Biodiversity and Conservation

In this chapter: levels of biodiversity; how many species exist; biodiversity in India; patterns (latitudinal gradients, species-area relationship); importance of species diversity; loss of biodiversity and its causes; reasons to conserve; in situ and ex situ conservation.

Levels of biodiversity

The term biodiversity was popularised by the sociobiologist Edward Wilson to describe the combined diversity at all levels of biological organisation. The three most important levels:

How many species are there on Earth, and in India?

• The IUCN (2004) puts the total number of plant and animal species described so far at slightly more than 1.5 million.
• Robert May's global estimate places species diversity at about 7 million. He extrapolated from the ratio of tropical to temperate species in well-studied groups such as insects.
• More than 70 per cent of all recorded species are animals; plants (including algae, fungi, bryophytes, gymnosperms and angiosperms) make up no more than 22 per cent.
• Among animals, insects are the most species-rich group, making up more than 70 per cent of the total animals: out of every 10 animals, 7 are insects.
• The number of fungi species in the world is more than the combined total of fishes, amphibians, reptiles and mammals.
• Prokaryote diversity is hard to estimate because conventional taxonomic methods do not work for them and many cannot be cultured in the lab.
• India has only 2.4 per cent of the world's land area, but its share of global species diversity is about 8.1 per cent. India is one of the 12 mega diversity countries of the world.
• Nearly 45,000 species of plants and twice as many (about 90,000) species of animals have been recorded from India.

Patterns of biodiversity

Latitudinal gradients

• Diversity generally decreases from the equator towards the poles. Tropics (23.5° N to 23.5° S) harbour more species than temperate or polar areas.
• Colombia, near the equator, has nearly 1,400 species of birds; New York (41° N) has 105; Greenland (71° N) only 56. India, with much of its land in the tropics, has more than 1,200 species of birds.
• A forest in a tropical region like Ecuador has up to 10 times as many species of vascular plants as a forest of equal area in a temperate region like the Midwest of the USA.
• The Amazonian rain forest in South America has the greatest biodiversity on Earth: more than 40,000 species of plants, 3,000 of fishes, 1,300 of birds, 427 of mammals, 427 of amphibians, 378 of reptiles and more than 1,25,000 invertebrates.

Why are the tropics so diverse?

1. Speciation is generally a function of time. Temperate regions were subjected to frequent glaciations in the past; tropical latitudes remained relatively undisturbed for millions of years and had a long evolutionary time for species diversification.
2. Tropical environments are less seasonal, relatively more constant and predictable, which promotes niche specialisation and greater species diversity.
3. There is more solar energy available in the tropics, which contributes to higher productivity and, indirectly, to greater diversity.

Species-area relationship

• Alexander von Humboldt, the German naturalist and geographer, observed during his explorations in the wilderness of South American jungles that within a region, species richness increased with increasing explored area, but only up to a limit.
• The relation is a rectangular hyperbola; on a logarithmic scale it is a straight line: log S = log C + Z log A, where S = species richness, A = area, Z = slope of the line (regression coefficient), C = Y-intercept.
• Regardless of the taxonomic group or region, the slopes are amazingly similar: Z lies in the range 0.1 to 0.2.
• For species-area relationships among very large areas like entire continents, the slope is much steeper, with Z values of 0.6 to 1.2. For frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is 1.15.

Importance of species diversity to the ecosystem

• Communities with more species tend to be more stable than those with fewer species. A stable community should not show too much variation in productivity from year to year; it must be resistant or resilient to occasional disturbance; and it must be resistant to invasions by alien species.
• David Tilman's long-term ecosystem experiments using outdoor plots: plots with more species showed less year-to-year variation in total biomass. He also showed that increased diversity contributed to higher productivity.
• Rivet popper hypothesis (by the Stanford ecologist Paul Ehrlich): an ecosystem is compared to an aeroplane and its species to the rivets joining the parts. If every passenger pops a rivet (causing a species to become extinct), it may not affect flight safety initially, 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 obviously a more serious threat than the loss of a few rivets on the seats or windows.

Loss of biodiversity

• The colonisation of tropical Pacific islands by humans is said to have led to the extinction of more than 2,000 species of native birds.
• The IUCN Red List (2004) documents the extinction of 784 species (including 338 vertebrates, 359 invertebrates and 87 plants) in the last 500 years.
• Recent extinctions include the dodo (Mauritius), quagga (Africa), thylacine (Australia), Steller's sea cow (Russia), and three subspecies of tiger (Bali, Javan, Caspian).
• The last twenty years alone have witnessed the disappearance of 27 species.
• More than 15,500 species worldwide face the threat of extinction. Presently, 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species and 31 per cent of all gymnosperm species face the threat of extinction.
• There have been five episodes of mass extinction since the origin and diversification of life. The current, sixth extinction is different: current species extinction rates are estimated to be 100 to 1,000 times faster than in pre-human times, and human activities are responsible.
• Loss of biodiversity leads to: (a) decline in plant production, (b) lowered resistance to environmental perturbations such as drought, and (c) increased variability in certain ecosystem processes such as plant productivity, water use, and pest and disease cycles.

Causes: "the evil quartet"

1. Habitat loss and fragmentation: the most important cause. Tropical rain forests once covered more than 14 per cent of the Earth's land surface; now they cover no more than 6 per cent. The Amazon rain forest, so huge that it is called the "lungs of the planet", is being cut and cleared for cultivating soya beans or for conversion to grasslands for raising beef cattle. When large habitats are broken up into small fragments, mammals and birds requiring large territories, and certain animals with migratory habits, are badly affected.
2. Over-exploitation: when "need" turns to "greed". Steller's sea cow and the passenger pigeon became extinct in the last 500 years due to over-exploitation by humans. Many marine fish populations around the world are over-harvested.
3. Alien species invasions: the introduction of the Nile perch into Lake Victoria in East Africa led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish. Invasive weeds such as Parthenium (carrot grass), Lantana and water hyacinth (Eichhornia) threaten native species. The illegal introduction of the African catfish Clarias gariepinus for aquaculture poses a threat to the indigenous catfishes in Indian rivers.
4. 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 species becomes extinct, its unique assemblage of parasites meets the same fate. Another example is the co-evolved plant-pollinator mutualism, where the extinction of one leads to the extinction of the other.

Why conserve biodiversity?

How do we conserve biodiversity?

In situ (on site) conservation

• Protecting the whole ecosystem protects its biodiversity at all levels: save the entire forest to save the tiger.
• Biodiversity hotspots: regions with very high levels of species richness and high degree of endemism (species confined to that region and not found anywhere else). Initially 25 hotspots were identified; subsequently nine more were added, bringing the total to 34.
• Three of these hotspots cover India's exceptionally high biodiversity regions: Western Ghats and Sri Lanka, Indo-Burma, and the Himalaya.
• Although all the hotspots together cover less than 2 per cent of the Earth's land area, the number of species they collectively harbour is extremely high, and strict protection could reduce the ongoing mass extinctions by almost 30 per cent.
• In India, ecologically unique and biodiversity-rich regions are legally protected as biosphere reserves, national parks and sanctuaries. India now has 14 biosphere reserves, 90 national parks and 448 wildlife sanctuaries (NCERT figures).
• Sacred groves: tracts of forest set aside, with all trees and wildlife venerated and given total protection. Found in the Khasi and Jaintia Hills in Meghalaya, the Aravalli Hills of Rajasthan, the Western Ghat regions of Karnataka and Maharashtra, and the Sarguja, Chanda and Bastar areas of Madhya Pradesh (NCERT's wording; Sarguja and Bastar are now in Chhattisgarh, and Chanda is Chandrapur in Maharashtra). In Meghalaya, the sacred groves are the last refuges for a large number of rare and threatened plants.

Ex situ (off site) conservation

• Threatened animals and plants are taken out of their natural habitat and placed in special settings where they can be protected and given special care: zoological parks, botanical gardens, wildlife safari parks.
• Many animals that have become extinct in the wild continue to be maintained in zoological parks.
• Gametes of threatened species can be preserved in viable and fertile condition for long periods using cryopreservation. Eggs can be fertilised in vitro, and plants can be propagated using tissue culture. Seeds of different genetic strains of commercially important plants can be kept for long periods in seed banks.

International efforts

• The Earth Summit, the historic Convention on Biological Diversity held in Rio de Janeiro in 1992, called upon all nations to take appropriate measures for conservation of biodiversity and sustainable utilisation of its benefits.
• The World Summit on Sustainable Development, held in 2002 in Johannesburg, South Africa: 190 countries pledged their commitment to achieve, by 2010, a significant reduction in the current rate of biodiversity loss at global, regional and local levels.

Common traps: (1) India: 2.4 per cent of land, 8.1 per cent of species. (2) Z is 0.1 to 0.2 normally, 0.6 to 1.2 for continents, 1.15 for frugivorous birds and mammals. (3) Habitat loss is the most important cause of extinction. (4) The Nile perch introduced into Lake Victoria led to the extinction of more than 200 species of cichlid fish. (5) Rivet popper hypothesis is Paul Ehrlich's, not Tilman's. (6) Hotspots cover less than 2 per cent of land. (7) Amazon: 20 per cent of the oxygen. (8) Sacred groves in Maharashtra are in the Western Ghat regions, useful for local students to remember.

NEET focus

• Examples for each level of biodiversity (reserpine, rice, mango, amphibians of the Western Ghats).
• All the numbers: 1.5 million, 7 million, 70 per cent, 22 per cent, 2.4 and 8.1 per cent, 45,000 and 90,000.
• Latitudinal gradient reasons; species-area equation and Z values.
• Tilman and Ehrlich; the evil quartet with examples; extinct species list.
• Utilitarian and ethical arguments; hotspots; in situ and ex situ; Rio 1992 and Johannesburg 2002.

Practice questions