Biodiversity and Conservation: Class 12 Biology · Chapter 13

Sit under a big banyan or peepal tree for an hour and keep a list. Barbets and bulbuls arrive for the figs, tiny wasps crawl into the fruits, ants patrol the bark, a squirrel raids the upper branches and fungi slowly rot the fallen leaves. A single old tree can support hundreds of kinds of living things, and it is only one of millions of species on Earth.

This chapter asks four questions about that variety. How much life is there, and where is it richest? Does an ecosystem need all of its species to work well? Why are species vanishing faster now than at any time in human history? And what can we do to slow the loss?

13.1 Biodiversity

Biodiversity (जैव विविधता) is short for biological diversity: the variety of life at every level of organisation, from small differences in DNA between two individuals to the contrast between a desert and a rain forest. The word was coined in 1985 by Walter Rosen of the US National Research Council while he planned a scientific forum, and the Harvard biologist Edward O. Wilson, a founder of sociobiology, made it famous when he edited the book that came out of that meeting.

Three levels get most attention, and each one sits inside the next (Fig. 13.1).

Genetic diversity (आनुवंशिक विविधता) is the variation in genes within a single species. Individuals and populations carry different alleles, so they differ in size, colour, yield, resistance to disease or tolerance of heat and drought. India's humped zebu cattle show this well. The hardy Gir of Gujarat, the high-yielding Sahiwal of Punjab and the tiny Vechur of Kerala are breeds of one species, yet a farmer can tell them apart at a glance. Such variation is the raw material of evolution and of plant and animal breeding. In the 1970s, resistance to the grassy stunt virus of rice turned up in one population of a wild rice, Oryza nivara, collected in Uttar Pradesh. Breeders transferred it into rice varieties grown across Asia.

Species diversity (जातीय विविधता) is the variety of species in an area. Its simplest measure is species richness, the number of species present, but ecologists also ask how evenly the individuals are shared among those species. A teak plantation and a natural deciduous forest may clothe the same hillside. The plantation is dominated by a single tree; the natural forest mixes dozens of trees with climbers, shrubs and herbs, and it feeds far more birds and insects.

Ecological diversity (पारिस्थितिक विविधता) is the variety of ecosystems, the habitats, communities and processes, within a region. Gujarat alone holds the salt flats of the Rann of Kachchh, the dry teak forests and grasslands of Gir, mangroves along its creeks and coral reefs in the Gulf of Kachchh. A region with that range of ecosystems is ecologically more diverse than an equally large area covered by one kind of forest.

Keep all three levels in mind. Conservation (Section 13.2) tries to protect them together, because losing variety at one level weakens the others: a species squeezed into a single small population loses genetic diversity, and an ecosystem that loses species changes character.

13.1.1 How Many Species are there on Earth and How Many in India?

The question 'How many species are there?' has two answers. The easy one is the number already described: every new species is formally published, and global catalogues now list a little over two million. The hard one is the number that exist, since most species have never even been collected. Your textbook quotes the IUCN's 2004 figure of about 1.5 million described plant and animal species. The total has grown since, and taxonomists still add around 15,000 to 20,000 new species every year.

One way to estimate the unknown works by proportion. Suppose butterflies, which collectors have studied everywhere, turn out to have four tropical species for every temperate one. Beetles, by contrast, are thoroughly listed only in temperate countries, where generations of naturalists have collected them. If beetles follow the butterflies' ratio, multiplying the temperate beetle list by four estimates the tropical one (Example). The method matters because the tropics, where lists are least complete, hold most of the species still undescribed.

The most widely cited modern estimate comes from a 2011 study led by Camilo Mora, which used the regular way species group into genera, families and orders to arrive at about 8.7 million species of eukaryotes. Earlier figures spread widely: the ecologist Robert May argued for about 7 million, while guesses about tropical insects once pushed totals to 20–50 million or more. On the more cautious figures, only a quarter to a third of Earth's species have names.

Which groups dominate?

Roughly every second named species is an insect (Fig. 13.2). Insects make up about 70 % of all named animals, and animals in turn make up more than 70 % of all named species. Vertebrates, the group we notice most, are only a few per cent of the list, and there are more named species of fungi than of fishes, amphibians, reptiles and mammals put together. The plant kingdom in the old, broad sense that also counts fungi and algae accounts for only about 22 %.

Microbiologists who sample soil or seawater and sort the DNA they find into species-like groups turn up enormous numbers of prokaryote types, probably millions. None of these appear in the counts above. Bacteria and archaea rarely look different under a microscope and most refuse to grow in culture, so they cannot be described in the classical way.

India's share

India covers only about 2.4 % of the world's land area, yet it holds roughly 7–8 % of all recorded species. That makes it one of the megadiverse countries, the small group of nations that between them hold most of Earth's species. Your textbook counts 12 such countries, a figure from an early study; the list most widely used today, prepared by Conservation International, names 17, including India, China, Brazil, Indonesia and Australia.

Our national surveys keep extending the list. The Botanical Survey of India records at least 45,000 species of plants, including fungi and algae. By the early 2020s the Zoological Survey of India's count of animal species had passed one lakh. Each year Indian and foreign scientists describe a few hundred more species from India.

Scale India's list up by the same fraction and the country could hold several times as many species as it has recorded (Example). The bottleneck is people: each new species needs a trained taxonomist to collect, compare and describe it, and India, like most tropical countries, has far too few. That shortage is urgent, because forests, wetlands and grasslands are being cleared now, and many small insects, fungi and soil animals will probably vanish unrecorded.

13.1.2 Patterns of Biodiversity

Species are not scattered evenly over the planet. Two patterns hold for so many groups of organisms that ecologists treat them almost as rules.

Latitudinal gradients

Travel from the equator towards either pole and, in group after group, the number of species drops (Fig. 13.3). This latitudinal gradient makes the tropics, which lie within about 23.5° of the equator, the richest belt on Earth; only a handful of groups, such as aphids and penguins, buck the trend. A single hectare of rain forest in the Amazon or Borneo can contain more than 200 species of trees. A hectare of oak or beech forest in Europe usually holds fewer than 20, and the tundra has no trees at all. Ants, butterflies, frogs, birds and mammals follow the same trend. Much of India lies in or near the tropics, one reason it is so rich in species.

Why should the tropics be so rich? Ecologists favour three explanations, which can all be true at once:

1. More time. Ice ages repeatedly buried much of the temperate and polar land under ice or pushed its communities far towards the equator, and each time life there had to recolonise and start diversifying afresh. The tropics were never wiped clean in this way, so their lineages have been splitting into new species for far longer.
2. A steadier environment. Where there is no harsh winter and the seasons repeat reliably, a species can stake its living on something narrow and still survive every year. It might eat one kind of fruit, live in one layer of the canopy or feed at one hour of the night. Narrow niches let more species share the same space.
3. More energy. Strong sunshine all year drives fast plant growth, the highest productivity on land. A bigger food supply can support more consumers and so, indirectly, more kinds of them.

Species–area relationships

The second pattern concerns area. While exploring South America around 1800, the German naturalist Alexander von Humboldt noticed that larger stretches of country yielded more species. Each extra stretch, though, added fewer new species than the last. Ecologists have since found the same species–area relationship for plants, birds, beetles, snails and many other groups (Fig. 13.4).

On ordinary axes the points climb steeply and then bend over. They fit a power law, first written down by the Swedish botanist Olof Arrhenius in 1921.11Your textbook calls this curve a rectangular hyperbola. Strictly, the power law never reaches a ceiling: it keeps rising, more and more slowly, which on ordinary axes looks much like levelling off.

The slope $Z$ is the number that matters most for conservation. Its value depends mainly on what kind of areas are being compared (Table 13.1). Strikingly, nested plots within one region give much the same slope whether the counts are of plants, birds or snails, and in whichever part of the world they are made.

What does a steeper slope mean? $Z$ measures how fast species are added as area grows and, read backwards, how fast they are lost as area shrinks. From the power law, multiplying the area by 10 multiplies the number of species by ${10}^Z$: by about 1.4 when $Z=0.15$, but by about 14 when $Z=1.15$. A bigger patch within one region mostly contains more of the same species, so the slope is shallow. Two continents share few species, because their plants and animals evolved largely apart, so each extra continent's worth of area brings in a new set of species and the slope is steep.

13.1.3 The Importance of Species Diversity to the Ecosystem

Would a grassland with 16 kinds of plants work any better than one with 2? At Cedar Creek in Minnesota, USA, the ecologist David Tilman has spent decades testing exactly this, in outdoor plots that differ only in how many plant species were sown in them. Plots with more species produced more biomass on average, and their total biomass varied less from year to year (Fig. 13.6). In an earlier study of a severe drought, species-rich plots also lost a smaller share of their biomass. One reason is that species respond differently to the same stress: in a dry year some suffer while others hold steady or even expand into the gaps.

Results like these support an old idea, still debated for forests, rivers and seas: that diversity makes a community more stable. Ecologists use the word in a specific sense. A stable community:

• keeps its productivity fairly steady from one year to the next;
• either withstands occasional disturbances, such as droughts, fires or pest outbreaks, or bounces back from them quickly;
• is hard for species arriving from elsewhere to invade.

Why worry about losing one species among thousands? Paul Ehrlich of Stanford University answered with a thought experiment now known as the rivet popper hypothesis (Fig. 13.7). Think of an ecosystem as an aircraft and of its species as the rivets holding it together. The aircraft is built with spare strength, so a few missing rivets go unnoticed. But every rivet removed uses up some of that margin, and no one can say which removal will be the last safe one. Position matters too. A few rivets missing from a wing are more dangerous than many missing from the seats. In the same way, losing a key species that drives a major ecosystem function, such as a dominant pollinator, predator or nitrogen fixer, does more harm than losing a minor one.

The lesson is that species loss can stay invisible for years and then show itself all at once, as a collapsed fishery, a pest outbreak or a failed harvest. And because people draw food, water, fertile soil and a stable climate from ecosystems, their weakening is our problem too.

13.1.4 Loss of Biodiversity

The dodo of Mauritius, the quagga of southern Africa, the thylacine of Tasmania and Steller's sea cow of the Bering Sea are among the best-known species lost in the last few centuries. Three subspecies of tiger have gone too: the Bali, Javan and Caspian tigers. India has lost the pink-headed duck, last reliably seen in 1949, and the cheetah, which was hunted out of the country and declared extinct here in 1952, although it survives elsewhere.

Since 1500 CE the IUCN Red List has recorded roughly 900 extinctions (your textbook quotes 784, from the 2004 list), and the true number is certainly higher, because most species have never been assessed or even named. Earlier losses were huge too: when people first settled the islands of the tropical Pacific, as many as 2,000 bird species, many of them flightless rails, are thought to have died out. Speciation still produces new species, but far too slowly to make up for losses on this scale, and nearly all of today's losses trace back to people.

Some groups are losing species much faster than others. Amphibians are especially vulnerable: they breathe partly through moist skin and need both water and land. They have also been devastated by chytridiomycosis, a fungal disease that has contributed to the decline of at least 500 amphibian species. The Red List shows how large a share of the assessed species in some groups is now threatened (Table 13.3). In 2024 more than 45,000 of about 1.6 lakh assessed species were threatened; your textbook's figure of 15,500 dates from 2004, when far fewer species had been assessed.

Mass extinctions have happened before. The fossil record reveals five, all within the last 450 million years, in which a large fraction of species vanished in a geologically short time (Fig. 13.8). The most recent, 66 million years ago, ended the age of the non-bird dinosaurs. Many ecologists say a sixth extinction is now under way. Two things set it apart: its speed, an estimated 100 to 1,000 times the background rate before humans, and its cause, which is the activity of one species, ours. In 2019 a global assessment by IPBES, the intergovernmental science panel on biodiversity, concluded that about a million species of animals and plants face extinction. Many could vanish within decades.

Losing species changes how whole ecosystems behave, much as Tilman's plots would predict when run in reverse. A region that loses biodiversity tends to grow less plant matter and to cope worse with droughts and other shocks. Its processes also become more erratic: plant growth, the water taken up by vegetation, and the rise and fall of pests and diseases all swing more widely.

Causes of biodiversity losses

Four human-driven causes account for most of today's losses. The American biologist and geographer Jared Diamond nicknamed them the evil quartet.

Habitat loss and fragmentation heads the list. In the Western Ghats the lion-tailed macaque now lives mostly in scattered rain-forest fragments among tea and coffee estates, each too small to hold a healthy population. Fragmentation works this way everywhere (Fig. 13.9). Once roads, canals, railway lines, mines and farms split a forest into islands, wide-ranging animals such as tigers and elephants cannot find enough room in any one piece, and migratory animals find their routes cut. The edges of each fragment also turn hotter, drier and weedier than the interior. Outright destruction is larger still: tropical rain forests, once spread over about 14 % of Earth's land, now cover about 6 %, and clearing continues for cattle ranches, soybean farms and oil-palm plantations. Pollution can ruin a habitat even where no tree is felled.

Over-exploitation happens when we harvest a species faster than its populations can replace themselves. Steller's sea cow was hunted to extinction within 27 years of its discovery. The passenger pigeon, once perhaps the most numerous bird in North America, was shot and netted until the last one died in a zoo in 1914. In the sea, the cod fishery off Newfoundland, worked for centuries, collapsed so completely that Canada had to close it in 1992, and many other commercial fish stocks are overfished today. In India, sandalwood trees have been felled and smuggled so heavily that the IUCN lists the species as Vulnerable.

Alien species invasions. In the 1960s mesquite (Prosopis juliflora) was planted in the Banni grasslands of Kachchh to hold back the desert; it has since spread over much of the grassland that herders and wildlife depended on. Mesquite is an alien species (विदेशी जाति), one that people moved, on purpose or by accident, outside its native range. Most aliens never establish themselves, but some become invasive: free of the enemies that kept them in check at home, they spread and crowd out or prey on native species. The Nile perch, released into Lake Victoria in East Africa in the 1950s, is blamed for the loss of about 200 of the lake's unique cichlid fishes. India battles invasive weeds such as Parthenium (carrot grass), Lantana and water hyacinth (Eichhornia), and the illegally farmed African catfish Clarias gariepinus threatens our native catfishes.

Co-extinctions. A fig tree and its fig wasp show how one extinction can trigger another (Fig. 13.10). Most of India's many fig species can be pollinated only by their own species of tiny wasp, and each wasp can breed only inside the figs of its host. If the wasp disappears, its fig can no longer set seed, and the fig is doomed too. Ecologists call such knock-on losses co-extinctions. Specialist parasites, which die with their only host, are probably the commonest victims.

13.2 Biodiversity Conservation

Knowing why species are vanishing tells us where to act. Two questions follow: why should we conserve biodiversity at all, and how can we do it?

13.2.1 Why Should We Conserve Biodiversity?

The arguments fall into three groups, running from the purely practical to the purely moral (the three arguments).

The narrowly utilitarian argument counts what reaches us straight from wild or farmed species. Think of rice, dal and mangoes, timber for houses and fuelwood for stoves, cotton and jute, and raw materials for industry such as natural rubber, lac, gums, dyes and fragrant oils. Medicines are a striking case. The anti-cancer drugs vincristine and vinblastine come from the Madagascar periwinkle (sadabahar, common in Indian gardens). The antimalarial artemisinin comes from sweet wormwood, and the cancer drug paclitaxel from the bark of yew trees. A widely quoted estimate is that over a quarter of the world's medicines come from plants, and traditional systems such as Ayurveda, Siddha and Unani, along with folk healers elsewhere, draw on tens of thousands more. Searching biodiversity for useful genes, molecules and species is called bioprospecting. Species-rich countries stand to gain most from it, provided the benefits are shared fairly with the communities whose knowledge often guides the search; Chapter 10 discusses the problem of biopiracy.

The broadly utilitarian argument looks at ecosystem services (पारितंत्र सेवाएँ), the benefits we receive from ecosystems working as a whole. Forests store carbon, hold soil on slopes, recharge groundwater and release rain slowly into rivers. Wetlands and mangroves absorb floods and storm surges, birds and bats eat crop pests, and soil organisms recycle nutrients. Pollination alone is worth a great deal. About three-quarters of the leading kinds of food crops benefit from animal pollinators such as bees, butterflies, birds and bats, and the yearly harvest that depends on them is valued at hundreds of billions of US dollars. Nobody sends us a bill for these services, but replacing them with machines, fertilisers or hand pollination would be ruinously expensive, and some cannot be replaced at all. Then there are pleasures no price can capture: flamingos on a winter lake, the smell of earth after the first rain (much of it produced by soil bacteria), a tiger's pugmarks on a forest track.

The ethical argument turns the question round: instead of asking what a species is worth to us, it asks what we owe it. A tiger, a lichen or a soil bacterium is the product of millions of years of evolution and has intrinsic value, a worth that does not depend on our finding a use for it. If so, people have no right to wipe out species for convenience, and each generation holds the living world in trust for the next. Many Indian traditions say much the same, treating rivers, groves and animals as sacred.

13.2.2 How do we Conserve Biodiversity?

Picture two ways to save the great Indian bustard, a heavy grassland bird with perhaps 150 left in the wild. One is to protect the grasslands of Rajasthan where it lives, keeping out ploughs and the overhead power lines into which it often flies. The other is to collect its eggs and raise chicks in a breeding centre until they can be released. These are the two strategies of conservation (Fig. 13.11).

In situ (स्वस्थाने) conservation, meaning on site, protects species where they live by protecting the whole ecosystem, so every level of its biodiversity is saved at once. A well-guarded tiger forest also protects the deer, the trees and the soil fungi. Ex situ (बाह्यस्थाने) conservation, meaning off site, takes individuals, seeds or cells out of the wild and keeps them safe elsewhere. It is the emergency option for threatened species, those at high risk of dying out in the wild in the coming years, which the IUCN grades as Vulnerable, Endangered or Critically Endangered.

In situ conservation

Conservation money is limited, so where should it go first? In 1988 the ecologist Norman Myers suggested a rule of thumb: spend it where the most irreplaceable species face the greatest danger. He called such places biodiversity hotspots: regions crowded with endemic (स्थानिक) species, found nowhere else, that are also losing their habitat fast. Conservation International later turned the idea into two tests (Fig. 13.12). A hotspot must have at least 1,500 endemic species of vascular plants, and it must already have lost at least 70 % of its original natural vegetation.

The first list of 25 hotspots, published in 2000, grew to 34 in 2004, the number in your textbook, and to 36 by 2016. Together they cover only about 2.5 % of Earth's land surface, yet more than half of all plant species and over 40 % of land vertebrate species are endemic to them. By one estimate, strictly protecting them could cut the current rate of extinctions by about 30 %. India includes parts of four hotspots: the Himalaya, Indo-Burma (the north-east and the Andaman Islands), the Western Ghats and Sri Lanka, and Sundaland, which takes in the Nicobar Islands. Your textbook names the first three, which cover most of India's biodiversity-rich land.

Within India, important areas for biodiversity are protected by law. National parks protect habitats strictly and allow no grazing or harvesting. Wildlife sanctuaries protect animals but may permit some regulated activities. The newer conservation reserves and community reserves involve local people in managing land next to protected forests. By 2023 India had about 106 national parks, more than 550 wildlife sanctuaries and, in all, roughly a thousand protected areas covering a little over 5 % of its land. Your textbook gives older figures of 90 national parks and 448 sanctuaries.

Biosphere reserves take a broader view of conservation. Under UNESCO's Man and the Biosphere programme, each is a large landscape divided into zones (Fig. 13.13). A strictly protected core is ringed by a buffer, where research, education and limited use are allowed. Beyond lies a transition zone where people live, farm and work in ways that do not harm the core. India now has 18 biosphere reserves, from the Nilgiri, the first in 1986, to the Great Nicobar, the Sundarbans and the Cold Desert of Himachal Pradesh; your textbook mentions 14. A dozen or more of them belong to UNESCO's world network.

Long before national parks existed, Indian villages were protecting forests in their own way. A patch of woodland would be dedicated to a deity or the ancestors, and by custom no tree there could be cut and no animal hunted. Thousands of these sacred groves (पवित्र उपवन) survive. Kerala calls them kavu, Maharashtra devrai, Karnataka devara kaadu and Rajasthan's Aravalli villages orans. Communities in Sarguja and Bastar (now in Chhattisgarh) and around Chanda (Chandrapur) in Maharashtra know them as sarna or devgudi. In the Khasi and Jaintia Hills of Meghalaya, groves called law kyntang, such as Mawphlang near Shillong, are the last refuges of rare orchids and other plants that have vanished from the surrounding hills.

Ex situ conservation

When India's Gyps vultures crashed, the birds could not be left in the wild while the poison was still in use, so breeding centres such as the one at Pinjore in Haryana took them in. A breeding programme for the great Indian bustard followed in Rajasthan in 2019. Rescues like these are ex situ conservation, carried out in breeding centres, zoos, botanical gardens and safari parks. Some species exist today only because of it. Père David's deer vanished from the wild in China but lived on in a herd on an English estate, and it was returned to China in 1985. The Arabian oryx and Przewalski's horse were likewise bred in captivity after dying out in the wild, and both have since been reintroduced.

Not every rescue needs live animals. Sperm, eggs, embryos and tissue samples of threatened animals can be stored for decades in liquid nitrogen at −196 °C, a technique called cryopreservation. The stored gametes can later be used for in vitro fertilisation or artificial insemination. Plants can be multiplied in large numbers from small pieces of tissue by tissue culture. Seeds of crops, their wild relatives and many other plants are dried and frozen in seed banks. India's National Genebank, at the National Bureau of Plant Genetic Resources in New Delhi, holds more than four lakh seed samples. The Svalbard Global Seed Vault in Arctic Norway keeps back-up copies of collections from around the world.

Ex situ methods buy time, but they cannot replace habitat. A species kept only in zoos and freezers slowly loses genetic diversity and the behaviour it needs in the wild. The aim is always to return it to a safe, protected habitat, as India attempted when it brought cheetahs from Namibia and South Africa to Kuno National Park in 2022–23.

Working together across borders

Migrating birds, rivers and ocean fish cross national borders, so no country can conserve its biodiversity alone (the timeline). The Convention on Biological Diversity (CBD) was opened for signature at the UN 'Earth Summit' in Rio de Janeiro in 1992, and almost every country has since joined it. It commits nations to conserve biodiversity, use it sustainably and share its benefits fairly. Ten years later, at the 2002 World Summit on Sustainable Development in Johannesburg, 190 countries set themselves a deadline: to slow the loss of biodiversity substantially by 2010. That target, and the Aichi targets set for 2020, were largely missed. In 2022 the Kunming–Montreal Global Biodiversity Framework set new goals for 2030, including protecting 30 % of the world's land and seas.