Loss of biodiversity

Species are disappearing fast. The IUCN Red List (2004) records 784 extinctions over the last 500 years, including the dodo (Mauritius), the quagga (Africa), the thylacine (Australia), Steller's sea cow (Russia), and three subspecies of tiger (Bali, Javan and Caspian). The last twenty years alone saw 27 species disappear.

More than 15,500 species worldwide now face extinction, including a large share of the world's birds, mammals, amphibians and gymnosperms.

The earth has seen five mass extinctions in its history, long before humans. The current loss is being called the sixth extinction, and it is different: species are vanishing 100 to 1,000 times faster than in pre-human times, and human activities are the cause.

Losing biodiversity harms whole ecosystems: plant production falls, the system resists disturbances such as drought less well, and processes such as water use, pest and disease cycles become more variable.

The four major causes are called the 'Evil Quartet':

• Habitat loss and fragmentation, the most important cause. The tropical rain forests once covered 14% of the earth's land; today they cover no more than 6%. The vast Amazon forest, the 'lungs of the planet', is being cut and cleared for soya beans and cattle pasture. When a habitat is broken into small pieces, animals that need large territories, and migratory ones, suffer most.
• Over-exploitation: taking too much from nature. Steller's sea cow and the passenger pigeon were hunted to extinction, and many fish stocks are being over-harvested.
• Alien species invasions: species brought in from elsewhere can become invasive and wipe out natives. The Nile perch, introduced into Lake Victoria in East Africa, led to the extinction of more than 200 species of cichlid fish. Invasive weeds such as carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) damage our environment; the illegal introduction of the African catfish for aquaculture threatens native catfishes.
• Co-extinctions: when one species dies out, the species that depend on it, such as its parasites or its pollinator partner, die with it.