Biodiversity and Conservation | CBSE Class 12 Biology Notes

This note covers NCERT Class 12 Biology Chapter 13, Biodiversity and Conservation: what biodiversity is, how many species exist, the patterns in which they are distributed, why species are being lost, and how they can be conserved. The numbers in this chapter are asked directly in board and entrance examinations, so each one is given exactly as NCERT states it. Several are dated (the IUCN figures are from 2004), and they are marked as NCERT figures where that matters.

What is biodiversity, and at what levels does it exist?

Biodiversity is the combined diversity at all levels of biological organisation, from macromolecules within cells to biomes. The term was popularised by the sociobiologist Edward Wilson.

Definition: Biodiversity is the sum total of diversity that exists at all levels of biological organisation. The three levels that matter most are genetic, species and ecological diversity.

It took millions of years of evolution to build up this diversity. NCERT warns that we could lose it in less than two centuries if present rates of species loss continue.

Note: Do not write "genetic diversity" for the rice and mango figures and then give the Western Ghats example with it. Rice strains and mango varieties are genetic diversity within one species. The Western Ghats and Eastern Ghats comparison is species diversity.

How many species are there on Earth?

We know how many species have been recorded, because discoveries are published. We do not know how many exist.

• According to the IUCN (2004), the total number of plant and animal species described so far is slightly more than 1.5 million.
• Some extreme estimates of the true total range from 20 to 50 million.
• A more conservative and scientifically sound estimate by Robert May places global species diversity at about 7 million.

How do ecologists arrive at such an estimate?

Species inventories are more complete in temperate countries than in tropical ones, and most undiscovered species are in the tropics. Biologists therefore work in steps:

1. Choose a group of insects that has been studied exhaustively.
2. Compare the species richness of that group in temperate and in tropical regions.
3. Extrapolate this temperate-tropical ratio to other groups of animals and plants.
4. Add up the results to reach a gross estimate of the total number of species on earth.

What does the recorded inventory look like?

• 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, more than 70 per cent of the total. Out of every 10 animals, 7 are insects.
• The number of fungi species is more than the combined total of the species of fishes, amphibians, reptiles and mammals.

Three pie charts: invertebrates, vertebrates and plants. In the invertebrate chart the insect slice is by far the largest; the rest is shared by other animal groups, molluscs and crustaceans. In the vertebrate chart fishes form the largest slice, about half, and birds, reptiles, amphibians and mammals make up the rest. In the plant chart fungi and angiosperms are the two largest slices, with algae, mosses, ferns and allies, and lichens as small ones. What to notice: insects dominate the animal kingdom, and fungi outnumber all the vertebrate groups combined.

See Fig. 13.1 in your NCERT textbook

These figures leave out prokaryotes. Conventional taxonomic methods are not suitable for identifying microbial species, and many cannot be cultured in the laboratory. If biochemical or molecular criteria were used to delineate species, their diversity alone might run into millions.

How rich is India's biodiversity?

India has only 2.4 per cent of the world's land area but 8.1 per cent of the global species diversity. This makes it one of the 12 mega diversity countries of the world. Nearly 45,000 species of plants and twice as many of animals have been recorded from India.

Worked example 1. If Robert May's global estimate is accepted, only 22 per cent of all species have been recorded so far. Estimate how many plant species remain to be discovered in India.

Given: recorded plant species = 45,000; fraction recorded = 22 per cent. Method: total = recorded ÷ 0.22, then subtract the recorded number. Working: 45,000 ÷ 0.22 ≈ 2,04,500; 2,04,500 − 45,000 ≈ 1,59,500. Answer: about 1.6 lakh, which is why NCERT says "more than 1,00,000" plant species are yet to be discovered. The same calculation with 90,000 recorded animal species gives about 3.2 lakh, NCERT's "more than 3,00,000".

NCERT's image for this is worth remembering: nature's biological library is burning even before we have catalogued the titles of all the books in it.

Why does species diversity fall from the equator to the poles?

Diversity is not spread evenly across the earth. The best-known pattern is the latitudinal gradient: in general, species diversity decreases as we move from the equator towards the poles. With very few exceptions, the tropics (23.5° N to 23.5° S) harbour more species than temperate or polar areas.

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. NCERT lists 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. At least two million insect species may be waiting there to be discovered.

What are the three hypotheses for tropical richness?

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

What is the species-area relationship?

The German naturalist and geographer Alexander von Humboldt, exploring South American jungles, observed that within a region species richness increases with increasing explored area, but only up to a limit.

For a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes) the relation between species richness and area is a rectangular hyperbola. On a logarithmic scale it becomes a straight line:

Equation: log S = log C + Z log A, where S = species richness, A = area, Z = slope of the line (regression coefficient) and C = Y-intercept.

Draw two curves on axes of area (x) against species richness (y). On ordinary axes the curve rises steeply and then flattens: a rectangular hyperbola, labelled S = CA^Z (C times A to the power Z). On log-log axes the same relationship is a straight line, labelled log S = log C + Z log A. What to notice: the slope of the straight line is Z.

See Fig. 13.2 in your NCERT textbook

What does a steeper slope mean?

Z tells you how fast species richness rises as area increases. A steeper slope means that richness climbs much more sharply with area. This is what happens when very large areas such as whole continents are compared.

Worked example 2. An illustration with made-up values: suppose C = 10 and Z = 0.2. Find S for an area of 100 units and for 10,000 units.

Working: For A = 100, log S = log 10 + 0.2 × log 100 = 1 + 0.2 × 2 = 1.4, so S ≈ 25. For A = 10,000, log S = 1 + 0.2 × 4 = 1.8, so S ≈ 63. Answer: a hundredfold increase in area raises species richness only about two and a half times when Z is 0.2. With Z = 1.15 the same increase in area would raise it about 200 times.

Note: The curve is a rectangular hyperbola on ordinary axes and a straight line only on a log scale. Examiners ask for both, so state which axes you mean.

Does it matter to an ecosystem how many species it has?

Ecologists have not been able to give a definitive answer. For many decades they believed that communities with more species tend to be more stable. A stable community:

• does not show too much variation in productivity from year to year;
• is either resistant or resilient to occasional disturbances, natural or man-made;
• is resistant to invasions by alien species.

David Tilman's long-term ecosystem experiments using outdoor plots give some tentative answers. Plots with more species showed less year-to-year variation in total biomass, and increased diversity contributed to higher productivity.

What is the rivet popper hypothesis?

The Stanford ecologist Paul Ehrlich used an analogy. An aeroplane (the ecosystem) is held together by thousands of rivets (species). If every passenger pops a rivet to take home (causing a species to become extinct), flight safety may not be affected at first. As more rivets are removed, the plane becomes dangerously weak.

Which rivet is removed also matters. Losing rivets on the wings (key species that drive major ecosystem functions) is a more serious threat than losing a few on the seats or windows.

How fast are species being lost?

NCERT's figures on species loss:

• The IUCN Red List (2004) documents the extinction of 784 species in the last 500 years: 338 vertebrates, 359 invertebrates and 87 plants.
• The last twenty years alone saw the disappearance of 27 species.
• More than 15,500 species worldwide face the threat of extinction.
• Threatened at present: 12 per cent of all bird species, 23 per cent of mammals, 32 per cent of amphibians and 31 per cent of gymnosperms.

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. Extinctions are not random across groups: amphibians appear to be more vulnerable.

How is the Sixth Extinction different from the first five?

In the long history of life (more than 3 billion years) there were five episodes of mass extinction, all before humans appeared. The Sixth Extinction, now in progress, differs in its rate. Current extinction rates are estimated to be 100 to 1,000 times faster than in pre-human times, and human activities are responsible. Ecologists warn that if present trends continue, nearly half of all species on earth might be wiped out within the next 100 years.

Loss of biodiversity in a region may lead to:

1. a decline in plant production;
2. lowered resistance to environmental perturbations such as drought;
3. increased variability in certain ecosystem processes such as plant productivity, water use, and pest and disease cycles.

What are the four major causes of biodiversity loss?

The four causes are together called The Evil Quartet.

Fragmentation harms two kinds of animals in particular: mammals and birds that need large territories, and animals with migratory habits. Their populations decline when large habitats are broken up.

Note: Steller's sea cow appears twice in this chapter: in the list of recent extinctions (Russia) and as an example of over-exploitation. The Nile perch is the invader; the cichlids are the victims. Students often reverse them.

Why should we conserve biodiversity?

NCERT groups the reasons into three categories.

Bioprospecting means exploring molecular, genetic and species-level diversity for products of economic importance. Nations rich in biodiversity can expect to gain from it.

How do we conserve biodiversity?

There are two approaches. In in situ (on site) conservation, the whole ecosystem is protected, and its biodiversity at all levels is protected with it: we save the entire forest to save the tiger. In 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. It is the desirable approach when a species is endangered or threatened and needs urgent measures.

Draw a flow chart. Start with "Biodiversity conservation" and split it into two branches. Branch 1, in situ: biodiversity hotspots, biosphere reserves, national parks, wildlife sanctuaries, sacred groves. Branch 2, ex situ: zoological parks, botanical gardens, wildlife safari parks, seed banks, cryopreservation, tissue culture. What to notice: in situ protects places, ex situ protects selected species.

What are biodiversity hotspots?

Biodiversity hotspots are regions with very high levels of species richness and a high degree of endemism (species confined to that region and found nowhere else). They are also regions of accelerated habitat loss.

• Initially 25 hotspots were identified. Nine more were added, bringing the total to 34 (the NCERT figure).
• Three cover India's high-biodiversity regions: Western Ghats and Sri Lanka, Indo-Burma and Himalaya.
• All the hotspots together cover less than 2 per cent of the earth's land area.
• Strict protection of the hotspots could reduce the ongoing mass extinctions by almost 30 per cent.

How does India protect its biodiversity in situ?

Ecologically unique and biodiversity-rich regions are legally protected. NCERT gives India 14 biosphere reserves, 90 national parks and 448 wildlife sanctuaries. Learn these as the textbook's figures; the chapter summary rounds the last one to "more than 450".

India also has a tradition of sacred groves: tracts of forest set aside, where all the trees and wildlife were venerated and given total protection. NCERT lists them in:

• the Khasi and Jaintia Hills in Meghalaya;
• the Aravalli Hills of Rajasthan;
• the Western Ghat regions of Karnataka and Maharashtra;
• the Sarguja, Chanda and Bastar areas of Madhya Pradesh.

In Meghalaya, the sacred groves are the last refuges for a large number of rare and threatened plants.

What can ex situ conservation do today?

Many animals that are extinct in the wild are still maintained in zoological parks. Ex situ conservation has also moved beyond enclosures. Gametes of threatened species can be preserved in viable and fertile condition for long periods by cryopreservation. Eggs can be fertilised in vitro. Plants can be propagated by tissue culture. Seeds of different genetic strains of commercially important plants can be kept for long periods in seed banks.

Which international agreements does NCERT name?

Glossary

• Biodiversity — The sum total of diversity at all levels of biological organisation; the term was popularised by Edward Wilson.
• Genetic diversity — Variation at the genetic level within a single species over its distributional range.
• Endemism — The condition of a species being confined to one region and not found anywhere else.
• Latitudinal gradient — The general decrease in species diversity from the equator towards the poles.
• Species-area relationship — The rise in species richness with explored area, up to a limit; a rectangular hyperbola that becomes a straight line on a log scale.
• Regression coefficient (Z) — The slope of the log-scale species-area line; 0.1 to 0.2 within regions and 0.6 to 1.2 across very large areas.
• Rivet popper hypothesis — Paul Ehrlich's analogy in which species are the rivets holding an aeroplane (the ecosystem) together.
• The Evil Quartet — The four major causes of biodiversity loss: habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions.
• Co-extinction — The extinction of species associated in an obligatory way with a species that has become extinct.
• Bioprospecting — Exploring molecular, genetic and species-level diversity for products of economic importance.
• Biodiversity hotspot — A region with very high species richness and a high degree of endemism, identified for maximum protection.
• Sacred grove — A tract of forest set aside by a community, in which all trees and wildlife are venerated and fully protected.
• Cryopreservation — Preserving gametes of threatened species in viable and fertile condition for long periods.

Common errors and misconceptions

• Misconception: About 1.5 million species exist on earth. Correct: Slightly more than 1.5 million have been described (IUCN 2004). Robert May's estimate of the species that exist is about 7 million.
• Misconception: Plants are the largest group of recorded species. Correct: More than 70 per cent of recorded species are animals; plants are no more than 22 per cent.
• Misconception: The species-area relationship is a straight line. Correct: It is a rectangular hyperbola, and a straight line only when plotted on a logarithmic scale.
• Misconception: Z is always between 0.1 and 0.2. Correct: That range holds within regions. For very large areas such as continents Z is 0.6 to 1.2.
• Misconception: The Sixth Extinction is different because it is the largest. Correct: NCERT's point is the rate: 100 to 1,000 times faster than in pre-human times, and caused by human activities.
• Misconception: Zoological parks and botanical gardens are in situ conservation. Correct: They are ex situ. National parks, sanctuaries, biosphere reserves and sacred groves are in situ.
• Misconception: All 34 hotspots are in India, or India has four. Correct: NCERT names three that cover India's regions: Western Ghats and Sri Lanka, Indo-Burma, and Himalaya.

Exam-style questions with model answers

Q1. Name the three important components of biodiversity, with one example of each. [3 marks]

1. Genetic diversity: variation within a species, for example more than 50,000 genetically different strains of rice in India, or the varying reserpine content of Rauwolfia vomitoria across Himalayan ranges.
2. Species diversity: diversity at the species level, for example greater amphibian species diversity in the Western Ghats than in the Eastern Ghats.
3. Ecological diversity: diversity at the ecosystem level, for example India's deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows compared with Norway.

Q2. How do ecologists estimate the total number of species present in the world? [2 marks]

1. They compare the temperate and tropical species richness of an exhaustively studied group of insects.
2. They extrapolate this ratio to other groups of animals and plants to reach a gross estimate. Estimates range from 20 to 50 million at the extreme, and Robert May's more conservative estimate is about 7 million species.

Q3. Give three hypotheses explaining why the tropics show the greatest species richness. [3 marks]

1. Evolutionary time: tropical latitudes remained relatively undisturbed for millions of years, unlike temperate regions subjected to frequent glaciations, so there was a long time for species diversification.
2. Constant environment: tropical environments are less seasonal and more predictable, which promotes niche specialisation.
3. Solar energy: more solar energy is available in the tropics, giving higher productivity, which may contribute indirectly to greater diversity.

Q4. What is the significance of the slope of regression in a species-area relationship? [3 marks]

1. The relationship is log S = log C + Z log A, where Z is the slope (regression coefficient).
2. Within a region Z lies between 0.1 and 0.2 regardless of the taxonomic group or the region.
3. For very large areas such as entire continents the slope is much steeper, 0.6 to 1.2 (1.15 for frugivorous birds and mammals in tropical forests of different continents). A steeper slope means species richness rises much faster as area increases.

Q5. Explain the four major causes of biodiversity loss. [5 marks]

1. Habitat loss and fragmentation: the most important cause. Tropical rain forests have shrunk from more than 14 per cent of the land surface to no more than 6 per cent. Fragmentation badly affects mammals and birds needing large territories and animals with migratory habits.
2. Over-exploitation: Steller's sea cow and the passenger pigeon became extinct because of it; many marine fish populations are over-harvested.
3. Alien species invasions: the Nile perch introduced into Lake Victoria led to the extinction of more than 200 species of cichlid fish; Parthenium, Lantana and water hyacinth threaten native species; the African catfish Clarias gariepinus threatens indigenous catfishes.
4. Co-extinctions: when a host fish becomes extinct its parasites also do; in a coevolved plant-pollinator mutualism, extinction of one leads to extinction of the other.
5. These four are together called The Evil Quartet.

Q6. Assertion (A): The species-area relationship for a region is a straight line on a logarithmic scale. Reason (R): Species richness increases with explored area without any limit. (a) Both A and R are true and R explains A. (b) Both are true but R does not explain A. (c) A is true but R is false. (d) A is false but R is true. [1 mark]

(c) A is true but R is false. On a log scale the relationship is linear, but Humboldt observed that species richness increases with area only up to a limit.

Q7. What are sacred groves? What is their role in conservation? [3 marks]

1. Sacred groves are tracts of forest set aside under religious and cultural traditions, in which all the trees and wildlife are venerated and given total protection.
2. They are 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.
3. They are a form of in situ conservation. In Meghalaya they are the last refuges for a large number of rare and threatened plants.

Q8. Distinguish between in situ and ex situ conservation, and explain why biodiversity hotspots were identified. [5 marks]

1. In situ conservation protects species in their natural habitat by protecting the whole ecosystem, for example biosphere reserves, national parks, wildlife sanctuaries and sacred groves.
2. Ex situ conservation protects threatened species outside their natural habitat, for example zoological parks, botanical gardens, wildlife safari parks and seed banks, with techniques such as cryopreservation, in vitro fertilisation and tissue culture.
3. Hotspots were identified because nations cannot afford to conserve all their biological wealth, and the species needing protection far exceed the resources available.
4. They are regions of very high species richness and high endemism that are also losing habitat fast: 34 in the world, three covering India.
5. Together they cover less than 2 per cent of the land area, and their strict protection could reduce ongoing mass extinctions by almost 30 per cent.

Key takeaways

• Biodiversity exists at three main levels: genetic, species and ecological. The term was popularised by Edward Wilson.
• Slightly more than 1.5 million species have been described (IUCN 2004); Robert May estimates about 7 million exist.
• More than 70 per cent of recorded species are animals, and more than 70 per cent of those are insects; plants are no more than 22 per cent.
• India has 2.4 per cent of the world's land, 8.1 per cent of its species diversity, and is one of 12 mega diversity countries.
• Species diversity decreases from the equator to the poles; the three explanations are evolutionary time, a constant environment and more solar energy.
• log S = log C + Z log A; Z is 0.1 to 0.2 within regions and 0.6 to 1.2 across continents.
• The Evil Quartet: habitat loss and fragmentation, over-exploitation, alien species invasions and co-extinctions.
• Current extinction rates are 100 to 1,000 times faster than in pre-human times; this is the Sixth Extinction.
• In situ conservation protects the habitat; ex situ conservation protects the species outside it. NCERT counts 34 hotspots, three of them covering India.

Test yourself

Who popularised the term biodiversity?

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

What is Robert May's estimate of global species diversity?

Robert May's conservative and scientifically sound estimate places global species diversity at about 7 million species.

What share of the world's land area and species diversity does India have?

India has only 2.4 per cent of the world's land area but 8.1 per cent of the global species diversity.

How many bird species do Colombia, New York and Greenland have, according to NCERT?

Colombia, near the equator, has nearly 1,400 bird species; New York at 41° N has 105; and Greenland at 71° N has only 56.

What is the value of Z for frugivorous birds and mammals in the tropical forests of different continents?

The slope Z for frugivorous birds and mammals in the tropical forests of different continents is 1.15.

Whose experiments showed that plots with more species had less year-to-year variation in total biomass?

David Tilman's long-term ecosystem experiments using outdoor plots showed this, and also that increased diversity contributed to higher productivity.

How many species does the IUCN Red List (2004) record as extinct in the last 500 years?

The IUCN Red List (2004) documents the extinction of 784 species: 338 vertebrates, 359 invertebrates and 87 plants.

Which introduced fish caused the extinction of more than 200 cichlid species, and where?

The Nile perch, introduced into Lake Victoria in east Africa, led to the extinction of more than 200 species of cichlid fish.

Name the three biodiversity hotspots that cover India's regions.

The three hotspots covering India's high-biodiversity regions are Western Ghats and Sri Lanka, Indo-Burma, and Himalaya.

Where and when was the Convention on Biological Diversity (the Earth Summit) held?

The Convention on Biological Diversity, known as the Earth Summit, was held in Rio de Janeiro in 1992.