Illustrate with the help of an example how introduction of an alien species turns invasive and causes decline of an indigenous species.

Answer :

Introduction of Nile perch introduced into Lake Victoria, led eventually to the extinction of more than 200 species of Cichlid fish.
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African catfish / Clarias gariepinus, causes threat to indigenous catfish
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Parthenium/ carrot grass/ Lantana/ Water hyacinth/ Eicchornia sp.,causes threat to native species.