The accelerated rates of species extinction the world faces now are largely due to human activities. There are four major causes, known collectively as ‘The Evil Quartet’.

(i) Habitat loss and fragmentation — the most important cause. 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, called the lungs of the planet, is being cut and cleared for cultivating soya beans and for conversion to grasslands for raising beef cattle. Besides total loss, degradation of habitats by pollution also threatens survival. And when large habitats are broken into small fragments, mammals and birds requiring large territories, and 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. Steller's sea cow and the passenger pigeon were lost this way, and many marine fish populations around the world are over-harvested today.

(iii) Alien species invasions. When alien species are introduced unintentionally or deliberately, some turn invasive and cause the decline or extinction of indigenous species. The Nile perch introduced into Lake Victoria led to the extinction of more than 200 species of cichlid fish; carrot grass, Lantana and water hyacinth damage native Indian species; and the illegally introduced African catfish Clarias gariepinus threatens our indigenous catfishes.

(iv) Co-extinctions. When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct — a host fish's unique assemblage of parasites, or the partner in a co-evolved plant-pollinator mutualism.

The honest starting point. Whether the number of species in a community really matters to the functioning of the ecosystem is a question for which ecologists have not been able to give a definitive answer. But for many decades ecologists have believed that communities with more species generally tend to be more stable than those with fewer.

What stability means here. A stable community should not show too much variation in productivity from year to year; it must be either resistant or resilient to occasional disturbances, natural or man-made; and it must be resistant to invasions by alien species.

The experimental evidence. David Tilman's long-term ecosystem experiments using outdoor plots gave two tentative answers: plots with more species showed less year-to-year variation in total biomass, and increased diversity contributed to higher productivity.

The consequences of losing it. In general, 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 certain ecosystem processes such as plant productivity, water use, and pest and disease cycles. These are precisely the reverse of Tilman's findings, which is why they are read as confirmation.

The rivet popper analogy. Paul Ehrlich's image makes the risk intelligible: in an airplane, the ecosystem, all parts are joined by thousands of rivets, the species. Popping a few rivets may not affect flight safety initially, but as more are removed the plane becomes dangerously weak. And which rivet is removed matters: losing rivets on the wings — key species that drive major ecosystem functions — is a far more serious threat than losing a few from the seats or windows.

The conclusion. Although we may not understand completely how species richness contributes to the well-being of an ecosystem, we know enough to realise that rich biodiversity is not only essential for ecosystem health but imperative for the very survival of the human race on this planet.

The precedent is real. From the fossil record we learn that large-scale losses of species like the one we are currently witnessing have happened earlier, even before humans appeared. During the more than 3 billion years since the origin and diversification of life, there were five episodes of mass extinction.

The difference is in the rates. Current species extinction rates are estimated to be 100 to 1,000 times faster than in pre-human times. A mass extinction spread over hundreds of thousands of years leaves time for surviving lineages to adapt and radiate; one compressed into a few centuries does not.

The difference is in the cause. Our activities are responsible for the faster rates — the accusing finger is clearly pointing to human activities. Previous episodes followed geological or astronomical events over which nothing alive had any influence. This one is caused by a single species, which means, uniquely, that it could be slowed by decisions.

The projection. Ecologists warn that if present trends continue, nearly half of all the species on Earth might be wiped out within the next 100 years.

And a difference the chapter notes in passing. While it is doubtful whether any new species are being added through speciation into the Earth's treasury of species, there is no doubt about the continuing losses. In earlier extinctions, the surviving lineages eventually refilled the vacated niches. Under present conditions of habitat loss and fragmentation, that recovery process is itself impaired.

Careful analysis of records shows that extinctions across taxa are not random; some groups like amphibians appear to be more vulnerable to extinction. Presently 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species and 31 per cent of all gymnosperm species face the threat of extinction.

(i) They need two habitats, not one. Most amphibians breed in water and live as adults on land, so the loss or pollution of either habitat is fatal. Habitat loss and fragmentation, the most important cause of extinction, therefore strikes them twice.

(ii) Their skin is permeable. Amphibians respire and absorb water through a thin, moist skin, which leaves them directly exposed to pollutants, acidified water and ultraviolet radiation. The degradation of habitats by pollution threatens them more than it does birds or mammals.

(iii) They cannot escape fragmentation. Birds can fly across a cleared strip or a road; amphibians move slowly, over short distances, and often only in damp conditions. A fragmented landscape isolates their populations far more effectively.

(iv) Small ranges and high endemism. Many amphibian species are confined to a single hill range, stream or valley — which is why amphibian diversity differs so markedly even between the Western and Eastern Ghats. A species found in only one place cannot survive the loss of that place.

(v) Sensitivity to climate and disease. Their dependence on moisture and temperature for breeding makes them vulnerable to small climatic shifts, and their moist skin makes them susceptible to fungal disease.

The general principle. Vulnerability follows from ecology: narrow habitat requirements, restricted range, low mobility and direct physiological exposure all raise the risk. This is also why the rivet popper analogy warns that which rivet is removed matters as much as how many.

The distinction the chapter draws. Habitat loss and fragmentation are named together as the most important cause, but the mechanism described for fragmentation is separate: 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. The harm comes from the arrangement of what remains, not only from how much remains.

Why arrangement matters so much.

(i) Territory requirements. A large predator needs a minimum area to feed itself, and that is why it needs so much — the 10 per cent law leaves very little energy at the top of a food chain. Two fragments, each below that minimum, support no individuals at all, although together they hold enough area for several.

(ii) Migration is blocked. Animals with migratory habits depend on moving between areas at different seasons. A barrier across the route makes both areas unusable, even though neither has been destroyed.

(iii) Small populations are fragile. An isolated fragment holds a small population, which is vulnerable to chance events, to loss of genetic variation and to local catastrophe, with no immigration possible to replenish it. Recall from Chapter 11 that immigration is one of the four processes that maintain population density.

(iv) Edge effects. Fragmenting an area multiplies its boundary. Conditions at an edge — light, wind, temperature, invading species — differ from the interior, so the usable interior habitat shrinks by far more than the area actually cleared.

The practical consequence. This is why in situ conservation aims to protect whole ecosystems — we save the entire forest to save the tiger — and why connecting reserves by corridors can matter as much as enlarging them.