What is biodiversity? Three levels
Biodiversity means the variety of living things at every level of life. The word was made popular by the scientist Edward Wilson.
- Genetic diversity: variety of genes inside one species. India has more than 50,000 kinds of rice and about 1,000 kinds of mango. The medicinal plant Rauwolfia vomitoria makes different strengths of the drug reserpine in different places.
- Species diversity: the number of different species. The Western Ghats have more amphibian species than the Eastern Ghats.
- Ecological diversity: variety of ecosystems: deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows. India has far more of these than a country like Norway.
How many species?
About 1.5 million species have been described so far, but scientists estimate the total may be around 7 million. More than 70% of recorded species are animals, and insects are the biggest group. India has only about 2.4% of the world's land but about 8% of its species, so it is one of the 12 megadiversity countries.
Patterns of biodiversity
Latitudinal gradient
Diversity falls as we move from the equator towards the poles. Colombia near the equator has around 1,400 bird species; Greenland far north has about 56. The Amazon rain forest has the richest life on Earth.
Why the tropics are richer: (1) they were not disturbed by ice ages for millions of years, so species had a long time to form; (2) the climate is steady and predictable, so species can specialise; (3) more sunlight all year means more productivity.
Species–area relationship
The explorer Alexander von Humboldt saw that the number of species grows with the area explored, but only up to a point. On a log–log graph it is a straight line:
log S = log C + Z log A
S = species richness, A = area, Z = slope (regression coefficient), C = Y-intercept. For small regions Z is about 0.1–0.2, whatever the group or place. For very large areas like whole continents Z is steeper, about 0.6–1.2.
Importance of biodiversity
Stability and productivity
Long-term field experiments (David Tilman) showed that plots with more species varied less in total biomass year to year and were more productive. A rich ecosystem is more stable.
Rivet popper hypothesis
Paul Ehrlich compared an ecosystem to an aeroplane and species to its rivets. Losing a few rivets may not matter, but losing many, or one key rivet (a key species on the "wing"), can bring the whole system down.
Why should we care?
- Narrowly utilitarian: direct gains such as food, fibre, firewood, medicines (more than 25% of drugs come from plants), dyes and resins. Searching nature for useful molecules is called bioprospecting.
- Broadly utilitarian: ecosystem services such as oxygen from forests, pollination by bees and birds, clean water, fertile soil and simple beauty and joy.
- Ethical: every species has a right to exist; we share the planet and must hand it on in good shape.
Loss of biodiversity: the evil quartet
The IUCN recorded about 784 extinctions in the last 500 years, such as the dodo (Mauritius), the quagga (Africa), the thylacine (Australia) and Steller's sea cow (Russia). Today's rate is thought to be 100 to 1,000 times faster than the natural rate, mostly because of humans: people call it the sixth mass extinction.
Loss of species reduces plant production, lowers resistance to drought and pests, and makes processes like water use and disease cycles less stable.
Four causes (evil quartet)
- Habitat loss and fragmentation: the biggest cause. Tropical rain forests once covered about 14% of land, now about 6%. When a large habitat is cut into small pieces, animals needing big areas (big cats, migratory birds) decline.
- Over-exploitation: taking too much. Steller's sea cow and the passenger pigeon were hunted out. Many marine fish are over-harvested.
- Alien species invasions: Nile perch put into Lake Victoria wiped out more than 200 cichlid fish species. Carrot grass (Parthenium), Lantana and water hyacinth harm Indian ecosystems. The African catfish Clarias gariepinus threatens native catfish.
- Co-extinction: when a species dies out, species that depend on it die too, such as parasites of a host, or a plant and its only pollinator.
Endangered species and the Red Data Book
The International Union for Conservation of Nature (IUCN) keeps the Red List, first printed as the Red Data Book. It records how close each species is to extinction.
| Category | Meaning in simple words |
|---|---|
| Least Concern (LC) | Common, not at risk now |
| Near Threatened (NT) | Could soon become threatened |
| Vulnerable (VU) | High risk in the wild |
| Endangered (EN) | Very high risk |
| Critically Endangered (CR) | Extremely high risk |
| Extinct in the Wild (EW) | Lives only in zoos or farms |
| Extinct (EX) | No member left anywhere |
VU, EN and CR together are called threatened. Indian examples: the tiger and the Ganges river dolphin are endangered; the great Indian bustard and the gharial are critically endangered. The Red Data Book helps in planning protection, spreading awareness and finding species that need urgent help.
Conservation: in situ and ex situ
In situ (on site) conservation protects species in their natural home, along with the whole ecosystem. It is the best way because the species keeps its natural links. Examples: hotspots, protected areas, sacred groves.
Ex situ (off site) conservation protects species outside their home, used when a species is so threatened that it needs special care:
- Zoological parks, botanical gardens, wildlife safari parks.
- Captive breeding and release (for example, vultures and gharials in India).
- Cryopreservation: storing gametes of threatened species at very low temperature (−196 °C in liquid nitrogen) so they stay alive for a long time.
- Seed banks for crop and wild seeds, tissue culture of plants and in vitro fertilisation.
Nations work together too: the Earth Summit (Rio de Janeiro, 1992) created the Convention on Biological Diversity, and the World Summit on Sustainable Development (Johannesburg, 2002) set a target to reduce the rate of loss.
Biodiversity hotspots
A hotspot is a region with (1) a very high number of species, (2) many endemic species (found nowhere else), and (3) heavy threat, having lost much of its original plant cover. The idea was given by Norman Myers.
At first 25 hotspots were named; later more were added to make 34 (now about 36). Together they cover less than 2% of Earth's land but hold a very large share of all species. Protecting them strongly could cut the current extinction rate by up to 30%.
Hotspots touching India: Western Ghats and Sri Lanka, Indo-Burma and the Himalaya (the Nicobar Islands fall in the Sundaland hotspot).
Protected areas: national parks, sanctuaries, biosphere reserves, Ramsar sites
- National park: set aside only for wildlife. Private rights, grazing, farming and tree felling are not allowed. Examples: Jim Corbett (Uttarakhand, the first, 1936), Kaziranga (Assam), Gir (Gujarat).
- Wildlife sanctuary: protects animals, but some human activities such as collecting wood or forest produce may be allowed if they do not harm wildlife. Examples: Bharatpur (Keoladeo) was a bird sanctuary before becoming a national park; Periyar (Kerala).
- Biosphere reserve: a large area to protect the whole ecosystem and support local people. It has three zones: core (no human activity), buffer (research, education, limited use) and transition (villages, farming, tourism). Examples: Nilgiri, Sundarbans, Nanda Devi, Gulf of Mannar. India has 18.
- Ramsar site: a wetland of international importance under the Ramsar Convention (Iran, 1971). Wetlands clean water, control floods and host migratory birds. Indian examples: Chilika Lake (Odisha), Keoladeo (Rajasthan), Loktak Lake (Manipur), Wular Lake (Jammu and Kashmir), Sambhar Lake (Rajasthan). India has more than 85 Ramsar sites, and the number keeps growing.
India has over 100 national parks and more than 550 wildlife sanctuaries.
Sacred groves
Sacred groves are patches of forest that local communities protect because of religious or cultural beliefs. Nobody cuts trees or hunts there, so they act as safe homes for many rare and endemic plants and animals.
Examples: the Khasi and Jaintia Hills of Meghalaya, the Aravalli Hills of Rajasthan, the Western Ghats in Karnataka and Maharashtra, and Sarguja, Chanda and Bastar in Madhya Pradesh and Chhattisgarh. In Meghalaya, some sacred groves are the last shelter for many rare plants. They show that community traditions can be powerful conservation tools.
What the board exam asks
Common questions: explain the species–area relationship and write the equation; give reasons for greater diversity in the tropics; explain the rivet popper hypothesis; list and explain the evil quartet with examples; define hotspots and name those in India; differentiate in situ and ex situ, and national park, sanctuary and biosphere reserve; what are sacred groves; what is the Red Data Book. Short case-based questions often describe an alien species or a lost pollinator and ask you to name the cause.
Key formulas and definitions
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Worked examples
1. For a group of plants, C = 20 and Z = 0.2. How many species are expected in an area of 100,000 m² (log 100,000 = 5)?
log S = log 20 + 0.2 × 5 = 1.301 + 1 = 2.301. S = 10^2.301 ≈ 200 species.
2. With Z = 0.2, if the area becomes 32 times larger, by how much does species number grow?
S ∝ A^Z, so S grows by 32^0.2 = 2 times (since 2⁵ = 32). A 32-fold bigger area only doubles the species.
3. An island of 10 km² has 40 species of lizards. With Z = 0.3, estimate species on an island of 1,000 km².
Ratio of areas = 100. S₂ = 40 × 100^0.3 = 40 × 10^0.6 ≈ 40 × 3.98 ≈ 159 species, about 160.
4. Name the cause of loss in each case: (a) Nile perch in Lake Victoria, (b) a parasite dies when its host fish goes extinct, (c) passenger pigeon hunted out, (d) forest cut for a highway.
(a) Alien species invasion, (b) co-extinction, (c) over-exploitation, (d) habitat loss and fragmentation.
5. A species' wild numbers fall so much that it survives only in zoos. Which Red List category is it? Is it 'threatened'?
Extinct in the Wild (EW). It is beyond the three threatened categories (VU, EN, CR); only ex situ populations remain.
6. Classify each as in situ or ex situ: Kaziranga National Park, seed bank, sacred grove in Meghalaya, cryopreserved sperm of a rare deer, Chilika Ramsar site.
In situ: Kaziranga, sacred grove, Chilika. Ex situ: seed bank, cryopreserved sperm.
7. A villager wants to graze cattle and collect firewood near a protected forest. Where can each happen: national park core, sanctuary, biosphere reserve transition zone?
National park: neither is allowed. Sanctuary: collecting wood may be allowed if it does not harm wildlife; grazing is usually limited. Biosphere transition zone: yes, people live and use land there in a sustainable way.
Common mistakes
- Saying 'hotspot' just means many species. It also needs high endemism and high threat.
- Mixing up national park and sanctuary: a national park allows no human use; a sanctuary may allow some.
- Thinking Z is the same for small regions and continents. It is about 0.1–0.2 for small regions and 0.6–1.2 for whole continents.
- Calling zoos and seed banks in situ. They are ex situ because the species is kept outside its natural home.