Biodiversity

Biodiversity is the combined diversity of life at all levels of biological organisation, from the genes within one species to whole ecosystems. This page covers the three levels of biodiversity, how many species live on earth and in India, the latitudinal and species-area patterns, why species diversity matters to an ecosystem, and the causes of biodiversity loss. It follows the Class 12 NCERT chapter Biodiversity and Conservation. NEET often asks the species-area equation, the Evil Quartet and the numbers on species and extinctions.

1. Biodiversity and its Levels

1.1 The variety of life

• Life on earth is enormously diverse. Even for humans, the variety of organisms that share the planet never stops being a source of wonder.
• Some numbers show how large this diversity is:

• Ecologists and evolutionary biologists ask several questions about this diversity:

1. Why are there so many species?
2. Did such great diversity exist throughout earth's history?
3. How did this diversification come about?
4. How and why is this diversity important to the biosphere, and would the biosphere work differently with much less diversity?
5. How do humans benefit from the diversity of life?

1.2 Meaning of biodiversity

• Diversity (heterogeneity) exists not only among species but at all levels of biological organisation, from macromolecules within cells to biomes.
• ★ Exam imp The term biodiversity was popularised by the sociobiologist Edward Wilson.
• He used it for the combined diversity at all the levels of biological organisation.

1.3 The three levels of biodiversity

• It took millions of years of evolution to build this diversity. At present rates of species loss, it could be lost in less than two centuries.
• Biodiversity and its conservation are now vital environmental issues of international concern, because biodiversity is critical for our survival and well-being.
• Life originated on earth nearly 3.8 billion years ago, and life forms have diversified enormously since then.
• Conservation efforts aim to protect diversity at all three levels: genetic, species and ecosystem.

2. How Many Species are there on Earth and in India?

2.1 Recorded species and estimated species

• Every species discovered and named has a published record. So we know how many species have been recorded, but not how many exist.
• ★ Exam imp According to the IUCN (International Union for Conservation of Nature and Natural Resources, 2004), slightly more than 1.5 million plant and animal species have been described.
• How many species are still undiscovered is unclear. Estimates vary widely, and many are only educated guesses.
• For many taxonomic groups, species inventories are more complete in temperate countries than in tropical ones.
• An overwhelmingly large share of the species still to be discovered lives in the tropics.

How biologists estimate the total number of species

1. Choose a group of insects that has been studied exhaustively.
2. Compare its species richness in temperate and tropical regions, as a statistical temperate-tropical ratio.
3. Extrapolate this ratio to other groups of animals and plants.
4. Arrive at a gross estimate of the total number of species on earth.

• Some extreme estimates range from 20 to 50 million species.
• ★ Exam imp A more conservative and scientifically sound estimate by Robert May places global species diversity at about 7 million.
• So nearly 6 million species may still be waiting to be discovered and named.

2.2 Which groups have the most species?

• More than 70 per cent of all recorded species are animals.
• Plants, including algae, fungi, bryophytes, gymnosperms and angiosperms, form no more than 22 per cent.
• ★ Exam imp Insects are the most species-rich group of animals: more than 70 per cent of all animals. So 7 out of every 10 animals are insects.
• The number of fungi species is more than the combined species of fishes, amphibians, reptiles and mammals. Fungi outnumber all vertebrate species put together.
• Figure 1 shows the proportionate number of species in the major groups.

2.3 Prokaryotes are not counted

• These estimates give no figures for prokaryotes. Biologists are not sure how many prokaryotic species exist.
• Conventional taxonomic methods are not suitable for identifying microbial species.
• Many microbial species are simply not culturable under laboratory conditions.
• If biochemical or molecular criteria are used to delineate species, prokaryotic diversity alone might run into millions.

2.4 India's biodiversity

• ★ Exam imp India has only 2.4 per cent of the world's land area, but 8.1 per cent of global species diversity.
• This makes India one of the 12 megadiversity countries of the world.
• Nearly 45,000 species of plants and twice as many species of animals have been recorded from India.
• By May's estimate, only 22 per cent of all species have been recorded so far.
• Applying this proportion, India probably has more than 1,00,000 plant species and more than 3,00,000 animal species yet to be discovered and described.
• Completing this inventory needs immense trained manpower (taxonomists) and a long time.
• Many of these species may become extinct even before we discover them: nature's biological library is burning before its books are catalogued.

Numbers to Remember

• Described species worldwide: slightly more than 1.5 million (IUCN, 2004)
• Extreme estimates: 20 to 50 million; Robert May's estimate: about 7 million
• Animals: more than 70 per cent of recorded species; plants: no more than 22 per cent
• Insects: more than 70 per cent of animals (7 in 10)
• India: 2.4 per cent of land, 8.1 per cent of species; one of 12 megadiversity countries
• India: about 45,000 plant species and about twice as many animal species recorded
• Recorded so far (May): 22 per cent; India's undiscovered: over 1,00,000 plants and 3,00,000 animals

3. Patterns of Biodiversity

3.1 Latitudinal gradient

• Plants and animals are not spread evenly over the world. Many groups show interesting patterns of diversity.
• ★ Exam imp The best-known pattern is the latitudinal gradient: in general, species diversity decreases as we move away from the equator towards the poles.
• With very few exceptions, the tropics (latitudes $23.5^{\circ }$ N to $23.5^{\circ }$ 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 an equal area of temperate forest in the Midwest of the USA.
• ★ Exam imp The largely tropical Amazonian rain forest of South America has the greatest biodiversity on earth.

• Scientists estimate that at least two million insect species in these rain forests are still waiting to be discovered and named.

Why do the tropics have more species? Ecologists and evolutionary biologists have proposed three important hypotheses.

3.2 Species-area relationship

• ★ Exam imp Alexander von Humboldt, the German naturalist and geographer, explored the wilderness of South American jungles.
• He observed that within a region, species richness increases with the explored area, but only up to a limit.
• For many taxa (angiosperm plants, birds, bats, freshwater fishes), the relation between species richness and area is a rectangular hyperbola.
• On a logarithmic scale, the relationship becomes a straight line:

• $S$ = species richness; $A$ = area.
• $Z$ = slope of the line (the regression coefficient).
• $C$ = Y-intercept.

• For very large areas, such as entire continents, the slope is much steeper: $Z$ = 0.6 to 1.2.
• Example: for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, $Z$ = 1.15.
• A steeper slope means species richness rises much faster as area increases.

Numbers to Remember

• Tropics: $23.5^{\circ }$ N to $23.5^{\circ }$ S
• Birds: Colombia nearly 1,400; India more than 1,200; New York ($41^{\circ }$ N) 105; Greenland ($71^{\circ }$ N) 56
• Ecuador forest: up to 10 times the vascular plants of an equal Midwest USA forest
• Amazon: over 40,000 plants, 3,000 fishes, 1,300 birds, 427 mammals, 427 amphibians, 378 reptiles, over 1,25,000 invertebrates; at least 2 million insects undiscovered
• $Z$: 0.1 to 0.2 (regions); 0.6 to 1.2 (continents); 1.15 (frugivorous birds and mammals)

4. Importance of Species Diversity to the Ecosystem

• Does the number of species in a community matter to how the ecosystem works? Ecologists have not given a definitive answer.
• For many decades, ecologists believed that communities with more species tend to be more stable than those with fewer species.

• How these attributes are linked to species richness is not known.
• ★ Exam imp David Tilman's long-term ecosystem experiments using outdoor plots gave some tentative answers.
• Tilman found that plots with more species showed less year-to-year variation in total biomass.
• He also showed that increased diversity contributed to higher productivity.
• We may not fully understand how species richness helps an ecosystem. Still, rich biodiversity is essential for ecosystem health and imperative for the survival of the human race.
• People still ask whether a few extinctions matter. Would Western Ghats ecosystems work less well if one tree frog species were lost? Would life change if earth had 15,000 instead of 20,000 ant species?

4.1 The rivet popper hypothesis

• ★ Exam imp Paul Ehrlich, a Stanford ecologist, used an analogy called the rivet popper hypothesis to answer such questions.

1. Removing a few rivets may not affect flight safety at first.
2. As more and more rivets are removed, the plane becomes dangerously weak over time.
3. Which rivet is removed also matters: losing wing rivets (key species) is a more serious threat than losing a few rivets on the seats or windows.

• In short, communities with high diversity are believed to be less variable, more productive and more resistant to biological invasions.

5. Loss of Biodiversity

5.1 The record of extinctions

• It is doubtful whether any new species are being added through speciation, but species are clearly being lost.
• The biological wealth of the planet is declining rapidly, and human activities are the main cause.
• Human colonisation of the tropical Pacific Islands is said to have led to the extinction of more than 2,000 species of native birds.
• ★ Exam imp 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.
• Extinctions across taxa are not random. Some groups, like amphibians, appear more vulnerable to extinction.
• More than 15,500 species worldwide face the threat of extinction; more than 650 of them are from India.

5.2 The Sixth Extinction

• Fossil records show that large-scale losses of species also happened before humans appeared.
• In the long period (more than 3 billion years) since life originated and diversified, there were five episodes of mass extinction.
• ★ Exam imp The Sixth Extinction, now in progress, differs in its rate: current extinction rates are 100 to 1,000 times faster than in pre-human times.
• Our activities are responsible for these faster rates.
• If present trends continue, nearly half of all species on earth might be wiped out within the next 100 years.

5.3 Effects of biodiversity loss

Loss of biodiversity in a region may lead to:

• (a) a decline in plant production;
• (b) lowered resistance to environmental perturbations such as drought;
• (c) increased variability in certain ecosystem processes, such as plant productivity, water use, and pest and disease cycles.

5.4 Causes of biodiversity loss: the Evil Quartet

• The present accelerated rates of extinction are largely due to human activities.
• There are four major causes. 'The Evil Quartet' is the sobriquet (nickname) used for them.

(i) Habitat loss and fragmentation

• ★ Exam imp This is the most important cause driving animals and plants to extinction.
• The most dramatic examples come from tropical rain forests. They once covered more than 14 per cent of the earth's land surface but now cover no more than 6 per cent.
• They are being destroyed fast: about 1000 hectares more are lost in the short time it takes to read a chapter on this topic.
• The Amazon rain forest is so huge that it is called the 'lungs of the planet'. It harbours probably millions of species.
• It is being cut and cleared to grow soya beans or converted to grasslands for raising beef cattle.
• Besides total loss, degradation of many habitats by pollution also threatens many species.
• Fragmentation: when large habitats are broken into small pieces, mammals and birds needing large territories, and animals with migratory habits, are badly affected. Their populations decline.

(ii) Over-exploitation

• Humans have always depended on nature for food and shelter. When 'need' turns to 'greed', natural resources are over-exploited.
• Many extinctions in the last 500 years were due to over-exploitation, for example Steller's sea cow and the passenger pigeon.
• Many marine fish populations are over-harvested today, which endangers some commercially important species.

(iii) Alien species invasions

• Alien species may be introduced unintentionally or deliberately. Some turn invasive and cause the decline or extinction of indigenous species.
• ★ Exam imp The Nile perch, introduced into Lake Victoria in east Africa, eventually caused 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 and threaten native species.
• The recent illegal introduction of the African catfish Clarias gariepinus for aquaculture threatens the indigenous catfishes of our rivers.

(iv) 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 also dies out.
• In a coevolved plant-pollinator mutualism, the extinction of one invariably leads to the extinction of the other.

Numbers to Remember

• Pacific Islands: more than 2,000 native bird species lost
• IUCN Red List (2004): 784 extinctions in 500 years (338 vertebrates, 359 invertebrates, 87 plants)
• Recent times: nearly 700 species extinct; more than 650 Indian species threatened
• Last 20 years: 27 species lost; threatened worldwide: more than 15,500 species
• Threatened: birds 12, mammals 23, amphibians 32, gymnosperms 31 per cent
• Mass extinctions: 5 in more than 3 billion years; present rate 100 to 1,000 times faster
• Nearly half of all species could vanish in the next 100 years
• Rain forests: from more than 14 to no more than 6 per cent of land; 1000 hectares lost in the time taken to read one chapter
• Nile perch: more than 200 cichlid species lost

6. Pairs to Match and Examples

Pairs to Match

Examples to Remember

7. Quick Revision

• Biodiversity: combined diversity at all levels of organisation; term popularised by Edward Wilson.
• Levels: genetic (Rauwolfia, rice, mango), species (Western over Eastern Ghats amphibians), ecological (India over Norway).
• Described species: over 1.5 million (IUCN 2004); May's estimate: about 7 million; extreme estimates 20 to 50 million.
• Animals over 70 per cent, insects over 70 per cent of animals, plants no more than 22 per cent; fungi outnumber all vertebrates.
• Prokaryotes are left out: methods unsuitable and many unculturable; molecular criteria suggest millions.
• India: 2.4 per cent land, 8.1 per cent species, 12 megadiversity countries, 45,000 plants and twice as many animals.
• Latitudinal gradient: diversity falls from equator to poles; Colombia 1,400 birds, Greenland 56.
• Tropics richer due to time (no glaciations), constancy (niche specialisation) and more solar energy.
• Humboldt: species richness rises with area up to a limit; $\log S=\log C+Z\log A$.
• $Z$: 0.1 to 0.2 for regions, 0.6 to 1.2 for continents, 1.15 for frugivores across continents.
• Stable community: steady productivity, resistant or resilient to disturbance, resistant to alien invasions.
• Tilman: more species, less variation in biomass, higher productivity; Ehrlich: rivet popper hypothesis.
• 784 extinctions in 500 years (IUCN 2004); over 15,500 species threatened; some groups, like amphibians, appear more vulnerable.
• Sixth Extinction: 100 to 1,000 times faster than pre-human rates, caused by humans.
• Evil Quartet: habitat loss and fragmentation (most important), over-exploitation, alien species, co-extinctions.

8. Solved Examples

Common Mistakes to Avoid