Biodiversity is the diversity of biological organization ranging from cellular macromolecules to biomes. Edward Wilson popularized the term ‘biodiversity’.
Levels of Biodiversity
- Genetic diversity: Diversity shown by a single species at the genetic level. For example, Rauwolfia vomitoria (Himalaya) shows genetic variation in the potency and concentration of the chemical reserpine. India has more than 50,000 different strains of rice and 1,000 varieties of mango.
- Species diversity: Diversity at the species level. For example, the Western Ghats have greater amphibian species than the Eastern Ghats.
- Ecological diversity: Diversity at the ecosystem level. For example, in India, deserts, rain forests, mangroves, coral reefs, wetlands, estuaries, and alpine meadows are seen.
Number of Species on Earth (Global Species Diversity)
- According to IUCN (2004), more than 1.5 million species have been described so far.
- According to Robert May’s Global estimate, about 7 million species would exist on Earth (considering species to be discovered in the tropics, i.e., only 22% of total species have been recorded so far).
- Animals are more diverse (above 70%) than plants, including Plantae and Fungi (22%).
- Among animals, insects are the most species-rich group (70%, i.e., out of every 10 animals, 7 are insects).
- The number of fungi species is more than the combined total of species of fishes, amphibians, reptiles, and mammals.
- India has only 2.4% of the 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 animals have been recorded from India.
- Applying May’s global estimates, India would have more than 100,000 plant species and 300,000 animal species. Biologists are not sure about the total number of prokaryotic species because:
- Conventional taxonomic methods are not suitable for identifying microbial species.
- In laboratories, many species cannot be cultured.
Patterns of Biodiversity
Latitudinal Gradients
- Species diversity decreases from the equator to the poles.
- Tropics (latitudinal range of 23.5° N to 23.5° S) have more species than temperate or polar areas.
- For example, the number of bird species in different latitudes:
- Colombia (near equator): about 1,400 species.
- India (in tropics): over 1,200 species.
- New York (41° N): 105 species.
- Greenland (71° N): 56 species.
- The tropical forest region like Ecuador has up to 10 times the number of vascular plant species compared to a temperate forest region like the Midwest of the USA.
- Tropical Amazonian rainforest (South America) has the greatest biodiversity on Earth, containing:
- Over 40,000 species of plants.
- 3,000 species of fishes.
- 1,300 species of birds.
- 427 species of mammals.
- 427 species of amphibians.
- 378 species of reptiles.
- Over 125,000 species of invertebrates.
- Biodiversity (species richness) is highest in the tropics because:
- Tropics had more evolutionary time.
- Relatively constant environment (less seasonal).
- They receive more solar energy, which contributes to greater productivity.
Species-Area Relationship
- According to Alexander von Humboldt’s study in South American jungles, within a region, species richness increases with increasing explored area, but only up to a limit. The relation between species richness and area gives a rectangular hyperbola.
- The equation is: S = CAz, where:
- S = Species richness
- A = Area
- C = Y-intercept
- Z = Slope of the line (regression coefficient)
- On a logarithmic scale, the relationship is a straight line described by the equation: Log S = log C + Z log A.
- Generally, for small areas, the Z value is 0.1 to 0.2. For large areas (e.g., entire continents), the slope is steeper (Z value: 0.6 to 1.2). For example, for frugivorous birds and mammals in tropical forests of different continents, the Z value is 1.15.
Importance of Species Diversity
- According to David Tilman, plots with more species show less year-to-year variation in total biomass.
- Increased diversity contributes to higher productivity, essential for ecosystem health and the survival of the human race.
- ‘Rivet Popper Hypothesis’: Proposed by Stanford ecologist Paul Ehrlich, it uses an analogy to understand the importance of biodiversity. 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 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 seats or windows.