When some foreign species of plants or animals are introduced to some place, they are termed Allien species and most often they are invasive and cause decline or extinction of indigenous species.
- Nile perch introduced into Lake Victoria in east Africa led eventually to the extinction of an ecologically unique assemblage of more than 200 species of Cichlid fish in the lake.
- Parthenium, Lantana, Eichornia are exotic species of plants that were introduced to India and have caused environmental damage. They pose threat to the survival of many of our native species.
- Clarius gariepnius – African Catfish was introduced in ndia for aquacultural purposes, today its a threat to the indeginous catfish of Indian waters.
- Gambusia fish introduced in Ponds and lake in India to curb mosquito larvae , lead to depletion of indigenous species of fish.