Biodiversity is the combined diversity at all levels of biological organisation, from macromolecules within cells to biomes — a term popularised by the sociobiologist Edward Wilson. Of these levels, three are the most important.

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

(ii) Species diversity. Diversity at the species level. Indian example: the Western Ghats have a greater amphibian species diversity than the Eastern Ghats.

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

Why all three are conserved together. Conservation efforts are aimed at protecting diversity at all these levels, because each carries something the others cannot. Genetic diversity is a species' capacity to adapt; species diversity is the working membership of a community; ecological diversity is the range of conditions under which life is organised at all.

What is known directly. Since there are published records of all the species discovered and named, we know how many species have been recorded so far: according to the IUCN (2004), slightly more than 1.5 million plant and animal species have been described. What is not known is how many remain to be discovered.

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

The method used to bridge it. Biologists make a statistical comparison of the temperate-tropical species richness of an exhaustively studied group of insects — a group whose inventory is nearly complete in both regions. They then extrapolate that ratio to other groups of animals and plants, whose temperate species are known but whose tropical species are not, 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.

The limitation of the method. These estimates give no figures at all for prokaryotes, because conventional taxonomic methods are not suitable for identifying microbial species and many species are simply not culturable under laboratory conditions. If biochemical or molecular criteria were accepted for delineating species in this group, their diversity alone might run into millions.

The figure is dominated by one group: among animals, insects are the most species-rich taxonomic group, making up more than 70 per cent of all animals — out of every 10 animals on this planet, 7 are insects. So the real question is why animals, and especially insects, have diversified so far.

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

(ii) Far more ways of making a living. Almost all plants do the same thing: they 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 offers a distinct niche, so animal niches vastly outnumber plant niches.

(iii) Co-evolution with plants and with each other. Herbivorous insects specialise on particular plant species and their chemical defences, and predators, parasites and pollinators each specialise in turn. Every plant species can therefore support several animal species specialised on it, multiplying diversity.

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

(v) Flight and the exoskeleton. These two insect innovations opened habitats unavailable to other organisms and allowed exploitation of the air and of extremely dry places.

A caution about the figures. Part of the difference is a matter of how we count rather than what exists. The number of fungi species in the world is more than the combined total of the species of fishes, amphibians, reptiles and mammals; and the estimates give no figures for prokaryotes, whose diversity alone might run into millions. Sampling effort has also been far greater on conspicuous animals than on plants and microbes.

Reason 1 — latitude. Much of India's land area lies in the tropical latitudes. With very few exceptions the tropics harbour more species than temperate or polar areas, and India, being largely tropical, has more than 1,200 species of birds compared with 105 in New York and 56 in Greenland.

Reason 2 — ecological diversity. India has deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows — a greater ecosystem diversity than a Scandinavian country like Norway. Each ecosystem carries its own species, so a country that contains many ecosystems contains many species, quite apart from its total area.

Reason 3 — topography and isolation. The Himalaya, the Western Ghats and the peninsular ranges create steep gradients of altitude and rainfall over short distances, and isolated pockets in which species become endemic. This is why three of the world's biodiversity hotspots — Western Ghats and Sri Lanka, Indo-Burma and Himalaya — cover India's exceptionally high biodiversity regions.

Reason 4 — genetic diversity built by cultivation. Thousands of years of agriculture in varied conditions have produced more than 50,000 genetically different strains of rice and 1,000 varieties of mango.

The obligation this creates. Being one of the 12 mega diversity countries of the world means India holds, in trust, a share of global biodiversity three times larger than its share of the land. Much of it is undescribed — probably more than 1,00,000 plant species and more than 3,00,000 animal species remain to be discovered here — and a large fraction faces extinction before discovery. The obligation is therefore both scientific, to complete the inventory, and practical, to protect habitat: India's in situ conservation efforts are reflected in its biosphere reserves, national parks, wildlife sanctuaries and sacred groves.

The difference is genetic diversity. A single species can show high diversity at the genetic level over its distributional range, and India has more than 50,000 genetically different strains of rice precisely because generations of farmers selected for different soils, rainfalls, pests and tastes. The second field holds twenty samples of that variation; the first holds one.

Why it matters even when yields match.

(i) Resistance to a new threat. A new pest or disease that can attack the single variety can attack every plant in the first field, because they are genetically identical. In the second field some strains are likely to resist, so the loss is partial rather than total.

(ii) Tolerance of a bad year. Drought, salinity, an unseasonal flood or an early frost will affect twenty strains unequally. The mixed field gives a lower peak but a far more reliable floor — which is the same reasoning as Tilman's finding that plots with more species showed less year-to-year variation in total biomass.

(iii) Raw material for future breeding. Every improved variety is bred from existing genetic variation. The traditional strains carry genes for disease resistance, drought tolerance and grain quality that no one has yet had reason to use; once lost, they cannot be recreated. This is exactly what makes bioprospecting valuable, and why the rich genetic diversity of Indian rice has been a target of patent claims.

The honest qualification. The single-variety field is not a mistake. It is easier to sow, manage, harvest and sell, and high-yielding varieties fed a rapidly growing population when traditional breeding could not. The point is that its yield is bought by spending a resource — genetic diversity — that does not appear in the accounts, and cannot be bought back.