MP Board · Class 12 · Biology · Biodiversity and ConservationDiscuss the 'Evil Quartet' representing the four major causes of biodiversity losses in detail. (5 Marks)

Biodiversity is currently facing accelerated rates of extinction, largely attributed to human activities. The four major causes of biodiversity loss are collectively known as the 'Evil Quartet'. These are discussed below:

1. Habitat Loss and Fragmentation

• This is the single most important cause driving animals and plants to extinction.
• The destruction of natural habitats due to deforestation, urbanisation, agricultural expansion, and mining leaves organisms homeless and leads to population decline.
• When large habitats are broken or fragmented into smaller pieces due to human activities (such as building roads or constructing dams), mammals and birds requiring large territories and migratory animals are badly affected, leading to a decline in population.
• The tropical rainforests, which once covered 14 percent of the earth's land surface, now cover no more than 6 percent.

2. Over-exploitation

• When 'need' turns to 'greed', it leads to over-exploitation of natural resources.
• Many species extinctions in the last 500 years (such as Steller's sea cow, passenger pigeon) were due to over-exploitation by humans.
• Currently, many marine fish populations around the world are over-harvested, endangering the continued existence of commercially important species.

3. Alien Species Invasions

• When alien species (non-native/exotic species) are introduced unintentionally or deliberately anywhere, some of them turn invasive and cause decline or extinction of indigenous species.
• For example, the Nile perch introduced into Lake Victoria in East Africa led to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake.
• In India, the illegal introduction of the African catfish Clarias gariepinus for aquaculture poses a threat to the indigenous catfishes in our rivers.
• Invasive weeds like Water hyacinth (Eichhornia crassipes) and Lantana have caused immense damage to native species.

4. Co-extinctions

• When a species becomes extinct, the plant and animal species associated with it in an obligatory relationship also become extinct.
• For example, when a host fish species becomes extinct, its unique assemblage of parasites also meets the same fate.
• In the case of a coevolved plant-pollinator mutualism, extinction of one invariably leads to the extinction of the other.