An alien species is any plant or animal species that is introduced in a new environment and where it flourishes and takes over the indigenous flora or fauna. Introduction of alien species causes decline or extinction of indigenous species due to tough competition for utilization of resources. The most famous example is that of water hyacinth. It was brought to India because of its beautiful leaves. However, once it got introduced, it spread like weed and was very difficult to control it.
Another example is that of the Nile perch. Nile Perch was introduced in lake Victoria and it led to the extinction of more than 200 species of Cichlid fish.
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