Concepts: Importance of biodiversity and its loss

3.1 Does diversity matter to the ecosystem

Whether the number of species in a community matters to how the ecosystem works is a question ecologists cannot answer definitively. For decades they held that communities with more species tend to be more stable than species-poor ones. Stability here means three things: productivity should not vary too much from year to year, the community should resist or recover from occasional disturbances, and it should resist invasion by alien species.

David Tilman's long-term experiments in outdoor plots give tentative answers. Species-rich plots varied less in total biomass from one year to the next, and adding species raised productivity. The links between richness and stability are still being worked out, but the practical conclusion is clear enough: biodiversity matters for the health of ecosystems and for human survival.

Paul Ehrlich's rivet popper hypothesis makes the point with an analogy. An ecosystem is like an aeroplane held together by thousands of rivets, each a species. If passengers keep pulling out rivets to take home, the plane may fly normally at first, but as rivets are removed it becomes dangerously weak. Which rivet goes also matters: losing rivets from the wings, the key species that drive major functions, is far more serious than losing a few from the seats.

3.2 The record of loss

Species are being lost steadily while new ones are barely appearing. Human activity is the main cause. The colonisation of the tropical Pacific islands is said to have driven more than 2,000 native bird species extinct. The IUCN Red List of 2004 records 784 extinctions in the last 500 years, including 338 vertebrates, 359 invertebrates and 87 plants. Recent losses include the dodo of Mauritius, the quagga of Africa, the thylacine of Australia, Steller's sea cow of Russia and the Bali, Javan and Caspian subspecies of tiger. The last twenty years alone account for 27 species. Extinctions are not spread evenly across groups: amphibians are more vulnerable than most. More than 15,500 species worldwide now face the threat of extinction, including 12 per cent of bird species, 23 per cent of mammals, 32 per cent of amphibians and 31 per cent of gymnosperms.

Fossil records show five earlier mass extinctions. The present sixth one differs in speed: current extinction rates are estimated at 100 to 1,000 times the pre-human rate, and human activity is responsible. If present trends continue, nearly half of all species on earth could be gone within the next hundred years.

3.3 Consequences of losing biodiversity

Three broad effects follow from losing biodiversity. Plant production drops. A species-poor community also survives drought and similar shocks less easily. A third effect is that swings appear in how much plants produce, how water is used, and how pest and disease cycles behave.

3.4 The evil quartet

Four causes account for most of the accelerated extinction, and textbooks call them the evil quartet.

Habitat loss and fragmentation is the most important. Tropical rain forests once covered more than 14 per cent of the earth's land surface and now cover no more than 6 per cent. The Amazon, sometimes called the lungs of the planet, is being cleared for soya bean cultivation and for grassland to raise beef cattle. Pollution degrades habitats too. When large habitats break into small fragments, mammals and birds that need large territories and animals with migratory habits suffer most, and their populations decline.

Over-exploitation comes next. When need turns into greed, resources are harvested beyond replacement. Steller's sea cow and the passenger pigeon were lost this way, and many marine fish populations are now over-harvested.

Alien species invasions are the third cause. Species moved deliberately or accidentally sometimes become invasive and drive native species to decline or extinction. Lake Victoria lost more than 200 cichlid fish species after the Nile perch was introduced there. Indian examples include the invasive weeds Parthenium, Lantana and Eichhornia, and the African catfish Clarias gariepinus, brought in illegally, now threatens native catfishes.

Co-extinctions close the list. When a species disappears, the species obliged to live with it may follow. Host-specific parasites die with their host, and in a tightly coevolved plant-pollinator pair, the loss of one can take the other. This last cause is easy to overlook because it works quietly: the second extinction may come years after the first, and the connection is rarely obvious at the time.