1. Alien species turn invasive and cause decline or extinction of indigenous species. Nile perch introduced into Lake Victoria in east Africa led to the extinction of more than 200 species of cichlid sh in the lake.
2. When a species become extinct the plants and animals associated with it in an obligatory way also become extinct, coevolved. Plant-pollinator mutualism where extinction of one species leads to the extinction of the other.
3. It happen when need turns greed and over utilization of natural resources in an irrecoverable way. Extinction of Steller’s sea cow, passenger pigeon was due to humans.