Clarias gariepinus, the African catifish has been illegally introduced for aquaculture in India. It is threatening native catfishes of Indian rivers.

Extinction of one species automatically causes extinction of the other species in certain obligatory mutualistic relationships. This is called co-extinction, e.g., mutualism between Pronuba yuccasella and Yucca.

A stable community is self-perpetuating and final biotic community that is in perfect harmony with the physical environment. It does not show much of changes and is usually resilient to natural or man made disturbances. It is also resistant to invasion by alien species.

Natural or background extinction is a slow process of replacement of existing species with the better adapted species due to alternate evolution, changes in environmental conditions, predators and diseases. A small population due to inbreeding depression (reduces genetic variability) and normal population fluctuations during unfavourable periods like drought, harsh winter or severe summer.

Animals constitute more than 70% of all the species recorded whereas plants constitute around 22% of total number of species.

The number of species of aves, reptiles, amphibians and mammals are 1232, 456, 209 and 300 respectively.

• The biosphere of Earth is heterogeneous in nature and shows immense diversity.
• There are more than slightly more than 1.5 million species known and still many species are yet to be discovered and described.

Biodiversity changes with the changes in altitude. It increases as one moves from higher to lower altitudes. Decrease in temperature and greater seasonal variability are the two major factors responsible for lower diversity at higher altitudes.

Robert May estimated that about 7 million species are present on earth.

India is one of the twelve of the twelve megadiversity regions of the world with 8.1% of genetic resources of the world. Wildlife Insitute of India has divided the country into 10 biogeographical regions-1. Trans-Himalayas, 2. Himalayas, 3. Desert, 4. Semi-arid, 5. Western Ghats, 6. Deccan Peninsula, 7. Gangetic Plain, 8. North East, 9. Coasts, 10. Island.