The invasive species, also called introduced species or foreign species, is any nonnative species that significantly changes or damages the ecosystem it invades. Such species may arrive in new areas through natural migration, but they are often introduced by the activities of other species. Human activities, such as those involved in global commerce and the pet trade, are considered to be the most common ways in which invasive plants, animals, microbes, and other organisms are transported to new habitats.
Most introduced species do not survive extended periods in new habitats, because they do not possess the necessary adaptations to adjust to the challenges posed by their new surroundings. Some introduced species may become invasive when they possess a built-in competitive advantage over native species in invaded areas. They change native food chains and in some cases even get to the top of the food chains, which means the ecosystem lacks natural enemy capable of keeping them in check. Under these circumstances, new arrivals can get the chance to reproduce in large numbers.
The ecological damage that tends to follow such invasions often reduces the ecosystem's biodiversity and causes economic harm to people who depend on the ecosystem's biological resources. Invasive species may be so good at catching preys that victim populations decline over time, and many victim species die out in the affected ecosystem. Other invasive species, in contrast, may prevent native species from obtaining food, living space, or other resources. Over time, invasive species can effectively replace native ones, often forcing the localized extinction of many native species. Invasive plants and animals may also serve as disease carriers that spread parasites (寄生虫) and viruses that may further do harm to the invaded area.
