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

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