Biodiversity is the combined diversity at all levels of biological organisation, a term popularised by the sociobiologist Edward Wilson. The three most important components are:

(i) Genetic diversity. A single species may show high diversity at the genetic level over its distributional range. India has more than 50,000 genetically different strains of rice and 1,000 varieties of mango, and the medicinal plant Rauwolfia vomitoria varies across Himalayan ranges in the potency and concentration of reserpine, its active chemical.

(ii) Species diversity. The diversity at the species level — for example, the Western Ghats have a greater amphibian species diversity than the Eastern Ghats.

(iii) Ecological diversity. Diversity at the ecosystem level — India, with its deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has a greater ecosystem diversity than a Scandinavian country like Norway.

Conservation efforts are aimed at protecting diversity at all these levels.

What is known with certainty. Since there are published records of all species discovered and named, we know how many have been recorded: according to the IUCN (2004), slightly more than 1.5 million plant and animal species have been described. But we have no clear idea how many species are yet to be discovered.

The problem. For many taxonomic groups, species inventories are more complete in temperate than in tropical countries — and an overwhelmingly large proportion of the species waiting to be discovered are in the tropics. The gap in our knowledge is concentrated exactly where the species are.

The method. Biologists take an exhaustively studied group of insects, whose inventory is nearly complete in both regions, and:

1. make a statistical comparison of its temperate-tropical species richness;
2. extrapolate that ratio to other groups of animals and plants, whose temperate species are known but whose tropical species are not;
3. and so arrive at a gross estimate of the total number of species on Earth.

The results. Estimates vary widely and many are only educated guesses. Some extreme estimates range from 20 to 50 million, but a more conservative and scientifically sound estimate made by Robert May places global species diversity at about 7 million.

What the method cannot cover. These estimates give no figures for prokaryotes, because conventional taxonomic methods are not suitable for identifying microbial species and many species are simply not culturable under laboratory conditions. On biochemical or molecular criteria, their diversity alone might run into millions.

(a) The tropics had more evolutionary time. Speciation is generally a function of time. Unlike temperate regions, which were subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years, and thus had a long evolutionary time for species diversification.

(b) Tropical environments are less seasonal. They are relatively more constant and predictable than temperate environments, and such constant environments promote niche specialisation, which leads to greater species diversity. A predictable environment allows a species to specialise narrowly and still survive the whole year, so more species can coexist.

(c) There is more solar energy available in the tropics, which contributes to higher productivity, and this in turn might contribute indirectly to greater diversity.

Why three hypotheses and not one answer. They explain different components of the same observation: (a) why there was time for species to arise, (b) why narrow specialists can persist once they arise, and (c) why there is enough resource to support many populations simultaneously. They are not rivals, and the chapter presents them as hypotheses because the question is still open.

What the slope is. Alexander von Humboldt observed that within a region, species richness increases with increasing explored area, but only up to a limit. For a wide variety of taxa — angiosperm plants, birds, bats, freshwater fishes — the relation is a rectangular hyperbola, which on a logarithmic scale becomes a straight line:

\( \log S = \log C + Z \log A \)

where S is species richness, A is area, Z is the slope of the line, or regression coefficient, and C is the Y-intercept.

Its significance. Z measures how fast species richness rises as area increases.

(i) Its remarkable constancy within regions. Z lies in the range of 0.1 to 0.2 regardless of the taxonomic group or the region: plants in Britain, birds in California and molluscs in New York state all give amazingly similar slopes. A shallow slope of this kind means that additional area mostly adds more of the same species, drawn from a finite regional pool that soon saturates.

(ii) Its steepness across continents. For very large areas such as entire continents, Z rises to 0.6–1.2, and for frugivorous birds and mammals in the tropical forests of different continents it is 1.15. A slope near or above 1 means richness rises almost in proportion to area, because each continent has had its own separate evolutionary history and contributes an almost entirely different, largely endemic set of species.

(iii) Its practical use. Read in reverse, the relationship predicts how many species will be lost when a habitat is reduced in area. This is the basis for estimating extinctions from deforestation, and for deciding how large a reserve must be to retain a given fraction of a region's species.

The accelerated rates of species extinction the world faces are largely due to human activities. There are four major causes, collectively called ‘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, the degradation of habitats by pollution also threatens survival. And when large habitats are broken up into small fragments, mammals and birds requiring large territories, and certain 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. Many extinctions in the last 500 years, such as Steller's sea cow and the passenger pigeon, were due to over-exploitation, 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 eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish; carrot grass, Lantana and water hyacinth damage our native species; and the recent illegal introduction of the African catfish Clarias gariepinus threatens indigenous catfishes in our rivers.

(iv) Co-extinctions. When a species becomes extinct, the plant and animal species associated with it in an obligatory way also become extinct. When a host fish becomes extinct, its unique assemblage of parasites meets the same fate; and in a co-evolved plant-pollinator mutualism, extinction of one invariably leads to the extinction of the other.

The honest position. 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 believed that communities with more species generally tend to be more stable.

What stability requires. 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 found that plots with more species showed less year-to-year variation in total biomass, and that increased diversity contributed to higher productivity. It is therefore believed that communities with high diversity tend to be less variable, more productive and more resistant to biological invasions.

The evidence from loss. 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 — the exact reverse of Tilman's findings.

The rivet popper hypothesis. Paul Ehrlich's analogy captures the risk: in an airplane, the ecosystem, all parts are joined by thousands of rivets, the species. Popping a rivet may not affect flight safety initially, but as more are removed the plane becomes dangerously weak over time. And which rivet is removed is also critical: loss of rivets on the wings — key species that drive major ecosystem functions — is a more serious threat than loss of a few rivets on 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.

What they are. India has a history of religious and cultural traditions that emphasised the protection of nature. In many cultures, tracts of forest were set aside, and all the trees and wildlife within them were venerated and given total protection. These tracts are called sacred groves.

Where they are found.

• Khasi and Jaintia Hills in Meghalaya
• Aravalli Hills of Rajasthan
• Western Ghat regions of Karnataka and Maharashtra
• Sarguja, Chanda and Bastar areas of Madhya Pradesh

Their role in conservation.

(i) They are a form of in situ conservation. Because the whole tract is protected, biodiversity at all levels is conserved — species, genetic variation and ecological interactions alike, including species never described.

(ii) They are refuges for rare and threatened species. In Meghalaya, the sacred groves are the last refuges for a large number of rare and threatened plants — species that survive nowhere else in their region.

(iii) They preserve undisturbed habitat. Having escaped logging and grazing for centuries in many cases, they retain mature vegetation of a kind that has vanished from the surrounding landscape, and so serve as reference points for what the region once held.

(iv) They cost the state nothing and are locally enforced. This matters greatly, because many nations find it economically not feasible to conserve all their biological wealth, and the number of species waiting to be saved far exceeds the conservation resources available.

The biotic components work on water before it can do damage — slowing it, absorbing it and holding the soil in place.

(i) The canopy intercepts rainfall. Leaves and branches break the force of falling rain so that it reaches the ground gently instead of striking bare soil. Much soil erosion is caused by the impact of raindrops themselves, and a forest largely prevents it.

(ii) Litter and humus absorb water. The layer of detritus and the humus formed by humification act as a sponge. Humus is colloidal in nature, so it holds water as well as nutrients, and rain is taken in and released slowly. This converts a sudden flood peak into a steady flow that continues long after the rain has stopped.

(iii) Roots bind the soil. Root systems form a dense mesh that physically holds soil particles on slopes and along river banks. Stratification helps here: a forest with trees, shrubs, herbs and grasses binds the soil at several depths at once.

(iv) Transpiration removes soil water. Plants return large volumes of water to the atmosphere, lowering the water table and leaving the soil able to absorb the next rainfall instead of shedding it.

(v) Detritivores keep the soil open. Earthworms and other soil animals, in fragmenting detritus, also loosen the soil — which is why the earthworm is called the farmer's friend. A loose, open soil absorbs water; a compacted one sheds it.

Why this belongs to the broadly utilitarian argument. None of these services is bought or sold, yet clearing the vegetation transfers the cost to people downstream as floods, silted reservoirs and lost topsoil. This is exactly the sense in which biodiversity plays a major role in the ecosystem services that nature provides.

First, note where the animal figure comes from: among animals, insects are the most species-rich taxonomic group, making up more than 70 per cent of all animals, so that out of every 10 animals on this planet, 7 are insects. The question is therefore largely about insects.

(i) Mobility. Animals can move, disperse into new habitats, colonise islands and isolated valleys, and so become geographically isolated — the usual first step towards speciation. A plant is fixed where its seed happens to land.

(ii) Far more niches available. Almost all plants do the same thing: photosynthesise, and compete for light, water and minerals. Animals are heterotrophs, and every species of plant, fungus, animal and microbe is a potential food source, each offering a distinct way of making a living. Animal niches therefore vastly outnumber plant niches.

(iii) Co-evolution. Herbivorous insects specialise on particular plants and on their chemical defences, and predators, parasites and pollinators specialise in turn. A single plant species can thus support several specialised animal species, multiplying animal diversity on a plant base.

(iv) Small body size and short generation time. Insects are small, so a single tree or pond contains many separate niches, and they reproduce quickly, so reproductive isolation and genetic divergence accumulate faster.

(v) Key innovations. Flight and the exoskeleton opened habitats, including the air and very dry places, that were unavailable to other groups.

(vi) Behaviour and sexual selection. Animals can develop mating preferences, calls, displays and courtship rituals, which can produce reproductive isolation between populations even without geographical separation.

A caution about the figures. Part of the difference reflects how we count. The number of fungi species in the world exceeds the combined total of fishes, amphibians, reptiles and mammals; the estimates give no figures at all for prokaryotes, whose diversity alone might run into millions; and sampling effort has always been greatest on conspicuous animals.

Yes — the clearest case is a pathogen that exists only as an agent of human disease. The smallpox virus was deliberately eliminated from nature by a global vaccination campaign, and similar efforts are far advanced against the poliovirus and the guinea worm. Such organisms have no host other than humans and no ecological role outside the human body.

A second, more common case: eradicating an invasive species from a region it has invaded. The Nile perch in Lake Victoria led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish. Removing the Nile perch, or Lantana, water hyacinth, carrot grass or the illegally introduced African catfish from Indian waters, is deliberate elimination — although note that this is local eradication, not global extinction: the species survives in its native range, where it belongs and does no such damage.

How the first case is justified.

On the narrowly utilitarian argument: smallpox gave us nothing — no food, fibre, firewood, medicine or industrial product — and killed hundreds of millions of people.

On the broadly utilitarian argument: it performed no ecosystem service. It was not a pollinator, a decomposer, a food source or a link in any food chain.

On the ethical argument, with more difficulty: the ethical argument holds that every species has an intrinsic value even if it has no economic value to us, and taken strictly that would forbid this too. The usual resolution is that our moral duty to the millions of species with whom we share the planet cannot extend to accepting mass human death from an organism whose entire existence consists of causing it, and that the duty to pass on our biological legacy in good order concerns functioning ecosystems rather than every genome individually.

Two cautions. First, extinction is irreversible, which is why smallpox virus stocks are still retained in two secure laboratories rather than destroyed — research may yet need them. Second, the argument does not extend to species that merely inconvenience us: mosquitoes transmit malaria but are also pollinators and prey for many animals, and eliminating a widespread wild species risks co-extinctions and consequences we cannot predict.