The evil quartet

Today's accelerated extinctions are largely caused by humans. The four major causes are nicknamed 'The Evil Quartet'.

Habitat loss and fragmentation is the most important cause. Tropical rain forests, which once covered over 14 per cent of the land surface of the Earth, now cover no more than 6 per cent; 1000 more hectares are lost in the time it takes to read this chapter.

The Amazon rain forest, called the 'lungs of the planet' and home to probably millions of species, is being cleared to grow soya beans or to make grassland for beef cattle. Pollution also degrades many habitats.

When large habitats are broken into small fragments, mammals and birds that need large territories, and animals with migratory habits, are badly hit and their populations decline.

Over-exploitation: when 'need' turns to 'greed', species are harvested to extinction. Steller's sea cow and the passenger pigeon were lost this way in the last 500 years, and many marine fish populations are over-harvested today.

Alien species invasions: introduced species can turn invasive. After the Nile perch was introduced into Lake Victoria in east Africa, over 200 species of cichlid fish, an ecologically unique group, eventually went extinct.

In India, the invasive weeds carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) damage the environment, and the illegally introduced African catfish Clarias gariepinus threatens native catfishes in our rivers.

Co-extinctions: when a species dies out, species bound to it in an obligatory way die too. A host fish takes its unique parasites with it, and in a coevolved plant-pollinator mutualism the loss of one leads to the loss of the other.