Big picture
Biodiversity is the variety of life at every level of biological organisation, from genes to species to ecosystems. It is unevenly spread: the tropics are far richer than the poles, and a few regions called hotspots hold a large share of all species.
Human activities are causing species to disappear far faster than the natural rate. This chapter explains how biodiversity is measured and distributed, why it matters, what is destroying it, and how we can conserve it. Facts and examples carry most of the marks.
Learning goals
Explain genetic, species and ecological diversity with examples.
Describe global and Indian biodiversity.
Explain latitudinal gradients and the species-area relationship.
Explain why biodiversity is important for ecosystems (rivet popper hypothesis).
Explain the loss of biodiversity and its four main causes.
Explain the narrowly utilitarian, broadly utilitarian and ethical reasons for conservation.
Explain in situ and ex situ conservation with examples.
Core ideas
1. Levels of biodiversity
The term biodiversity was popularised by the sociobiologist Edward Wilson.
Level | Meaning | Example |
|---|---|---|
Genetic diversity | Variation in genes within a species | The medicinal plant *Rauwolfia vomitoria 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 | Variety of species in a region | The Western Ghats have a greater amphibian species diversity than the Eastern Ghats |
Ecological diversity | Variety of ecosystems in a region | India, with its deserts, rainforests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has greater ecosystem diversity than a Scandinavian country like Norway |
2. How many species?
According to the IUCN (2004), more than 1.5 million species have been described so far.
Robert May estimated the global species diversity at about 7 million, using a scientifically sound extrapolation based on the ratio of tropical to temperate species in well-studied groups.
More than 70% of all species recorded are animals, and among animals, insects are the most species-rich group (more than 70% of animals). Plants (including algae, fungi, bryophytes, gymnosperms and angiosperms) make up no more than 22%.
India has only 2.4% of the world's land area but 8.1% of its species diversity, making it one of the 12 megadiversity countries. Nearly 45,000 species of plants and twice as many animals have been recorded in India.
3. Patterns of biodiversity
Latitudinal gradients. 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.
Example (bird species): Colombia, near the equator, has nearly 1,400 species of birds; New York, at 41°N, has 105; Greenland, at 71°N, only 56. India, largely in the tropics, has more than 1,200 species of birds. The Amazon rainforest has the greatest biodiversity on Earth.
Why the tropics are richer:
Speciation is a function of time. Temperate regions were subjected to frequent glaciations in the past, while tropical latitudes remained relatively undisturbed for millions of years, giving a long evolutionary time for species diversification.
Tropical environments are less seasonal, relatively more constant and predictable, which promotes niche specialisation and greater species diversity.
More solar energy is available in the tropics, which contributes to higher productivity, which in turn may contribute indirectly to greater diversity.
Species-area relationship. Alexander von Humboldt observed that within a region, species richness increases with the explored area, but only up to a limit. The relationship is a rectangular hyperbola, and 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 (the regression coefficient) and C = Y-intercept.
For small areas, Z typically lies between 0.1 and 0.2, regardless of the taxonomic group or region.
For very large areas such as whole continents, the slope is much steeper, with Z between 0.6 and 1.2. For example, for frugivorous (fruit-eating) birds and mammals in tropical forests of different continents, Z = 1.15.
4. Importance of species diversity
David Tilman's long-term experiments on outdoor plots found that plots with more species showed less year-to-year variation in total biomass. Increased diversity also contributed to higher productivity. Rich biodiversity is essential for ecosystem health and the survival of humans.
Rivet popper hypothesis (proposed by Paul Ehrlich): an ecosystem is like an aeroplane, and its species are like the rivets joining its parts. If every passenger pops a rivet to take home (species become extinct), the plane may not be affected at first, but as more rivets are removed, it becomes dangerously weak. The loss of rivets on key parts such as the wings (key species that drive major ecosystem functions) is a more serious threat than the loss of a few rivets elsewhere.
5. Loss of biodiversity
The IUCN Red List (2004) documents the extinction of 784 species (338 vertebrates, 359 invertebrates and 87 plants) in the last 500 years. Some examples:
Dodo (Mauritius), quagga (Africa), thylacine (Australia), Steller's sea cow (Russia).
Three subspecies of tiger: Bali, Javan and Caspian.
The last twenty years alone have seen the disappearance of 27 species. Currently, more than 15,500 species worldwide face the threat of extinction. Amphibians appear to be more vulnerable than other groups.
Effects of loss of biodiversity: decline in plant production; lower resistance to environmental disturbances such as drought; and increased variability in ecosystem processes such as plant productivity, water use and pest and disease cycles.
6. Causes: the evil quartet
Cause | Explanation and examples |
|---|---|
Habitat loss and fragmentation | The most important cause. Tropical rainforests once covered more than 14% of the Earth's land surface and now cover no more than 6%. The Amazon rainforest ("lungs of the planet") is being cut for soyabean cultivation and grasslands for beef cattle. When large habitats are broken into fragments, animals needing large territories (and migratory animals) are badly affected |
Over-exploitation | Human greed for natural resources. Steller's sea cow and the passenger pigeon became extinct due to over-exploitation; many marine fish populations are being over-harvested |
Alien species invasions | Species introduced from elsewhere can become invasive. The introduction of the Nile perch into Lake Victoria (East Africa) led to the extinction 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 threatens native catfishes in Indian rivers |
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 parasites also become extinct; in plant–pollinator mutualism, the extinction of one causes the extinction of the other |
7. Why conserve biodiversity?
Reason | Explanation |
|---|---|
Narrowly utilitarian | Humans derive direct economic benefits: food (cereals, pulses, fruits), firewood, fibre, construction material, industrial products (tannins, lubricants, dyes, resins, perfumes) and medicines. More than 25% of drugs currently sold worldwide come from plants, and 25,000 species of plants contribute to traditional medicines. Bioprospecting explores molecular, genetic and species-level diversity for products of economic importance |
Broadly utilitarian | Biodiversity provides ecosystem services. The Amazon forest is estimated to produce about 20% of the total oxygen in the Earth's atmosphere through photosynthesis. Pollination by bees, bumblebees, birds and bats; aesthetic pleasures such as walking through forests, watching birds and flowers |
Ethical | Every species has an intrinsic value, even if it has no economic value to us. We have a moral duty to care for their well-being and pass on our biological legacy in good order to future generations |
8. Conservation of biodiversity
In situ conservation (on site): protecting species in their natural habitat, conserving the whole ecosystem.
Biodiversity hotspots: regions with very high species richness and high endemism (species found only in that region). Initially 25 were identified; now there are 34 hotspots worldwide. India has three: Western Ghats and Sri Lanka, Indo-Burma, and Himalaya. Although all the hotspots together cover less than 2% of the Earth's land area, the number of species they harbour is extremely high; strict protection could reduce ongoing mass extinctions by almost 30%.
Protected areas: ecologically and biologically important regions legally protected, such as biosphere reserves, national parks and wildlife sanctuaries. (NCERT records India's figures as 14 biosphere reserves, 90 national parks and 448 wildlife sanctuaries; the current numbers are higher, so use the figures in your textbook.)
Sacred groves: tracts of forest set aside by local communities, where all trees and wildlife are venerated and protected because of religious beliefs. They are found in the Khasi and Jaintia Hills (Meghalaya), the Aravalli Hills (Rajasthan), the Western Ghats regions of Karnataka and Maharashtra, and the Sarguja, Chanda and Bastar areas (Madhya Pradesh and Chhattisgarh). In Meghalaya, sacred groves are the last refuges for many rare and threatened plants.
Ex situ conservation (off site): threatened animals and plants are taken out of their natural habitat and protected in special settings.
Zoological parks, botanical gardens and wildlife safari parks.
Cryopreservation: gametes of threatened species can be preserved in viable and fertile condition for long periods at −196 °C (in liquid nitrogen).
In vitro fertilisation of eggs, and tissue culture propagation of plants.
Seed banks: seeds of commercially important plants can be kept for long periods.
International efforts: the Earth Summit held in Rio de Janeiro in 1992 called on all nations to take measures to conserve biodiversity and use it sustainably (the Convention on Biological Diversity). The World Summit on Sustainable Development in Johannesburg, South Africa, in 2002, saw 190 countries pledge to reduce the current rate of biodiversity loss by 2010.
A reliable approach to questions
For levels of biodiversity, give the standard example: Rauwolfia (genetic), Western Ghats amphibians (species), India vs Norway (ecological).
For latitudinal gradients, give the bird numbers and the three reasons.
For the species-area relationship, write the equation and the ranges of Z.
For causes of loss, name all four and give an example for each.
For conservation, classify the method as in situ or ex situ and give examples.
Connections that matter
Ecosystem explains the functions that biodiversity supports.
Organisms and populations covers interactions like mutualism, which underlie co-extinction.
Biotechnology and its applications discusses biopiracy and bioprospecting.
Common mistakes
Saying species diversity increases towards the poles. It decreases.
Mixing up the values of Z. Small areas 0.1–0.2; continents 0.6–1.2.
Calling a national park ex situ conservation. It is in situ; zoos and seed banks are ex situ.
Naming over-exploitation as the most important cause. Habitat loss and fragmentation is the most important.
Crediting the rivet popper hypothesis to Tilman. It was proposed by Paul Ehrlich.
Revision checklist
I can explain the three levels of biodiversity with examples.
I can state global and Indian biodiversity figures.
I can explain latitudinal gradients and the species-area relationship.
I can explain the rivet popper hypothesis and Tilman's findings.
I can explain the four causes of biodiversity loss with examples.
I can explain the reasons for conservation and in situ and ex situ methods.