Biodiversity: concept and patterns

The essentials

Biodiversity is the diversity of life at all levels of biological organisation. The term was popularised by the sociobiologist Edward Wilson. The important levels are:

• Genetic diversity: variation in genes within a species. The medicinal plant Rauwolfia vomitoria in different Himalayan ranges shows variation in the potency and concentration of the chemical reserpine. India has a very large number of genetically different strains of rice and varieties of mango.
• Species diversity: diversity at the species level; for example, the Western Ghats have greater amphibian species diversity than the Eastern Ghats.
• Ecological diversity: diversity at the ecosystem level; India, with deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has greater ecosystem diversity than a Scandinavian country such as Norway.

More than 1.5 million species have been described, but many more remain undiscovered, especially insects and microbes in the tropics. Robert May estimated the global species diversity at about 7 million. Although India has only 2.4 per cent of the world's land area, it accounts for about 8.1 per cent of global species diversity and is one of the 12 megadiversity countries.

Patterns of biodiversity:

• Latitudinal gradient: species diversity decreases as we move away from the equator towards the poles. Tropics (23.5° N to 23.5° S) have more species. Explanations: tropical areas have remained relatively undisturbed for millions of years, giving long evolutionary time; tropical environments are less seasonal and more predictable, promoting niche specialisation; and more solar energy is available, contributing to higher productivity.
• Species–area relationship: Alexander von Humboldt observed that within a region species richness increases with explored area, but only up to a limit. On a log scale the relationship is a straight line: log S = log C + Z log A, where S is species richness, A is area, Z is the slope (regression coefficient) and C is the Y-intercept. Z generally lies between 0.1 and 0.2 regardless of taxonomic group or region; for very large areas such as entire continents, the slope is much steeper, about 0.6 to 1.2.

Importance of species diversity: David Tilman's long-term outdoor plot experiments showed that plots with more species had less year-to-year variation in total biomass and higher productivity. Paul Ehrlich's rivet popper hypothesis compares an ecosystem to an aeroplane and species to rivets: losing a few rivets may not matter at first, but over time the aircraft becomes dangerously weak, and losing rivets on key parts (key species driving major functions) is a more serious threat.