When alien species are introduced, some of them become invasive, compete with the native species and cause extinction of indigenous species.For example:
(i)Partheniun, Lantana and Eichhornia are the exotic species of plants that have invaded India and caused environmental damage.They pose threats to the survival of many of our native species.
(ii)Introduction of African catfish
(Clarias gariepinus) for aquaculture purposes is posind a threat to our indigenous catfish Clarias batrachus.