An invasive species is an introduced organism that becomes overpopulated and negatively alters its new environment. Although their spread can have beneficial aspects, invasive species adversely affect the invaded habitats and bioregions, causing ecological, environmental, and/or economic damage. Sometimes the term is used for native species that become invasive within certain ecosystems due to human alterations of the environment. An example of a native invasive species is the purple sea urchin which has decimated natural kelp forests along the northern California coast due to the historic overhunting of its natural predator, the California sea otter. In the 21st century, invasive species have become a serious economic, social, and environmental threat.
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. Animal examples include the New Zealand mud snail, feral pig, European rabbit, grey squirrel, domestic cat, carp, and ferret. Some popular reference sources now name Homo sapiens, especially modern-age humans, as an invasive species, but broad appreciation of human learning capacity and their behavioral potential and plasticity argues against any such fixed categorization.