Edward Wilson popularized the term ‘biodiversity’.
1. Genetic diversity: Diversity shown by a single species at genetic level. E.g. Rauwolfia vomitoria (Himalaya) shows genetic variation in the potency & concentration of the chemical reserpine. India has more than 50,000 different strains of rice and 1000 varieties of mango.
2. Species diversity: Diversity at species level. E.g. Western Ghats have greater amphibian species than Eastern Ghats.
3. Ecological diversity: Diversity at ecosystem level.
E.g. In India, deserts, rain forests, mangroves, coral reefs, wet lands, estuaries & alpine meadows are seen.
o According to IUCN (2004), more than 1.5 million species described so far.
o According to Robert May’s Global estimate, about 7 million species would have on earth. (He considered the species to be discovered in the tropics. i.e. only 22% of the total species have been recorded so far).
o Animals are more diverse (above 70%) than plants including Plantae and Fungi (22%).
o Among animals, insects are most species rich group (70%, i.e. out of every 10 animals, 7 are insects).
o Number of fungi species is more than the combined total of the species of fishes, amphibians, reptiles & mammals.
o India has only 2.4% of world’s land area, but has 8.1% of the species diversity. India is one of the 12 mega diversity countries of the world. Nearly 45,000 plant species and twice as many of animals have been recorded from India.
o Applying May’s global estimates, India would have more than 1 lakh plant species and 3 lakh animal species.
o Biologists are not sure about total number of prokaryotic species because
· Conventional taxonomic methods are not suitable for identifying microbial species.
· In laboratory, many species cannot be cultured.
i. Latitudinal gradients
- Species diversity decreases from the equator to the poles.
- Tropics (latitudinal range of 23.5o N to 23.5o S) have more species than temperate or polar areas.
E.g. Number of bird species in different latitudes:
o Colombia (near equator): about 1400 species.
o India (in tropics): > 1200 species.
o New York (41o N): 105 species.
o Greenland (71o N): 56 species.
- Tropical forest region like Equador has up to 10 times of vascular plant species as compared to a temperate forest region like the Midwest of USA.
- Tropical Amazonian rain forest (South America) is the greatest biodiversity on earth. It contains
o > 40000 species of plants
o 3000 species of fishes
o 1300 species of birds
o 427 species of mammals
o 427 species of amphibians
o 378 species of reptiles
o > 1,25,000 species of invertebrates
- Biodiversity (species richness) is highest in tropics because
o Tropics had more evolutionary time.
o Relatively constant environment (less seasonal).
o They receive more solar energy which contributes to greater productivity.
ii. Species- Area relationship
According to the study of Alexander von Humboldt in South American jungles, within a region, species richness increases with increasing explored area, but only up to a limit.
S= Species richness
Z= slope of the line (regression co-efficient)
- On a logarithmic scale, the relationship is a straight line described the equation Log S = log C + Z log A
- Generally, for small areas, the Z value is 0.1 to 0.2.
- But for large areas (e.g. entire continents), slope of the line is steeper (Z value: 0.6 to 1.2).
- E.g. for frugivorous birds and mammals in the tropical forests of different continents, the Z value is 1.15.
- According to David Tilman, plots with more species shows less year-to-year variation in total biomass.
- Increased diversity contributes to higher productivity. It is essential for ecosystem health and survival of human race.
- ‘Rivet popper hypothesis’: It is an analogy used to understand the importance of biodiversity.
It is proposed by Stanford ecologist Paul Ehrlich.
In an airplane (ecosystem), all parts are joined with many rivets (species). If passengers pop a rivet (extinction of a species), it may not affect flight safety (functioning of the ecosystem). But as more and more rivets are removed, the plane becomes dangerously weak. Loss of rivets on the wings (key species that drive major ecosystem functions) is more dangerous than loss of a few rivets on the seats or windows.