Biodiversity and Conservation

NCERT opens with an image worth keeping in mind: Class 11 taught you that the living world holds a huge range of forms, from the vast number of insect species to the four or so species of great apes. This variety at every level of biological organisation is biodiversity. The term was popularised by the sociobiologist Edward Wilson, and it describes the combined diversity at all levels, from the genes within one species to the ecosystems of a whole region. The examiner expects exactly three levels, each with an NCERT example.

Genetic diversity is the variation a single species shows over its distributional range. Two examples are standard. The medicinal plant Rauwolfia vomitoria, growing in different Himalayan ranges, shows genetic variation in the potency and concentration of the active chemical reserpine that it produces. India has more than 50,000 genetically different strains of rice and 1000 varieties of mango. Whenever a question mentions strains, varieties, cultivars or a chemical whose concentration differs between populations of one species, the answer is genetic diversity.

Species diversity is the diversity at the species level: the number and kinds of species in a region. NCERT's example is that the Western Ghats have greater amphibian species diversity than the Eastern Ghats.

Ecological (ecosystem) diversity is the variety of ecosystems. India, with its deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows, has greater ecosystem diversity than a Scandinavian country such as Norway. When a question lists habitats or ecosystem types (estuaries, alpine meadows, mangroves), the level is ecological diversity.

Two more lines from the introduction get asked. First, the biodiversity we see today is the product of about 3.5 billion years of evolution. Second, before humans appeared, the earth's biodiversity was much larger than it is now; ever since, human activity has been destroying it. NCERT then frames the chapter around the questions that follow: how many species there are, what patterns their distribution shows, why species are being lost and how we can conserve them. Keep this frame, because the sections of the chapter follow it exactly.

This section is a set of numbers and the examiner wants them exactly. According to the IUCN (2004), the total number of plant and animal species described so far is slightly more than 1.5 million. Many more, especially in the tropics, remain undescribed, so ecologists estimate the total by a statistical comparison of the temperate-tropical species richness of a well-studied group of insects, and then extrapolate that ratio to other groups of animals and plants. Robert May made a more conservative and scientifically sound estimate of about 7 million species globally. That is the answer to the NCERT exercise 'How do ecologists estimate the total number of species present in the world?'

The proportions matter too. 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, making up more than 70 per cent of the total, so out of every ten animals on the planet seven are insects. A striking comparison NCERT makes is that the number of fungal species in the world is more than the combined total of all species of fishes, amphibians, reptiles and mammals. NCERT also points out that these numbers are for the described species; the prokaryotes are not counted because conventional taxonomic methods are not suitable for them and many cannot be cultured in the laboratory. If biochemical or molecular criteria were used, the diversity of prokaryotes alone might run into millions.

Now India. India has only 2.4 per cent of the world's land area but its share of global species diversity is an impressive 8.1 per cent, which makes India one of the 12 mega diversity countries of the world. Nearly 45,000 species of plants and twice as many animals have been recorded from India. Applying May's global estimate that only 22 per cent of the world's species have been recorded, India probably has more than 100,000 plant species and more than 300,000 animal species yet to be discovered and described. NCERT closes with a sobering line: it would take many more centuries to describe all these species, but a large fraction of them face the threat of becoming extinct even before they are discovered.

The two pie charts of NCERT Fig. 13.1 come from this section. Chart (a) shows the proportionate number of species of major taxa of invertebrates and vertebrates: insects are by far the largest sector; molluscs, crustaceans and other animal groups take smaller slices; and the vertebrates (fishes, amphibians, reptiles, birds and mammals) together are a small sector. Chart (b) shows plants: angiosperms are the largest sector, followed by fungi, algae, lichens, mosses, ferns and allies, and gymnosperms with the smallest slice. Board papers have asked students to identify labelled sectors in these charts.

Biodiversity is not spread evenly across the planet; it follows patterns, and NCERT names two. The first 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 (the belt between 23.5° N and 23.5° S) hold more species than temperate or polar areas. The examiner wants the bird numbers. Colombia, located near the equator, has nearly 1,400 species of birds; New York, at 41° N, has 105; and Greenland, at 71° N, has only 56. India, with much of its land in the tropical latitudes, has more than 1,200 species of birds. A forest in a tropical region like Ecuador has up to ten times as many species of vascular plants as a forest of equal area in a temperate region like the Midwest of the USA. The largest biodiversity on earth is in the tropical Amazonian rain forest of South America, home to 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 125,000 of invertebrates. Scientists estimate that this forest might hold at least two million insect species yet to be discovered.

Why do the tropics have so much more? Ecologists and evolutionary biologists have proposed several hypotheses; NCERT gives three and a 3-mark question asks for exactly these.

1. Time for speciation. Speciation is generally a function of time. Temperate regions were subjected to frequent glaciations in the past, which repeatedly wiped out communities, whereas the tropical latitudes have remained relatively undisturbed for millions of years and so have had a long evolutionary time for species diversification.
2. Constant, predictable environment. Tropical environments, unlike temperate ones, are less seasonal, relatively more constant and predictable. Such constant environments promote niche specialisation and so lead to a greater species diversity.
3. More solar energy. There is more solar energy available in the tropics, which contributes to higher productivity; this in turn contributes indirectly to greater diversity.

When the question is 'Where would you expect more biodiversity, tropics or polar regions, and why?', name the tropics and give these three reasons. When it says 'Write any two hypotheses', give two of the three with the key words glaciation, niche specialisation and solar energy.

The second pattern was found by the German naturalist and geographer Alexander von Humboldt during his pioneering explorations in the wilderness of South American jungles. Within a region, species richness increases with the area explored, but only up to a limit. The relationship between species richness and area, for a wide variety of taxa (angiosperm plants, birds, bats, freshwater fishes), turns out to be a rectangular hyperbola. On a logarithmic scale the relationship becomes a straight line, described by the equation

log S = log C + Z log A

where S is species richness, A is area, Z is the slope of the line (the regression coefficient) and C is the Y-intercept. The same relationship in ordinary form is S = CA^Z. You should be able to write both forms and to name each symbol.

The examiner's favourite point is the value of Z. Ecologists have discovered that, regardless of the taxonomic group or the region, the value of Z lies in the range of 0.1 to 0.2. This is the 'significance of the slope of regression' asked in NCERT exercise 4: the slope is remarkably similar across groups and regions, which tells us that the way species richness grows with area is a general property of nature and not something special to one group. But when the species-area relationship is analysed among very large areas, such as an entire continent, the slope is much steeper, with Z values in the range of 0.6 to 1.2. For example, for frugivorous (fruit-eating) birds and mammals in the tropical forests of different continents, the slope is 1.15. So the answer to 'what is the significance of the slope' has two parts: the small, consistent value for small areas within a region, and the steep value for continental comparisons.

NCERT Fig. 13.2 is the graph you must be able to draw and read. It has two curves on one set of axes: the species-area relationship on a linear scale rises steeply and then flattens (the rectangular hyperbola), and the same data plotted on log scales gives a straight line. The X-axis is Area and the Y-axis is Species richness. Board questions have presented this figure and asked for the equation of each curve, the names of the symbols and the range of Z, so learn the figure with its labels and not just the equation.

Does the number of species in a community matter to how the ecosystem functions? For many decades ecologists believed that communities with more species were generally more stable than those with fewer species. A stable community, in NCERT's words, should not show too much variation in productivity from year to year, must be either resistant or resilient to occasional disturbances (natural or man-made), and must be resistant to invasions by alien species. These three features answer the old question 'List the features that make a stable biological community.'

The experimental evidence comes from David Tilman, whose long-term ecosystem experiments used outdoor plots. He found that plots with more species showed less year-to-year variation in total biomass, and that in his experiments increased diversity contributed to higher productivity. Those two findings are the answer to 'Support your answer with the finding of Tilman' and to 'How did Tilman show experimentally that stability depends on species richness?' NCERT is honest that we do not yet have a definite answer to whether species richness and diversity are essential to ecosystem health, but it points out that rich biodiversity is not only essential for ecosystem health but imperative for the very survival of the human race.

The second idea is the rivet popper hypothesis of the Stanford ecologist Paul Ehrlich. Think of an aeroplane (the ecosystem) held together by thousands of rivets (the species). If every passenger travelling in it starts popping a rivet to take home, it may not affect flight safety at first (the ecosystem functions properly), but as more and more rivets are removed the plane becomes dangerously weak over time. Which rivet is removed also matters: a rivet on the wing (a key species that drives major ecosystem functions) is a more serious threat to flight safety than a rivet on a seat or window inside the plane. The hypothesis therefore says that the loss of a few species may not hurt at once, that cumulative losses will, and that losing a keystone species does the most damage. A 2- or 3-mark question asks you to explain the analogy and name the ecologist; make sure every part of the analogy is mapped to its ecological meaning.

The biological wealth of the planet is being lost at an alarming rate, and NCERT gives the numbers from the IUCN Red List (2004): 784 species have gone extinct in the last 500 years, made up of 338 vertebrates, 359 invertebrates and 87 plants. The named examples are the dodo (Mauritius), the quagga (Africa), the thylacine (Australia), Steller's sea cow (Russia) and three subspecies of tiger, the Bali, Javan and Caspian tigers. The last twenty years alone have seen the disappearance of 27 species. Extinctions across taxa are not random: some groups such as amphibians appear to be more vulnerable. Adding to the extinct species, more than 15,500 species world-wide face the threat of extinction. Presently 12 per cent of all bird species, 23 per cent of all mammal species, 32 per cent of all amphibian species and 31 per cent of all gymnosperm species face the threat of extinction.

The fossil record shows five episodes of mass extinction of species in the past. The sixth extinction, now in progress, is different in two ways that a 3-mark question wants: its rate is 100 to 1,000 times faster than the pre-human times, and it is caused mainly by human activities, whereas the earlier five were the result of natural causes. NCERT's warning is that if the present trends continue, nearly half of all the species on earth might be wiped out within the next 100 years.

What does the loss of biodiversity do to an ecosystem? NCERT lists three consequences, and 'How is biodiversity important for ecosystem functioning?' (exercise 6) is answered by turning them around. Loss of biodiversity in a region may lead to (i) a decline in plant production, (ii) lowered resistance to environmental perturbations such as drought, and (iii) increased variability in certain ecosystem processes such as plant productivity, water use, and pest and disease cycles. Put with Tilman's findings and the rivet popper hypothesis, these three points make up any 3- or 5-mark answer on the importance of biodiversity to ecosystem functioning.

The accelerated rates of species extinction the world is facing now are largely due to human activities. NCERT names four major causes and calls them, borrowing a phrase from the evolutionary biologist Jared Diamond, the Evil Quartet. This is the single most examined part of the chapter; a 3-mark question asks for any three with an example each and a 5-mark question for all four.

(i) Habitat loss and fragmentation. This is the most important cause driving animals and plants to extinction. The best example is the tropical rain forests: once covering more than 14 per cent of the earth's land surface, they now cover no more than 6 per cent. The Amazon rain forest, so huge that it is called the 'lungs of the planet', is being cut and cleared to cultivate soya beans or to convert to grasslands for raising beef cattle. Besides total loss, the degradation of many habitats by pollution also threatens the survival of many species. When large habitats are broken up into small fragments because of human activities, mammals and birds that require large territories and certain animals with migratory habits are badly affected, leading to population declines.

(ii) Over-exploitation. Humans have always depended on nature for food and shelter, but when 'need' turns to 'greed' it leads to over-exploitation of natural resources. Many species extinctions in the last 500 years, such as Steller's sea cow and the passenger pigeon, were due to over-exploitation by humans. Presently many marine fish populations around the world are over-harvested, endangering the continued existence of some commercially important species.

(iii) Alien species invasions. When alien species are introduced, unintentionally or deliberately, some turn invasive and cause the decline or extinction of indigenous species. The Nile perch introduced into Lake Victoria in east Africa led eventually to the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in the lake. In India, the invasive weeds carrot grass (Parthenium), Lantana and water hyacinth (Eichhornia) have caused environmental damage and posed a threat to native species. The illegal introduction of the African catfish Clarias gariepinus for aquaculture purposes is posing a threat to the indigenous catfishes of Indian rivers. When asked for one plant and one animal alien species that threaten Indian natives, give Parthenium (or Lantana or Eichhornia) and Clarias gariepinus.

(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 meets the same fate. Another example is a coevolved plant-pollinator mutualism, where the extinction of one invariably leads to the extinction of the other. This is the link the board used in 2019: mutualists tend to coevolve (the fig and its wasp from Chapter 11 is the example to quote), and the loss of one partner is a cause of biodiversity loss.

NCERT groups the reasons for conserving biodiversity under three headings, and the 5-mark question wants all three with examples.

Narrowly utilitarian arguments are the obvious ones: humans derive countless direct economic benefits from nature. These are food (cereals, pulses, fruits), firewood, fibre, construction material, industrial products (tannins, lubricants, dyes, resins, perfumes) and products of medicinal importance. More than 25 per cent of the drugs currently sold in the market are derived from plants, and 25,000 species of plants contribute to the traditional medicines used by native peoples around the world. Nobody knows how many more medicinally useful plants there may be in tropical rain forests waiting to be explored. With increasing resources put into bioprospecting (exploring molecular, genetic and species-level diversity for products of economic importance), nations endowed with rich biodiversity can expect to reap enormous benefits. Bioprospecting is a term the examiner may ask you to define in one line.

Broadly utilitarian arguments say that biodiversity plays a major role in many ecosystem services that nature provides. The fast-dwindling Amazon forest is estimated to produce, through photosynthesis, 20 per cent of the total oxygen in the earth's atmosphere; NCERT asks whether we can put a price on such a service. Pollination, without which plants cannot give us fruits or seeds, is another service through bees, bumblebees, birds and bats. There are also the intangible benefits: the aesthetic pleasures of walking through thick woods, watching spring flowers in full bloom or waking up to a bulbul's song in the morning. Other ecosystem services that questions expect you to name are control of floods and soil erosion, nutrient cycling and climate regulation. When a question asks 'how is control of floods and soil erosion achieved by the biotic components' (exercise 8), the answer is that plant roots bind the soil and hold it against runoff, the canopy and litter break the force of rain, and vegetation slows and absorbs surface water so that streams do not flood.

Ethical arguments relate to what we owe to millions of plant, animal and microbe species with whom we share the planet. Philosophically or spiritually, we need to realise that every species has an intrinsic value, even if it is not of current or any economic value to us. We have a moral duty to care for their well-being and to pass on our biological legacy in good order to future generations.

A frequent 3-mark variant is 'Compare narrowly utilitarian and broadly utilitarian approaches'; the difference is that the first counts direct, priced products that humans take from nature, while the second counts the unpriced services that a functioning ecosystem provides to everyone.

NCERT gives two broad approaches. In situ conservation is on-site conservation: the threatened or endangered species is protected in its natural habitat, so that the entire ecosystem is protected. Ex situ conservation is off-site: the species is taken out of its natural habitat and placed in a special setting where it can be protected and given special care. The board's standard 2-mark question is the difference between the two, and the standard 5-mark question wants each described with examples.

When we conserve and protect the whole ecosystem, its biodiversity at all three levels is protected. Faced with the conflict between development and conservation, many nations find it unrealistic and economically not feasible to conserve all their biological wealth, so the number of species waiting to be saved has to be prioritised. Conservationists have identified biodiversity hotspots: regions with very high levels of species richness and a high degree of endemism (species confined to that region and found nowhere else). Initially 25 hotspots were identified; nine more were added later, bringing the total to 34. Three of these hotspots, the Western Ghats and Sri Lanka, Indo-Burma and Himalaya, cover our country's exceptionally high biodiversity regions. These hotspots are also regions of accelerated habitat loss. Although all the hotspots put together cover less than 2 per cent of the earth's land area, the number of species they collectively harbour is extremely high, and strict protection of them could reduce the ongoing mass extinctions by almost 30 per cent. That is why the Western Ghats are a hotspot: very high species richness, high endemism (for example, of amphibians) and rapid habitat loss.

In India, ecologically unique and biodiversity-rich regions are legally protected as biosphere reserves, national parks and sanctuaries. NCERT's figures are 14 biosphere reserves, 90 national parks and 448 wildlife sanctuaries. India has also a long history of religious and cultural traditions that emphasised protection of nature: in many cultures tracts of forest were set aside and all the trees and wildlife within them were venerated and given total protection. These are sacred groves, 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. In Meghalaya the sacred groves are the last refuges for a large number of rare and threatened plants. Their role in conservation is that religious sentiment gives total protection to every organism inside, so they preserve species that have vanished elsewhere. Sacred groves are in situ conservation, and a question that pairs a sacred grove with a botanical garden is asking you to contrast in situ with ex situ.

In ex situ conservation the threatened animals and plants are taken out of their natural habitat and placed in a special setting where they can be protected and given special care. Zoological parks, botanical gardens and wildlife safari parks serve this purpose. Many animals that have become extinct in the wild continue to be maintained in zoological parks. Ex situ conservation has moved well beyond keeping threatened species in enclosures. Now the gametes of threatened species can be preserved in viable and fertile condition for long periods by cryopreservation techniques (storage at very low temperature, in liquid nitrogen at about −196 °C), eggs can be fertilised in vitro, and plants can be propagated using tissue culture methods. Seeds of different genetic strains of commercially important plants are kept for long periods in seed banks. Pollen banks store pollen in the same way, and gene banks hold the genetic material of many strains.

The 2019 board paper asked for six advantages of the ex situ approach, and the list to give is: (1) threatened animals and plants get protection and special care outside the dangers of their habitat; (2) animals extinct in the wild survive in zoos and can later be reintroduced; (3) gametes of threatened species can be stored by cryopreservation for long periods; (4) eggs can be fertilised in vitro, which allows captive breeding; (5) plants can be multiplied rapidly by tissue culture; (6) seed banks preserve many genetic strains of commercially important plants. Each advantage is one marking point, so write them as a numbered list.

The difference from in situ conservation is that in situ protects the species together with its whole ecosystem and is cheaper and more natural, while ex situ protects the individual species intensively and can rescue species whose habitat is already gone. Exam answers should say that in situ is the better approach where the habitat survives and ex situ is the fallback when it does not.

Biodiversity knows no political boundaries, and its conservation is a collective responsibility of all nations. Two international efforts are in NCERT. The historic Convention on Biological Diversity ('The Earth Summit') held in Rio de Janeiro in 1992 called upon all nations to take appropriate measures for the conservation of biodiversity and the sustainable utilisation of its benefits. In a follow-up, the World Summit on Sustainable Development held in 2002 in Johannesburg, South Africa, saw 190 countries pledge their commitment to achieve by 2010 a significant reduction in the current rate of biodiversity loss at global, regional and local levels. A 1-mark item may ask the place and year of either summit, so learn both pairs.