Introduction of Alien species

Plants and animals that are not native to an ecosystem can wreak havoc on the naturally occurring species within that system. Exotic species often out-compete native species for resources and occupy much of the available habitat. There are many examples such as coypu, muskrat, Canada goose, Himalayan balsam, Caulerpa taxifolia algae, purple loosestrife and kudzu.

Other exotics prey on native species or usurp or destroy their habitats. For example, brown snakes have eradicated virtually all bird life on Guam, and zebra mussels prey on and replace native mussel beds in USA. The economical impact of exotics on agriculture, fisheries, and other aspects is estimated in considerable billions of dollars each year; the impact on biodiversity is immeasurable. Exotic species can be introduced to an ecosystem intentionally or accidentally.

A ctenophore - Mnemiopsis leidyi, carnivorous pelagic animal was introduced to the Black sea
from East coast of the North America together with ballast waters by oil tankers. Within few year it consumed most part of the meroplancton (larvae and eggs of the invertebrates and fishes). According to the official statistic, the fishery in the region got great losses.