When alien species are introduced with the native species it can cause either decline or extinction of indigenous species.
The greatest impact is caused by introduced species that change an entire habitat, because many native species thrive only in a particular habitat.
Other invaders, though they do not change a habitat, endanger single species or even entire groups of them in various ways.
Often invaders interact with one another to generate a problem where either species alone would be harmless.
e.g. A case study in East Africa suggest that when an alien exotic species Nile perch was introduced in lake Victoria it led to the extinction of 200 species of cichlids in the lake.
In India when an aquatic invasive weed Eichhornia was introduced it resulted in the decrease in the level of biological oxygen demand of water, it resulted in the death of the aquatic lives .
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