Biodiversity and Conservation

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XII, Chapter 13.

How many rivets can a plane lose?

Imagine you are on an aeroplane and someone is quietly removing rivets from it. Lose a few from a seat and nothing happens. Lose a few more and you hardly notice. But keep going, or remove them from the wing, and the whole plane is in danger. The ecologist Paul Ehrlich used this picture for ecosystems, with species as the rivets. This chapter asks how many species exist, how many we are removing and what we can do about it.

What biodiversity means: three levels

Biodiversity is the term popularised by the sociobiologist Edward Wilson for the combined diversity at all levels of biological organisation, from macromolecules inside cells up to biomes. It is usually studied at three levels.

It took millions of years of evolution to build this variety, and at the present rate of loss it could be erased in less than two centuries.

How many species are there?

Nobody knows exactly, which is why this section is all about estimates. Each number below has a source, and CUET tests the source along with the number.

Fungi alone outnumber the combined species of fishes, amphibians, reptiles and mammals. Prokaryotes are left out of these counts because conventional taxonomy fails for them, as many cannot be cultured. By molecular criteria, prokaryote diversity could run into millions.

India's share. India has just 2.4 per cent of the world's land area but 8.1 per cent of its species, and it is one of the 12 mega-diversity countries. About 45,000 plant species and twice as many animal species have been recorded. May's estimate implies that only about 22 per cent of species are recorded, so India probably has more than 100,000 plant species and more than 300,000 animal species still to be found.

Beyond the textbook. For the exam: about 1.5 million described and about 7 million estimated, as of 2004. In reality: the totals are moving targets. Scientists keep describing new species and revising estimates, so treat any number in this chapter as a value for a given year and not as a fixed constant. Why it matters: answer with the figure you have learnt, and read "approximately" into every estimate.

Patterns in nature: latitude and area

The latitudinal gradient. Species diversity falls as you go from the equator to the poles. The tropics, between 23.5° N and 23.5° S, hold more species than temperate or polar regions.

A tropical forest in Ecuador can have up to ten times as many vascular plant species as a temperate forest of the same area in the Midwest of the USA. The Amazon rain forest is the richest of all.

Why are the tropics so rich? Three hypotheses are offered.

Notice that "frequent glaciations" is a reason the temperate zone is poorer. It is a favourite wrong option when the question asks for a cause of tropical richness.

The species–area relationship. The German naturalist Alexander von Humboldt observed that within a region, species richness rises with the area explored, but only to a limit. Plotted directly, it is a rectangular hyperbola. On a log scale it becomes a straight line:

The relationship: S = C A^Z, or log S = log C + Z log A

Here S is species richness, A is area, Z is the slope of the line (the regression coefficient) and C is the Y intercept. The values are strikingly constant.

Worked example. A 100 hectare plot has 200 species and you survey a 10,000 hectare plot with Z = 0.2. The area is 100 times larger, so S₂/S₁ = 100^0.2 ≈ 2.51. The expected richness is 200 × 2.51 ≈ 502. So a hundredfold increase in area raises the species count only about two and a half times, which is what a small Z of about 0.2 means.

Pause and predict: with Z = 0.2, roughly by what percentage does species richness rise if you double the area?

Check your answer: 2^0.2 is about 1.15, so richness rises by about 15 per cent. Doubling the area does not double the species.

Beyond the textbook. For the exam: Z is 0.1 to 0.2 in a region and 0.6 to 1.2 across continents. In reality: the large-area slope is steeper because a very large area contains many separate habitats and regions, each with its own set of species. The relationship also works in reverse: when habitat shrinks, species numbers fall along the same curve. Why it matters: it is the arithmetic behind the most important cause of extinction in the next section.

Why do species matter to an ecosystem?

Communities with more species tend to be more stable. A stable community shows little year-to-year variation in productivity, resists or recovers from disturbance 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 productivity increased with diversity. This is the evidence behind Ehrlich's rivet popper hypothesis: in the aeroplane (ecosystem), the rivets (species) matter unequally, and the loss of rivets on the wings, the key species driving major ecosystem functions, is far more serious than the loss of rivets on a seat.

Biodiversity loss: the numbers

The IUCN Red List (2004) documents the extinction of 784 species in the last 500 years: 338 vertebrates, 359 invertebrates and 87 plants. Recent extinctions include the dodo (Mauritius), the quagga (Africa), the thylacine (Australia), Steller's sea cow (Russia) and three subspecies of tiger (Bali, Javan and Caspian). In the last 20 years alone, 27 species have disappeared. More than 15,500 species are currently threatened with extinction. That includes 12 per cent of all bird species, 23 per cent of mammals, 32 per cent of amphibians and 31 per cent of gymnosperms.

Earth has seen five mass extinctions over more than three billion years. The current one, the Sixth Extinction, differs in rate: it is 100 to 1,000 times faster than before humans, and it is caused by us. Ecologists warn that nearly half of all species could be wiped out within the next 100 years.

The four causes: the Evil Quartet

Pause and predict: a fig species depends on exactly one wasp species for pollination. What follows if the wasp is wiped out?

Check your answer: the fig cannot be pollinated and will eventually vanish too. That is a co-extinction, and the one-to-one link makes the risk higher.

Beyond the textbook. For the exam: the Amazon produces about 20 per cent of the Earth's atmospheric oxygen. In reality: this figure is widely disputed. The forest's own organisms and decomposers use up, through respiration and decomposition, nearly as much oxygen as its plants release, so the net contribution is small. The Amazon matters enormously for rainfall, climate and species, but not because it is the planet's oxygen tank. Why it matters: the "lungs of the planet" image is a way of saying that the forest is a vital ecosystem service provider, and the 20 per cent number is still the answer expected in CUET.

Why conserve? Three arguments

How to conserve: on site and off site

In situ conservation protects the species in its natural habitat, and so protects the whole ecosystem: you save the forest to save the tiger. Ex situ conservation takes threatened species out of their habitat and cares for them elsewhere.

Hotspots. A biodiversity hotspot is a region with very high species richness and very high endemism, which means species found nowhere else. Initially 25 hotspots were identified, and the number is now 34. Three cover India: Western Ghats and Sri Lanka, Indo-Burma and the Himalaya. Together, the hotspots cover less than 2 per cent of the Earth's land area, yet strict protection of them could reduce the ongoing mass extinctions by about 30 per cent.

Pause and predict: hotspots cover less than 2 per cent of land, yet protecting them could cut extinctions by about 30 per cent. How can so little land matter so much?

Check your answer: because they combine high richness with high endemism. The species there live nowhere else, so each patch saved protects species that no other place can.

Protected areas in India. India has 14 biosphere reserves, 90 national parks and 448 wildlife sanctuaries. Cultural traditions help too. Sacred groves, patches of forest set aside by local communities, exist in the Khasi and Jaintia Hills of Meghalaya, the Aravalli Hills of Rajasthan, the Western Ghat regions of Karnataka and Maharashtra, and Sarguja, Chanda and Bastar in Madhya Pradesh. In Meghalaya they remain the last refuge of many rare plants.

Beyond the textbook. For the exam: learn the counts of 14, 90 and 448 and the three Indian hotspots. In reality: both have moved on. The official numbers of Indian protected areas have grown since this count, so a newer source will give higher totals. Also, the Nicobar Islands belong to the Sundaland hotspot, so India is often said to have four hotspots, and the global list has since been lengthened beyond 34. The three kinds of protected area also differ: national parks give the strictest protection, with no grazing or private rights, while sanctuaries may permit limited human use such as collection of forest produce if it does not harm wildlife, and biosphere reserves are zoned into a core, a buffer and a transition area. Why it matters: the exam asks for the numbers above, but the distinction between the three kinds is real knowledge you can apply.

Ex situ methods. Cryopreservation keeps gametes of threatened species viable and fertile for long periods at very low temperature. Eggs can be fertilised in vitro, plants can be propagated by tissue culture, and genetic strains of commercially important plants are stored in seed banks.

International efforts. The Convention on Biological Diversity, known as the Earth Summit, was held in Rio de Janeiro in 1992. A decade later, the World Summit on Sustainable Development in Johannesburg in 2002 saw 190 countries pledge to achieve by 2010 a significant reduction in the current rate of biodiversity loss at global, regional and local levels.

Exam traps

60-second revision

Where this chapter connects

• Previous: Ecosystem describes the services, such as nutrient cycling and productivity, that biodiversity supports.
• Class XII link: Organisms and Populations explains invasive species, competition and keystone predators, which turn up here as causes of loss.
• Class XII link: Biotechnology and its Applications takes up bioprospecting and the question of who owns biological resources.
• Class XII link: Evolution explains how speciation built the diversity this chapter tries to protect.