, and microbes; some also 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. The definition of "native" is also sometimes controversial.
