MP Board · Class 12 · Biology · Biodiversity and ConservationDiscuss the major causes of biodiversity loss, often referred to as 'The Evil Quartet'. Explain each cause with suitable examples.

Introduction to Biodiversity Loss\nWhile it is well accepted that the loss of biodiversity is a serious global crisis, the current rates of species extinctions are primarily driven by human activities. Ecologists have grouped the major causes of biodiversity loss into four overarching threats, collectively known as 'The Evil Quartet'.

The Four Causes (The Evil Quartet)

1. Habitat Loss and Fragmentation:

   • Explanation: This is the most important cause driving animals and plants to extinction. The overwhelming rise in human population has increased resource demands, leading to the destruction and fragmentation of natural habitats.
   • Example: The tropical rainforests, once covering 14 percent of the earth's land surface, now cover no more than 6 percent. The Amazon rainforest is being cleared for cultivating soybeans or for conversion to grasslands for beef cattle.

2. Over-exploitation:

   • Explanation: When 'need' turns to 'greed', it leads to over-exploitation of natural resources. Whenever human populations have excessively depended on a species for food, shelter, or commercial value, it has led to its extinction.
   • Example: Steller’s sea cow and the passenger pigeon became extinct in the last few centuries due to over-exploitation by humans. Many marine fish populations are currently being over-harvested.

3. Alien Species Invasions:

   • Explanation: When alien (exotic) species are introduced unintentionally or intentionally, some of them turn invasive and cause decline or extinction of indigenous species.
   • 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. Water hyacinth (Eichhornia crassipes) clogged Indian water bodies, causing severe ecological damage.

4. Co-extinctions:

   • Explanation: When a species goes extinct, the plant and animal species associated with it in an obligatory way also become extinct.
   • Example: When a host fish species goes extinct, its unique assemblage of parasites also meets the same fate. Another example is the co-evolved plant-pollinator mutualism where extinction of one invariably leads to the extinction of the other.