explain alien-species invasion??

@ Sanjeev : Abhirami is right. You can refer to her answer. I would like to add some information related to it.

The unintentional or deliberate introduction of alien species causes the declination of the indigenous species. Example − Nile perch introduced in Lake Victoria led to the extinction of more than 200 species of cichlid fish in the lake.

@ Abhirami : Good work. Keep posting!!