The essentials
The colonisation of tropical Pacific islands by humans is said to have led to the extinction of more than 2000 species of native birds. Recent extinctions include the dodo (Mauritius), quagga (Africa), thylacine (Australia), Steller's sea cow (Russia) and three subspecies of tiger (Bali, Javan, Caspian). The current species extinction rate is estimated to be 100 to 1000 times faster than in pre-human times, and human activities are responsible. This is often called the sixth mass extinction. Loss of biodiversity in a region may lead to a decline in plant production, lowered resistance to environmental perturbations such as drought, and increased variability in ecosystem processes such as plant productivity, water use and pest and disease cycles.
Causes of biodiversity loss – the "evil quartet":
- Habitat loss and fragmentation: the most important cause. Tropical rain forests once covered 14 per cent of the earth's land surface but now cover no more than 6 per cent. The Amazon rain forest, called the "lungs of the planet" because it is so vast, is being cut and cleared for cultivating soybeans or for conversion to grasslands for raising beef cattle. When large habitats are broken into small fragments, mammals and birds needing large territories, and animals with migratory habits, are badly affected.
- Over-exploitation: Steller's sea cow and the passenger pigeon became extinct in the last 500 years due to over-exploitation by humans. Many marine fish populations are over-harvested.
- Alien species invasions: introduced species may become invasive. The Nile perch introduced into Lake Victoria in East Africa eventually led to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish. Carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) are invasive weeds in India. The illegal introduction of the African catfish Clarias gariepinus for aquaculture threatens indigenous catfishes in Indian rivers.
- Co-extinctions: when a species becomes extinct, 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 meets the same fate. Co-evolved plant–pollinator mutualism is another example.
Common mistakes
- Habitat loss, not over-exploitation, is the most important cause of extinction.
- Lantana and Parthenium are alien species, not native Indian plants.