Biodiversity and Its Conservation

The word biodiversity was popularised by the sociobiologist Edward Wilson. It is a compact way of saying the total variety of living things at every level of biological organisation, from the genes carried inside a single species up to the different kinds of ecosystems spread across a region. A complete definition does not stop at the number of species present. Biodiversity is described at three levels, and a full answer names all three and attaches an example to each.

Genetic diversity is the variation in genes found within one species. A species spread over a wide geographical range often shows a great deal of genetic difference from one population to another, and in medicinal plants this difference shows up as a difference in the chemicals the plant makes. The standard example is the medicinal plant Rauwolfia vomitoria, which grows across different Himalayan ranges. Plants from different ranges differ in the potency and the concentration of the active chemical reserpine that they produce, even though they all belong to the same species. India shows this kind of within-species variation on an enormous scale in its crops, with more than 50,000 genetically different strains of rice and roughly 1,000 varieties of mango. Genetic diversity is the raw material that natural selection acts on, so a species with a broad genetic base can cope with a new disease or a shifting climate, while a genetically uniform crop planted across a whole district can be destroyed by a single pathogen.

Species diversity is the variety of species present in a community or a region. Counting species is only part of it, because two forests with the same species count can still differ: one may be overwhelmingly dominated by a single species while the other spreads its individuals more evenly across many species. The example used for India compares two hill ranges, and the direction matters. The Western Ghats have a greater diversity of amphibian species than the Eastern Ghats. Reversing the two ranges is a common error.

Ecological diversity, also called ecosystem diversity, is the variety of ecosystem types found in an area. India contains deserts, rain forests, mangroves, coral reefs, wetlands, estuaries and alpine meadows inside a single country, so its ecosystem diversity is far greater than that of a Scandinavian country such as Norway, even though Norway has plenty of undisturbed natural habitat of its own. The three levels nest inside one another in a way worth fixing in your mind: genes sit inside a species, species sit inside an ecosystem, and ecosystems sit inside a landscape.

How many species share the planet with us? Slightly more than 1.5 million species have been described and formally named so far. Because huge stretches of the tropics have never been surveyed properly, the true figure must be much larger. Robert May arrived at an estimate statistically. The approach is to take a group such as insects that has been studied exhaustively, compare how many species it contains in temperate regions with how many it contains in the tropics, and then apply that ratio to the other groups of animals and plants that have not been surveyed nearly as thoroughly. Extrapolating in this way, May places the global species diversity at about 7 million species. Of the species already on record, more than 70 per cent are animals, while plants in the broad sense used here, which includes algae, fungi, bryophytes, gymnosperms and angiosperms, make up no more than 22 per cent. Among the animals, insects are overwhelmingly the most species-rich group, making up more than 70 per cent of all animal species. Put in everyday terms, out of every ten animals on this planet, seven are insects. The number of described fungal species by itself is greater than the combined total of fishes, amphibians, reptiles and mammals.

India occupies only about 2.4 per cent of the world land area, yet it holds about 8.1 per cent of the global species diversity, which is why it is counted among the mega diversity countries of the world. Around 45,000 species of plants and about twice as many species of animals have been recorded from India so far. If May's global estimate is applied to India, then more than 1,00,000 species of plants and more than 3,00,000 species of animals are probably still waiting to be discovered and described in this country, over and above everything already on record. The uncomfortable conclusion is that many Indian species could be driven to extinction before any biologist ever describes them.

Biodiversity is not scattered randomly over the earth. Two patterns are described in detail, and both come with numbers you should be able to reproduce from memory.

The first is the latitudinal gradient. Species diversity is at its maximum near the equator and decreases steadily as you move away from the equator towards either pole. The tropics, meaning the belt lying between 23.5 degrees North and 23.5 degrees South, harbour more species than temperate or polar regions of comparable size. Bird counts make the pattern impossible to miss. Colombia, which lies close to the equator, has nearly 1,400 species of birds. New York, at about 41 degrees North, has 105 species. Greenland, at about 71 degrees North, has only 56 species. India, with much of its land area lying in tropical latitudes, has more than 1,200 species of birds. The same gradient holds for plants: a forest in a tropical region such as Ecuador can contain up to ten times as many species of vascular plants as a temperate forest of equal area in the Midwest of the United States.

The richest single region on earth is the largely tropical Amazonian rain forest of South America. It contains more than 40,000 species of plants, 3,000 species of fishes, 1,300 species of birds, 427 species of mammals, 427 species of amphibians, 378 species of reptiles and more than 1,25,000 species of invertebrates. On top of that, scientists estimate that at least two million insect species living in these forests are still waiting to be discovered and named.

Why should the tropics be so much richer? Three explanations are offered, and none of them is treated as the single complete answer. The first is evolutionary time. Temperate regions were repeatedly scoured and frozen by glaciations in the geological past, wiping out species and resetting communities, whereas tropical latitudes remained relatively undisturbed for millions of years. Tropical lineages therefore had a far longer uninterrupted stretch of time in which to diversify. The second is environmental constancy. Tropical environments are less seasonal, more constant and more predictable than temperate ones. A predictable environment allows a species to specialise on a narrow niche instead of having to survive as a generalist through wild seasonal swings, and narrow niches let more species pack into the same space. The third is solar energy. More solar energy is available in the tropics, which raises productivity, and higher productivity may indirectly support greater diversity.

The second pattern is the species-area relationship, first noticed by the German naturalist and geographer Alexander von Humboldt during his exploration of the South American jungles. Within a region, the number of species recorded increases as the area explored increases, but the increase slows down and eventually levels off. Plotted on ordinary arithmetic axes, species richness against area produces a curve shaped like a rectangular hyperbola: it climbs steeply at first and then bends over towards a plateau. This same shape appears for a wide variety of taxa, including angiosperm plants, birds, bats and freshwater fishes.

If the same data are plotted on a logarithmic scale, the hyperbola straightens into a line, and the relationship is written as

log S = log C + Z log A

Define every symbol whenever you write this equation, because the line of algebra says nothing on its own. S is species richness, that is, the number of species recorded. A is the area surveyed. Z is the slope of the straight line, also called the regression coefficient. C is the Y-intercept, the value of log S at the point where log A equals zero. The same relationship written without logarithms is S = C multiplied by A raised to the power Z, which you can write as S = CA^Z.

The value of Z is the part of the relationship that carries the most biological information. For species-area studies carried out within a region, the slope turns out to be surprisingly consistent no matter which taxonomic group is studied and no matter where the study is done, whether it is plants in Britain, birds in California or molluscs in New York state. In all such cases Z lies in the range 0.1 to 0.2. But when the analysis is stretched across very large areas such as entire continents, the line becomes much steeper, with Z lying in the range 0.6 to 1.2. For fruit-eating, or frugivorous, birds and mammals compared across the tropical forests of different continents, the slope works out to 1.15.

What does the slope actually mean in plain language? Z measures how strongly species number responds to a change in area. A small Z means that enlarging the area adds only a few new species, because you are largely sampling more individuals from the same regional pool of species. A large Z means that enlarging the area adds a great many new species, because you have crossed into a region with a completely separate set of species that shares almost nothing with the first. That is precisely why comparisons made across continents give such steep slopes: different continents evolved their own distinct faunas and floras. The relationship also delivers a hard practical warning for conservation. Since the number of species an area can hold depends on that area, cutting a habitat down to a fraction of its original size must eventually cut down the number of species it can support.

It is one thing to say the earth is losing species and quite another to explain why that should worry anybody. The argument rests on the link between diversity and the stability of a community. Ecologists judge a community to be stable if it satisfies three conditions. It should not show too much variation in productivity from one year to the next. It should be either resistant to occasional disturbances, whether natural or caused by humans, or else resilient after them. And it should be resistant to invasion by alien species. Resistance and resilience are not the same idea, and the difference is worth holding on to: resistance is the ability of a community to remain more or less unchanged when a disturbance hits it, while resilience is the ability to return to its original condition after being knocked out of it.

The general observation is that communities with more species tend to be more stable than communities with fewer species. The experimental support comes from David Tilman, who ran long-term ecosystem experiments on outdoor field plots. His results showed two things. Plots containing more species displayed less year-to-year variation in total biomass, which is stability in the strict sense of the first condition above. In the same experiments, increased diversity also contributed to higher productivity. Note carefully that these are two separate findings. Higher diversity gave both steadier and larger yields: the year-to-year variation in biomass is the stability result, while the amount of biomass produced is the productivity result.

The reason more species buffer a community is not mysterious. Different species respond differently to the same bad year. A drought that ruins one grass species may barely touch a deep-rooted neighbour, so total biomass across the whole plot dips less than any one species would on its own. A community with a single species has nothing to fall back on.

The best-known argument for conserving species is the rivet popper hypothesis, put forward by the ecologist Paul Ehrlich. Imagine an ecosystem as an aeroplane, and imagine every species in that ecosystem as one rivet holding the parts of the aeroplane together. Now suppose every passenger who boards decides to pop out one rivet and carry it home as a souvenir. Each popped rivet stands for one species driven to extinction. Removing the first few rivets may not affect flight safety at all, and this is exactly what makes species loss so deceptive: the ecosystem appears to go on functioning normally. But as more and more rivets are pulled out, the aircraft becomes dangerously weak over time, and at some point it fails.

Ehrlich added a second point to the analogy that carries as much weight as the first. Which rivet is removed also matters. Rivets popped from the wings threaten flight safety far more seriously than rivets popped from the seats or the window frames inside the cabin. Translated into ecology, this says species are not interchangeable. Losing a key species, such as a top predator, a major primary producer or a pollinator on which many plants depend, damages the ecosystem out of all proportion to the single name struck off the list, while the loss of a rare species with few connections may pass almost unnoticed. Because we usually cannot tell in advance which species is a wing rivet, the safe policy is to conserve them all. Rich biodiversity, in short, is not a luxury; it is essential for the health of ecosystems and for the survival of the human race, which depends on those ecosystems for food, clean water and breathable air.

Extinction is not new. The fossil record shows five earlier episodes of mass extinction in which large fractions of the earth's species disappeared. What is happening now is called the sixth extinction, and it differs from the previous five in two crucial ways. Its rate is estimated to be 100 to 1,000 times faster than the rate in pre-human times, and human activity is what is driving it. Ecologists warn that if present trends continue, nearly half of all species on earth could be wiped out within the next hundred years.

The scale of recent losses is documented. The IUCN Red List records the extinction of 784 species over the last 500 years, made up of 338 vertebrates, 359 invertebrates and 87 plants, and 27 of those disappearances occurred within the last twenty years alone. The examples worth memorising are the dodo of Mauritius, the quagga of Africa, the thylacine of Australia, Steller's sea cow of Russia and three subspecies of tiger, the Bali, Javan and Caspian tigers. The colonisation of the tropical Pacific islands by humans is thought to have wiped out more than 2,000 species of native birds by itself. Among species still surviving, roughly 12 per cent of bird species, 23 per cent of mammal species, 32 per cent of amphibian species and 31 per cent of gymnosperm species currently face the threat of extinction. Amphibians, notice, are the worst hit of the animal groups in that list.

Losing biodiversity in a region carries measurable consequences: a decline in plant production, lowered resistance of the ecosystem to environmental disturbances such as drought, and increased variability in ecosystem processes including plant productivity, water use, and pest and disease cycles.

Four causes of biodiversity loss are named, and they are collectively called the Evil Quartet. Learn them in order with one example each, since it is very easy to file a real example under the wrong cause.

1. Habitat loss and fragmentation. This is the single most important cause of the extinction of animals and plants. The most dramatic case is the tropical rain forest, which once covered more than 14 per cent of the earth's land surface and today covers no more than 6 per cent. The Amazon rain forest, sometimes described as the lungs of the planet and home to probably millions of species, is being cut and cleared for growing soya beans and for conversion into grassland for raising beef cattle. Outright destruction is not the only mechanism: pollution degrades many habitats until they can no longer support the species that lived in them. Fragmentation is a separate and often underrated part of this cause. When a large continuous habitat is broken into small isolated patches by roads, fields and settlements, mammals and birds that need large territories, and animals with migratory habits, are hit hardest, and their populations decline even though the total remaining area may still look substantial on a map.

2. Over-exploitation. Humans have always depended on nature for food and shelter, but when need turns into greed, exploitation becomes over-exploitation. Many extinctions of the last 500 years, including Steller's sea cow and the passenger pigeon, were caused by human over-exploitation. The problem is very much alive today: many marine fish populations around the world are being over-harvested, which endangers the continued existence of several commercially important species.

3. Alien species invasions. When species from outside are introduced into an area, whether accidentally or deliberately, some of them turn invasive and cause the decline or extinction of the native species. The classic case is the Nile perch introduced into Lake Victoria in East Africa, which eventually caused the extinction of an ecologically unique assemblage of more than 200 species of cichlid fish in that lake. In India, the invasive weeds carrot grass or Parthenium, Lantana and water hyacinth or Eichhornia have caused environmental damage and pose a threat to native species. The African catfish Clarias gariepinus, brought in for aquaculture, is now a threat to indigenous catfishes in Indian rivers.

4. Co-extinctions. When one species becomes extinct, the plant and animal species that are associated with it in an obligatory way go extinct along with it. If a host fish species disappears, its unique assemblage of parasites, which can live nowhere else, meets the same fate. A second example is a coevolved plant-pollinator mutualism, where the plant depends entirely on one pollinator and the pollinator depends entirely on that plant: the extinction of one inevitably drags the other down with it. The word obligatory is the key to spotting a co-extinction. If the dependent species could survive by switching to another host or another food plant, the case is not a co-extinction.

The reasons for conserving biodiversity are grouped under three headings, and the three are not interchangeable. The real difficulty lies in deciding which heading a given benefit belongs under, so learn the boundary between the categories rather than just the examples.

Narrowly utilitarian arguments rest on the direct economic products that humans take from nature. These are the things you could put a price on and sell: food in the form of cereals, pulses and fruits; firewood; fibre; construction material; and industrial products such as tannins, lubricants, dyes, resins and perfumes. Medicines belong here too, and the numbers are striking. More than 25 per cent of the drugs currently sold worldwide are derived from plants, and about 25,000 plant species contribute to the traditional medicines used by indigenous peoples around the world. Almost none of the millions of species on earth have been examined for their medicinal or other economic potential, which is why nations are now taking bioprospecting seriously. Bioprospecting means exploring molecular, genetic and species-level diversity in search of products of economic value, and countries endowed with rich biodiversity, India among them, stand to gain enormously from it.

Broadly utilitarian arguments rest on the services that biodiversity provides to the biosphere as a whole, services that nobody pays for but everybody depends on. Photosynthesis by plants is the obvious one. The rapidly shrinking Amazon forest is estimated to produce roughly 20 per cent of the total oxygen in the earth's atmosphere through photosynthesis. Pollination is another service, and it is easy to overlook: without pollinators such as bees, bumblebees, birds and bats moving pollen between flowers, flowering plants cannot set fruits and seeds, and a large share of human food supply would collapse with them. Alongside these measurable services sit intangible benefits that are real even though they cannot be costed, such as the pleasure of walking through thick woods, of watching spring flowers in full bloom, or of waking up to a bulbul singing. The line to remember is this: if the benefit is a product you can harvest and sell, the argument is narrowly utilitarian; if the benefit is a service or a quality that the ecosystem supplies to everyone, the argument is broadly utilitarian.

The ethical argument has nothing to do with usefulness at all. It holds that every species has an intrinsic value of its own, quite apart from whether it is of any current or future economic use to human beings. We share this planet with millions of plant, animal and microbial species, and on this view we have a moral duty to care for their well-being and to hand over our biological legacy in good order to the generations that come after us. This is the argument that covers the obscure beetle nobody will ever make a medicine from, and it is the easiest of the three to overlook.

Conservation methods fall into two clearly separated groups, and the distinction underlies every method described below. In situ conservation means on site: the threatened species is protected in its own natural habitat, and because you cannot protect the species without protecting where it lives, the whole ecosystem is conserved along with it. Ex situ conservation means off site: the threatened plant or animal is taken out of its natural habitat and placed in a special setting where it can be given protection and special care. In situ saves the address along with the resident; ex situ rescues the resident and gives it a new address.

Because resources are limited and cannot be spread thinly over every hectare of the planet, ecologists have identified regions where in situ protection buys the most. These are the biodiversity hotspots. A hotspot is a region marked by two features together: very high species richness and a high degree of endemism, endemism meaning that the species there are confined to that region and found nowhere else in the world. Initially 25 hotspots were identified globally, and the number was later raised to 34. Three of them cover the biodiversity-rich regions of India: the Western Ghats and Sri Lanka, the Indo-Burma region, and the Himalaya. Although all the hotspots put together account for less than 2 per cent of the earth's land area, the number of species they collectively hold is extremely high, and strict protection of just these areas could cut the ongoing mass extinctions by almost 30 per cent. Both figures matter: a very small area doing a very large amount of work.

In India, ecologically unique and biodiversity-rich regions are legally protected as biosphere reserves, national parks and wildlife sanctuaries. All three are in situ methods. A biosphere reserve is the largest and most inclusive of the three, typically containing a strictly protected core along with surrounding zones where regulated human use is allowed. A national park gives strict legal protection to the habitat and its wildlife, and grazing, cultivation and private land holding are not permitted inside it. A wildlife sanctuary is usually aimed at protecting particular animal species and allows certain limited human activities that do not harm the wildlife.

Sacred groves are a distinctly Indian form of in situ conservation and one that predates any modern law. India has a long tradition of religious and cultural practices that emphasised the protection of nature, and in many communities tracts of forest were set aside as sacred, with every tree and every animal inside them venerated and given complete protection. Such sacred groves survive in the Khasi and Jaintia Hills of Meghalaya, in the Aravalli Hills of Rajasthan, in the Western Ghats regions of Karnataka and Maharashtra, and in the Sarguja, Chanda and Bastar areas of Madhya Pradesh. Some of these groves shelter undisturbed forest of a quality that has vanished from the surrounding landscape.

Ex situ conservation covers everything done outside the natural habitat. Zoological parks, botanical gardens and wildlife safari parks are the familiar examples, and several animals that are already extinct in the wild survive only because they continue to be maintained in zoological parks. In recent years ex situ conservation has moved well beyond simply keeping threatened species in enclosures. Gametes of threatened species can now be preserved in a viable and fertile condition for long periods by cryopreservation, that is, storage at extremely low temperature. Eggs can be fertilised in vitro, and plants can be multiplied from small pieces of tissue by tissue culture methods. Seed banks allow seeds of different genetic strains of commercially important plants to be stored for very long periods, which conserves genetic diversity and not merely the species itself. A quick test: if the organism, or its gametes, seeds or tissues, has been physically removed from where it naturally lives, the method is ex situ.

Conservation of biodiversity also has an international dimension. The historic Convention on Biological Diversity, signed at the Earth Summit held in Rio de Janeiro in 1992, called on all nations to take appropriate measures for the conservation of biodiversity and for the sustainable use of its benefits. Ten years later, at the World Summit on Sustainable Development held in Johannesburg, South Africa, in 2002, one hundred and ninety countries pledged to achieve by 2010 a significant reduction in the then current rate of biodiversity loss at global, regional and local levels.