Causes of biodiversity losses (‘ The Evil Quartet’):

Causes of biodiversity losses (‘The Evil Quartet’):
  • The accelerated rates of species extinctions that the world is facing now are largely due to human activities. There are four major causes (‘ The Evil Quartet’ is the sobriquet used to describe them).
(i) Habitat loss and fragmentation:
  • This is the most important cause driving animals and plants to extinction. The most dramatic examples of habitat loss come from tropical rain forests. 
  • Once covering more than 14 per cent of the earth’s land surface, these rain forests now cover no more than 6 per cent. 
  • They are being destroyed fast. By the time you finish reading this chapter, 1000 more hectares of rain forest would have been lost. 
  • The Amazon rain forest (it is so huge that it is called the ‘lungs of the planet’) harbouring probably millions of species is being cut and cleared for cultivating soya beans or for conversion to grasslands for raising beef cattle. 
  • Besides total loss, the degradation of many habitats by pollution also threatens the survival of many species. 
  • When large habitats are broken up into small fragments due to various human activities, mammals and birds requiring large territories and certain animals with migratory habits are badly affected, leading to population declines. 
(ii) Over-exploitation: 
  • Humans have always depended on nature for food and shelter, but when ‘need’ turns to ‘greed’, it leads to over -exploitation of natural resources. 
  • Many species extinctions in the last 500 years few are- Steller’s sea cow, passenger pigeon (Ectopistes migratoriu) were due to overexploitation by humans. 
  • Presently many marine fish populations around the world are over harvested, endangering the continued existence of some commercially important species. 
(iii) Alien species invasions: 
  • When alien species are introduced unintentionally or deliberately for whatever purpose, some of them turn invasive, and cause decline or extinction of indigenous species. 
  • The Nile perch introduced into Lake Victoria in east Africa led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake. 
  • You must be familiar with the environmental damage caused and threat posed to our native species by invasive weed species like carrot grass (Parthenium), Lantana and water hyacinth (Eicchornia). 
  • The recent illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes is posing a threat to the indigenous catfishes in our rivers. 
(iv) Co-extinctions: 
  • When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct. 
  • When a host fish species becomes extinct, its unique assemblage of parasites also meets the same fate. 
  • Another example is the case of a coevolved plant-pollinator mutualism where extinction of one invariably leads to the extinction of the other.

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