Biodiversity and Conservation — Notes

Biodiversity, a term popularised by the sociobiologist Edward Wilson, is the combined diversity at all levels of biological organisation.

• Genetic diversity: a single species may show high diversity at the genetic level over its range. Rauwolfia vomitoria growing in different Himalayan ranges varies in the potency and concentration of the chemical reserpine. India has more than 50,000 genetically different strains of rice and 1,000 varieties of mango.
• Species diversity: diversity at the species level; the Western Ghats have a greater amphibian species diversity than the Eastern Ghats.
• Ecological diversity: diversity of ecosystems; India, with its deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has greater ecosystem diversity than a Scandinavian country like Norway.

How many species? According to the IUCN (2004), slightly more than 1.5 million plant and animal species have been described. Robert May placed the global estimate at about 7 million. More than 70 percent of all recorded species are animals; plants (including algae, fungi, bryophytes, gymnosperms and angiosperms) make up no more than 22 percent. Among animals, insects are the most species-rich group, making up more than 70 percent of the total; out of every 10 animals, 7 are insects. The number of fungal species exceeds the combined total of fishes, amphibians, reptiles and mammals.

India has only 2.4 percent of the world’s land area but 8.1 percent of global species diversity, making it one of the 12 mega diversity countries. Nearly 45,000 species of plants and twice as many animals have been recorded; applying May’s estimates, more than 1,00,000 plant and 3,00,000 animal species may yet be discovered.

Latitudinal gradients. Species diversity generally decreases from the equator towards the poles. The 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 at 41° N has 105; Greenland at 71° N has 56. India, with much of its land in tropical latitudes, has more than 1,200 bird species. 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 Amazon rain forest has the greatest biodiversity on Earth.

Why the tropics are richer:

1. Speciation is generally a function of time. Temperate regions were subjected to frequent glaciations in the past, whereas tropical latitudes remained relatively undisturbed for millions of years.
2. Tropical environments are less seasonal, more constant and predictable, which promotes niche specialisation.
3. More solar energy is available in the tropics, contributing to higher productivity, which might in turn contribute indirectly to greater diversity.

Species–area relationship. Alexander von Humboldt observed that within a region species richness increases with increasing explored area, but only up to a limit. The relation is a rectangular hyperbola, and on a logarithmic scale a straight line: $\log S = \log C + Z \log A$, where S is species richness, A area, C the Y-intercept and Z the slope (regression coefficient). Z lies in the range 0.1 to 0.2 regardless of taxonomic group or region; for very large areas such as entire continents the slope is much steeper, with Z of 0.6 to 1.2 (for example, frugivorous birds and mammals in tropical forests of different continents, slope 1.15).

Importance of species diversity. A stable community does not show too much variation in productivity from year to year, is resistant or resilient to occasional disturbances, and resists invasion by alien species. David Tilman’s long-term outdoor plot experiments showed that plots with more species had less year-to-year variation in total biomass, and that greater diversity contributed to higher productivity. Paul Ehrlich’s rivet popper hypothesis compares an ecosystem to an aeroplane whose rivets are species: losing a few rivets may not affect flight at first, but as more are removed the plane becomes dangerously weak, and losing rivets on the wings (key species that drive major ecosystem functions) is more serious than losing those on the seats.

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 (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). In the last twenty years alone, 27 species have disappeared. More than 15,500 species worldwide face the threat of extinction; at present 12 percent of bird species, 23 percent of mammal species, 32 percent of amphibian species and 31 percent of gymnosperm species face this threat.

The sixth extinction, now in progress, differs from the five earlier mass extinctions: current rates are estimated to be 100 to 1,000 times faster than in pre-human times, and human activities are responsible. If present trends continue, nearly half of all species might be wiped out within the next 100 years.

Effects of loss of biodiversity in a region: a decline in plant production; lowered resistance to environmental perturbations such as drought; and 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 percent of the Earth’s land surface and now cover no more than 6 percent. The Amazon rain forest, called the “lungs of the planet”, is being cut and cleared for cultivating soya beans or for conversion to grassland for raising beef cattle. When large habitats are broken into fragments, mammals and birds needing large territories and 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 through over-exploitation; many marine fish populations are over-harvested.
3. Alien species invasions — the Nile perch introduced into Lake Victoria in East Africa eventually led to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish. Invasive weeds such as carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) damage the environment; the illegal introduction of the African catfish Clarias gariepinus for aquaculture threatens indigenous catfishes in Indian rivers.
4. Co-extinctions — when a species becomes extinct, plant and animal species associated with it in an obligatory way also become extinct, e.g. the unique parasites of an extinct host fish, or a co-evolved plant and its pollinator.

Why conserve?

• Narrowly utilitarian: humans derive countless direct economic benefits — food (cereals, pulses, fruits), firewood, fibre, construction material, and industrial products (tannins, lubricants, dyes, resins, perfumes). More than 25 percent of the drugs currently sold in the market are derived from plants, and 25,000 plant species contribute to traditional medicines used by native peoples. Nations rich in biodiversity can expect benefits from bioprospecting (exploring molecular, genetic and species-level diversity for products of economic importance).
• Broadly utilitarian: biodiversity provides ecosystem services. The Amazon forest is estimated to produce, through photosynthesis, about 20 percent of the total oxygen in Earth’s atmosphere. Pollination by bees, bumblebees, birds and bats is another service, as are the aesthetic pleasures of nature.
• Ethical: every species has an intrinsic value, even if not of current economic value to us; we have a moral duty to care for their well-being and to pass on our biological legacy in good order to future generations.

In situ (on site) conservation protects a whole ecosystem and hence its biodiversity at all levels.

• Biodiversity hotspots are regions with very high levels of species richness and a high degree of endemism. Initially 25 were identified, and 9 more were added, giving 34. Three cover India’s exceptionally rich regions: Western Ghats and Sri Lanka, Indo-Burma and Himalaya. Hotspots together cover less than 2 percent of the Earth’s land area, yet harbour a very high number of species; strict protection could reduce ongoing mass extinctions by almost 30 percent.
• In India, ecologically unique and biodiversity-rich regions are legally protected as biosphere reserves, national parks and wildlife sanctuaries. (The textbook gives counts of 14, 90 and 448; official current counts are higher.)
• Sacred groves: tracts of forest set aside, with all trees and wildlife venerated and protected — in the Khasi and Jaintia Hills (Meghalaya), the Aravalli Hills (Rajasthan), the Western Ghat regions of Karnataka and Maharashtra, and the Sarguja, Chanda and Bastar areas of Madhya Pradesh. In Meghalaya they are the last refuges for many rare and threatened plants.
• Ramsar sites: wetlands of international importance designated under the Ramsar Convention (Ramsar, Iran, 1971) for conservation and wise use.
• Red Data Book: the IUCN Red List records the conservation status of species (for example critically endangered, endangered, vulnerable), guiding conservation priorities.

Ex situ (off site) conservation: threatened animals and plants are taken out of their natural habitat and protected in zoological parks, botanical gardens and wildlife safari parks; many animals extinct in the wild are maintained in zoos. Cryopreservation keeps gametes viable and fertile for long periods; eggs can be fertilised in vitro; plants can be propagated by tissue culture; seeds of commercially important plants are stored in seed banks.

International efforts: the Earth Summit at Rio de Janeiro (1992) called on all nations to take measures for conservation of biodiversity and sustainable use of its benefits; at the World Summit on Sustainable Development in Johannesburg (2002), 190 countries pledged to achieve, by 2010, a significant reduction in the current rate of biodiversity loss.