Chapter 9 — Our Environment

Look at a small pond near your school. In it live fish, frogs, snails, insects, floating plants and countless microscopic organisms. Around them is water, mud, dissolved oxygen, sunlight and a certain temperature. None of these organisms can be understood alone; the fish depends on the plants, the plants depend on the sunlight and minerals in the mud, and the mud is enriched by the wastes of the fish. This whole working unit of living things and their non-living surroundings, exchanging energy and materials, is called an ecosystem. The word was given by the British ecologist A. G. Tansley in 1935.

An ecosystem has two kinds of components. The biotic components are all the living organisms. They are grouped by how they get their food: producers (green plants, algae and some bacteria that make food by photosynthesis), consumers (animals that eat plants or other animals) and decomposers (bacteria and fungi that break down dead bodies and wastes into simple substances). The abiotic components are the non-living factors: sunlight, temperature, water, air, soil, minerals and the climate of the place.

Ecosystems can be very small or very large. A drop of pond water with its bacteria, a rotting log with its fungi and beetles, a field of paddy, a mangrove forest at the mouth of the Godavari, a lake like Kolleru, the Eastern Ghats forest, or the whole ocean are all ecosystems. Ecologists group them as natural ecosystems (forest, grassland, desert, pond, river, ocean) and artificial or man-made ecosystems (crop field, garden, aquarium, fish tank). Natural ecosystems maintain themselves; artificial ones need human care, such as watering, weeding and adding manure.

The important idea is interdependence. Remove the frogs from the pond and insects multiply; remove the plants and oxygen falls and fish die. Change an abiotic factor, for example by letting sewage into the pond so that oxygen drops, and the whole community changes. Every ecosystem, however large, has the same basic structure: a source of energy (the Sun), producers that capture it, consumers that pass it on, decomposers that return the materials to the soil and water, and the abiotic medium in which all this takes place. The rest of this chapter follows the energy and the materials through this structure.

Organisms in an ecosystem are placed in groups according to how they obtain their food. This grouping is the foundation of food chains and pyramids, so it must be understood clearly.

Producers are autotrophs: organisms that make their own organic food from carbon dioxide and water using light energy. Green plants, algae, phytoplankton in oceans and lakes, and blue-green bacteria are producers. They are the only living things that can bring the Sun's energy into the living world. All other organisms live, directly or indirectly, on the food that producers make. In the sea, microscopic phytoplankton do this work; on land it is done by grasses, crops, shrubs and trees.

Consumers are heterotrophs: they cannot make food and must eat other organisms. They are of several orders. Primary consumers or herbivores eat plants directly: grasshoppers, deer, rabbits, cattle, goats, caterpillars and zooplankton. Secondary consumers are carnivores that eat herbivores: frogs eating insects, small fish eating zooplankton, a cat eating a rat. Tertiary consumers eat secondary consumers: a snake eating a frog, a large fish eating a small fish. The animal at the end of a chain, with no natural predator, is the top carnivore, such as a lion, tiger, eagle or shark. Some animals eat both plants and animals and are called omnivores; humans, crows, bears and pigs are examples, and they can occupy more than one level.

Decomposers are bacteria and fungi that feed on dead plants, dead animals and the wastes of living organisms. They secrete enzymes that break complex organic matter into simple inorganic substances such as carbon dioxide, water, nitrates and phosphates, which return to the soil, water and air and are used again by producers. Without decomposers, dead bodies and dung would pile up and the minerals locked in them would never come back to the plants. Decomposers therefore close the cycle of materials. Animals that eat dead matter in large pieces, such as vultures, earthworms and dung beetles, are called scavengers or detritivores and they help the decomposers by breaking the material up first.

Note the difference between energy and matter here. Energy enters as sunlight, flows one way through producers and consumers, and finally leaves as heat. Matter, on the other hand, moves in a circle: from soil and air to producers, to consumers, to decomposers and back to soil and air. Both processes depend on all three groups being present.

A food chain is the sequence of organisms through which food energy passes in an ecosystem, each organism eating the one before it and being eaten by the one after it. The arrow in a food chain always points in the direction in which the energy moves: from the eaten to the eater. So we write grass → deer → tiger, never the other way. A food chain always begins with a producer, because that is where the energy of the Sun first enters living matter, and it ends with a top carnivore or with the decomposers that break everyone down.

Each step of the chain is a trophic level (trophe means food in Greek). Producers are the first trophic level (T₁), herbivores the second (T₂), carnivores eating herbivores the third (T₃), and so on. Food chains are usually short, having three, four or at most five links. The reason is energy: at each step about ninety per cent of the energy is lost, so after four or five steps there is too little energy left to support another level. This is studied in the section on the ten per cent law.

Ecologists recognise two main types of food chain. The grazing food chain starts with living green plants and goes to grazing herbivores and then carnivores; it is the type we usually draw. The detritus food chain starts with dead organic matter (detritus): fallen leaves, dead animals and dung. Here the first consumers are detritivores such as earthworms, termites, millipedes and fungi, followed by the animals that eat them. In a forest most of the energy in fact flows through the detritus chain, because most leaves fall and decay rather than being eaten alive.

Food chains are useful because they let us see who depends on whom. If pesticide sprayed on a crop kills the grasshoppers, the frogs that eat them starve, and the snakes that eat frogs move away or die. The chain also tells us how a substance eaten at the bottom can reach the top, which is the basis of biological magnification.

A few points to remember while drawing a food chain in the examination: begin with a named producer, not with 'plants' in general; keep the arrows pointing towards the consumer; label the trophic level of each organism; and choose organisms that really live in the same ecosystem. A food chain with grass, a fish and a tiger is wrong because they never meet.

In nature no animal eats only one kind of food and no organism is eaten by only one kind of predator. A grasshopper in a field is eaten by frogs, lizards, sparrows and spiders; a frog eats grasshoppers, moths, beetles and worms; a snake eats frogs, rats, lizards and eggs. When we draw all these feeding relationships together, the simple straight food chains become interconnected, and the network they form is called a food web. A food web is therefore a set of many food chains linked at shared organisms.

Consider a grassland. Grass is eaten by grasshoppers, rabbits, deer and mice. Grasshoppers are eaten by frogs, lizards and birds. Rabbits and mice are eaten by snakes, owls and foxes. Frogs are eaten by snakes and birds. Snakes are eaten by eagles and peacocks. If you draw arrows for each of these you get a web in which most animals are connected to several others. One organism may occupy different trophic levels in different chains: the snake is at T₃ when it eats a rabbit (grass → rabbit → snake) but at T₄ when it eats a frog (grass → grasshopper → frog → snake).

Food webs make ecosystems stable. If one food source becomes scarce, a consumer can switch to another and survive. Suppose a disease kills most of the rabbits in the grassland; the fox can still eat mice and birds, so the fox population does not crash, and the pressure on the remaining rabbits is reduced, letting them recover. In a simple chain the same disease would starve the fox. A rich web, with many species and many links, is a sign of a healthy ecosystem; a poor web with few species, such as a field of a single crop, is fragile and easily destroyed by one pest or one drought.

Food webs also show why removing one species can have effects far away in the web. When farmers killed sparrows in large numbers in China in 1958, believing they ate grain, the insects that sparrows had been eating multiplied and destroyed far more grain than the birds ever had. Similarly, the poisoning of vultures in India by the veterinary drug diclofenac led to a rise in stray dogs and rats feeding on carcasses, with a rise in rabies. These are consequences of breaking links in a web.

In the examination you may be asked to draw a food web from a list of organisms or to pick out food chains from a given web. Draw each organism once, connect it with arrows to everything it eats and everything that eats it, and then trace individual chains by following arrows from a producer to a top consumer.

All the energy used by life on Earth comes from the Sun. Of the sunlight that falls on a green plant, less than one per cent is captured by chlorophyll and converted into the chemical energy of glucose by photosynthesis. The rest is reflected, passes through, or warms the leaf. This captured energy is the plant's gross primary production. The plant itself uses a part of it in respiration to stay alive, grow and reproduce; what remains and is stored in its body as new tissue is the net primary production, and only this is available to the herbivores.

When a herbivore eats the plant, again only a fraction of the energy in the plant reaches the next level. Some of the plant is never eaten (roots, bark, thorns); part of what is eaten is not digested and leaves as dung; and most of what is absorbed is spent by the animal in respiration, movement and keeping warm, and is released as heat. Only the energy built into the animal's own body is available to the carnivore that eats it. The same losses repeat at every level.

This gives energy flow two important characteristics. First, it is unidirectional: energy moves from the Sun to producers to consumers and is finally lost as heat to the surroundings; it never goes back from a consumer to a plant or from heat to sunlight. Second, it decreases at every level. An ecosystem therefore needs a continuous supply of solar energy; a pond kept in the dark would die within weeks as its stored energy ran out. This is the difference between energy and nutrients: nutrients like carbon, nitrogen and phosphorus cycle round and round, but energy passes through once.

Energy flow obeys the laws of thermodynamics. The first law says energy is neither created nor destroyed but only changed in form: light becomes chemical energy in glucose, then mechanical energy in muscle, then heat. The second law says that at every change some energy is degraded to heat that cannot be used again; this is why each trophic level receives less than the one below.

An understanding of energy flow explains many everyday facts: why food chains are short, why there are far fewer tigers than deer and far fewer deer than plants, why a vegetarian diet can feed more people from the same land than a meat diet, and why the total mass of producers on Earth is far greater than that of all animals together.

The American ecologist Raymond Lindeman studied a lake in 1942 and measured how much energy passed from one trophic level to the next. He found that, as a rough rule, only about ten per cent of the energy available at one trophic level is transferred to the next; the remaining ninety per cent is used up in the life processes of that level or lost as heat and in undigested waste. This is the ten per cent law, and it is one of the most frequently examined ideas of this chapter.

Let us apply it. Suppose the plants of a grassland store 10,000 joules of energy from the Sun in their bodies. The grasshoppers that eat the grass will build about 1,000 J into their own bodies. The frogs that eat the grasshoppers will get about 100 J, the snakes that eat the frogs about 10 J, and an eagle that eats the snakes only about 1 J. Notice that after four transfers the energy has fallen from 10,000 to 1, that is to one ten-thousandth of the original. This is why food chains cannot go on for more than four or five links: there is simply not enough energy left to keep a sixth level alive.

The law is also used in reverse. If we know how much energy a top consumer needs, we can work out how much the plants must produce. A tiger needing 10 J of stored energy must eat deer containing 100 J, which must have eaten plants containing 1,000 J. This calculation shows why a tiger needs a large territory of forest: a huge quantity of vegetation is required to support even one tiger.

The ten per cent figure is an average, not an exact constant. Transfer may be as low as two per cent for some tough grasses eaten by cattle and as high as twenty per cent for a fish eating soft plankton. But in Class 10 problems, use ten per cent unless the question gives another figure. The method is always the same: divide by ten for each step up the chain, and multiply by ten for each step down.

The human lesson of the law is important for food policy. Grain fed to cattle loses ninety per cent of its energy before it reaches us as meat. In a country with many people and limited land, eating lower on the food chain, that is more plant food, supports more people. The same principle explains why fish-eating birds are rare compared to the fish, and fish rarer than the plankton.

The British ecologist Charles Elton in 1927 suggested drawing the trophic levels of an ecosystem one above the other, with producers at the bottom and the top carnivore at the top, and making the width of each bar proportional to some measurement of that level. Such a figure is an ecological pyramid (also called an Eltonian pyramid). Three measurements are used, giving three kinds of pyramid: the pyramid of number, the pyramid of biomass and the pyramid of energy. Each level of the pyramid is drawn as a horizontal bar, and the pyramid has as many bars as the chain has trophic levels.

The pyramid of number shows the number of individual organisms at each trophic level in a given area at a given time. In a grassland, a hectare may hold millions of grass plants, tens of thousands of grasshoppers, a few hundred frogs, a few dozen snakes and one or two eagles. The bars get narrower as we go up, giving an upright pyramid. This is the usual shape, and it follows directly from the ten per cent law: with less energy at each level, fewer animals can be supported.

But number can give a misleading picture because it ignores the size of the organisms. In a forest, one big tree supports thousands of insects, which support hundreds of insect-eating birds, which support a few hawks. Here the first level (a few trees) is narrower than the second (thousands of insects), so the pyramid bulges in the middle: it is spindle-shaped. In a parasitic chain, one tree supports many herbivorous birds, each bird carries many lice and each louse carries still more bacteria; the numbers increase at each level and the pyramid is inverted, narrow at the bottom and widest at the top.

To draw a pyramid of number, first list the organisms of the chain with their trophic levels, then count or estimate the number at each level, then draw bars whose widths are in the same ratio, one above the other, labelling each bar with the organism, the trophic level and the number. In the examination, describe the shape (upright, inverted or spindle) and give the reason.

The pyramid of number is easy to prepare, because counting is simpler than weighing or measuring energy, and it is a good first indication of the structure of the community. Its weakness is that it treats a tree and a blade of grass as one unit each, and so it can be upright, inverted or spindle-shaped for the same kind of ecosystem depending on which producers are chosen.

Because counting individuals ignores their size, ecologists prefer to compare trophic levels by biomass, the total dry mass of living matter at each level in a given area, measured in grams or kilograms per square metre. A pyramid of biomass shows this. In a grassland or a forest the biomass of producers is the greatest, herbivores have less, and carnivores still less, so the pyramid is upright. For a forest this corrects the misleading spindle shape of the pyramid of number: fifty large trees weigh far more than fifty thousand caterpillars.

In aquatic ecosystems, however, the pyramid of biomass is usually inverted. The producers of a pond or the sea are tiny phytoplankton that live only a few days but reproduce extremely fast. At any moment their standing mass is less than the mass of the zooplankton and fish that feed on them, because the fish live for years and accumulate mass while the plankton is consumed almost as soon as it is produced. The narrow base and wider upper levels give an inverted pyramid. The plankton supports the fish not by being heavy but by growing back quickly; over a whole year, the total mass produced by the plankton is much greater than that of the fish.

The most reliable picture is the pyramid of energy, which shows the amount of energy passing through each trophic level per unit area per unit time, in kilojoules per square metre per year. Because energy is lost at every transfer, the pyramid of energy is always upright; it can never be inverted for any ecosystem, whether on land or in water. It directly illustrates the ten per cent law: if producers fix 10,000 kJ, herbivores contain 1,000 kJ, carnivores 100 kJ and top carnivores 10 kJ. The pyramid of energy also takes account of the rate of production and so correctly represents the pond, where the plankton though small in mass passes a great deal of energy up the chain.

Compare the three pyramids. The pyramid of number is easiest to make but can be upright, inverted or spindle-shaped. The pyramid of biomass is better, upright on land and inverted in water. The pyramid of energy is the best measure of the working of an ecosystem and is always upright. In an examination answer, always state the shape, the ecosystem and the reason for the shape.

The pyramids teach one lesson in three ways: the amount of life that an ecosystem can support falls sharply with each step up the chain, and so the higher an animal feeds, the rarer and more vulnerable it is.

Energy flows through an ecosystem once and is lost, but the materials that make up living bodies are used again and again. Carbon, oxygen, hydrogen, nitrogen, phosphorus, sulphur and the minerals are taken up from air, water and soil by producers, passed to consumers, and returned to the environment when organisms die or excrete. The return is the work of the decomposers, chiefly bacteria and fungi, and the whole circular movement is called a biogeochemical cycle (bio for life, geo for earth, chemical for the substances).

Decomposition happens in stages. First, detritivores such as earthworms, termites, woodlice and millipedes break dead leaves and bodies into small fragments, a step called fragmentation, and rain washes out soluble salts, called leaching. Then fungi and bacteria secrete digestive enzymes on to the fragments and break the proteins, fats and carbohydrates into simple molecules, a step called catabolism. Part of the material is turned into dark, spongy humus, which holds water and slowly releases nutrients, and finally the humus is broken down to release minerals such as nitrates, phosphates and sulphates into the soil, a step called mineralisation. Plants absorb these minerals and the cycle begins again.

Take the carbon cycle as an example. Plants take carbon dioxide from the air in photosynthesis and build it into sugars. Animals eat the plants; both plants and animals give back carbon dioxide in respiration. When they die, decomposers release the remaining carbon as carbon dioxide. Some carbon is buried and becomes coal and petroleum, and returns only when these are burnt. In the nitrogen cycle, bacteria in the soil and in the root nodules of pulses fix atmospheric nitrogen into ammonia and nitrates, plants use them to make protein, animals eat the protein, and decomposers and denitrifying bacteria return the nitrogen to the soil and air.

The rate of decomposition depends on temperature and moisture. It is fast in a warm, wet monsoon forest and slow in a cold or dry place; this is why leaf litter piles up on a Himalayan slope but vanishes quickly in a Kerala forest. Farmers use decomposition deliberately when they make compost or vermicompost from crop waste and cow dung; the finished manure is humus rich in nutrients.

Substances that decomposers can break down, such as paper, cloth, food waste and wood, are biodegradable. Substances they cannot break down, such as plastics, glass, aluminium foil and many pesticides, are non-biodegradable; they remain in the environment for hundreds of years and can enter food chains. This distinction matters when we consider pollution and biological magnification in the next sections.

Some chemicals released by humans into the environment are non-biodegradable and also fat-soluble. Once an organism takes them in, they are not excreted but are stored in its fatty tissues. When that organism is eaten, the predator receives all the chemical stored in every prey animal it eats over its lifetime, and stores that too. So the concentration of the chemical rises at each higher trophic level, and the top carnivore ends up with a dose thousands or even millions of times greater than that in the water or soil. This progressive increase in the concentration of a harmful substance along a food chain is called biological magnification or biomagnification.

The classic example is DDT (dichloro-diphenyl-trichloroethane), an insecticide widely used from the 1940s to kill mosquitoes and crop pests. In a lake in the United States, DDT sprayed on surrounding land washed into the water at a concentration of about 0.003 parts per million (ppm). Plankton took it up and held 0.04 ppm. Small fish eating plankton had 0.5 ppm, large fish 2 ppm, and fish-eating birds such as grebes and pelicans about 25 ppm, an increase of nearly ten thousand times. The birds did not die directly, but DDT interfered with their calcium metabolism so that they laid eggs with very thin shells, which broke under the sitting mother. Populations of the bald eagle, peregrine falcon and brown pelican collapsed.

Biomagnification needs three conditions: the substance must be long-lasting (persistent), must be stored in the body rather than excreted, and must be passed on when the organism is eaten. Besides DDT, the same happens with other chlorinated pesticides such as BHC and aldrin, with heavy metals such as mercury, lead and cadmium, and with industrial chemicals such as PCBs. At Minamata in Japan, mercury discharged by a factory into the bay was magnified through fish, and people who ate the fish suffered nerve damage, paralysis and death, the disease now called Minamata disease.

Humans are at the top of many food chains and therefore at the greatest risk. Traces of DDT and other pesticides have been found in the fat, blood and even breast milk of people in India. Because the chemicals cannot be removed by washing or cooking once they are inside the animal's tissue, the only real protection is to prevent their entry into the environment. The lesson from these cases led India to ban DDT for agriculture and to restrict many other persistent pesticides.

The dangers of persistent pesticides were first brought to public attention by the American marine biologist Rachel Carson in her book Silent Spring, published in 1962. The title imagines a spring season in which no birds sing because pesticides have killed them all. Carson collected reports from across the United States: robins dying on lawns sprayed against Dutch elm disease, fish floating dead in rivers after aerial spraying of forests, eagles and falcons failing to breed. She traced each case to chlorinated hydrocarbon pesticides such as DDT, which had been hailed as miracle chemicals and sprayed freely on fields, marshes, towns and even on people.

Carson explained the biology behind the deaths. DDT does not break down; it is carried by wind and water far from where it is sprayed; it dissolves in fat and builds up in animal bodies; and it becomes more concentrated at each step of the food chain. She showed that spraying to kill one pest often killed its natural enemies as well, so that a worse outbreak followed, and that insects rapidly developed resistance, forcing ever larger doses. She did not ask for a total ban on pesticides but for their careful, limited use and for the study of biological control, that is using a pest's own predators, parasites and diseases against it.

The chemical industry attacked the book, but public opinion and scientific review supported Carson. The United States banned DDT for agricultural use in 1972 and several other countries followed. The book is regarded as the starting point of the modern environmental movement and led to laws on pesticide testing and to the creation of environmental protection agencies in many countries.

India learnt similar lessons. Heavy use of DDT and BHC in cotton and paddy fields caused pest resistance and residues in food, milk and water. India restricted DDT to public-health use against malaria mosquitoes and banned several organochlorine pesticides. Farmers in Andhra Pradesh, especially in cotton-growing districts such as Guntur and Warangal, suffered from pest resistance, crop failure and pesticide poisoning in the 1990s, and this experience pushed the state towards integrated pest management: pheromone traps, neem-based sprays, trap crops, encouraging predatory birds and insects, and spraying chemicals only when the pest count crosses a threshold. The village of Punukula in Khammam district became known for giving up chemical pesticides entirely.

The message of Silent Spring is the message of this chapter: the parts of an ecosystem are connected, a poison put in at one point spreads through the web, and the damage often appears far from its cause and long after it.

Humans are part of every ecosystem, but our activities are on such a scale that we can change or destroy an ecosystem within a few years. The story of Kolleru lake, between the Krishna and Godavari deltas in Andhra Pradesh, shows how this happens. Kolleru is one of the largest freshwater lakes in India. It receives water from the Budameru and Tammileru streams and from over sixty drains and channels, acts as a natural flood-balancing reservoir for the two river deltas, and drains into the Bay of Bengal through the Upputeru. For centuries it was a rich ecosystem: reeds and water plants, fish and prawns, and an enormous population of birds. Every winter tens of thousands of migratory birds, including pelicans, painted storks, flamingos, teals and pintails, arrived from as far as Siberia to feed and nest. In 1999 the lake was declared a wildlife sanctuary and in 2002 a wetland of international importance under the Ramsar Convention.

From the 1970s the lake began to shrink. Farmers extended paddy fields into the shallow margins. Then, as fish farming became profitable, thousands of fish and prawn tanks were dug inside the lake bed by enclosing parts of it with bunds. By the 1990s more than half the lake was under aquaculture ponds. The sewage of Eluru, Vijayawada and Gudivada, pesticide and fertiliser run-off from surrounding fields, and effluents from factories flowed into the remaining water. Water hyacinth spread across the surface.

The effects followed the logic of this chapter. Bunds blocked the free flow of water, so floods in the deltas worsened. Excess nutrients from sewage, fish feed and fertiliser caused dense growth of algae and hyacinth; when they died and decomposed, oxygen in the water was used up and fish died, a process called eutrophication. Pesticides entered the food chain and reached the fish-eating birds. The number of migratory birds fell sharply, the natural fishery on which thousands of traditional fishermen depended collapsed, and drinking-water wells around the lake became polluted.

In 2006, following orders of the courts, the state demolished most of the fish tanks inside the sanctuary limits, an operation known as Operation Kolleru. Birds began to return within a few seasons, showing that an ecosystem can recover if the pressure is removed. But conflict remains between the livelihood of the people who farm and fish there and the protection of the lake, and the case is still argued over the boundary of the sanctuary. Kolleru teaches that a wetland is not waste land: it controls floods, recharges ground water, supports fisheries and gives shelter to wildlife, and that these services are lost when the wetland is converted for short-term gain.

Kolleru is one case among many. Across Andhra Pradesh and the country, the same pattern is repeated in different ecosystems. Deforestation, the clearing of forests for farming, timber, mining, dams and roads, removes the producers of the forest ecosystem at one stroke. With the trees go the animals that lived in them; the soil, no longer held by roots, is washed away by rain; streams that once flowed all year become seasonal because the forest floor no longer soaks up the monsoon; and the carbon stored in the wood goes into the air. The Eastern Ghats forests of Visakhapatnam and Srikakulam districts have thinned from shifting cultivation, bauxite mining and firewood cutting, and the villages on their slopes suffer more landslides and drier summers as a result.

Pollution introduces harmful substances into air, water and soil. Untreated sewage and industrial effluents in the Musi, Krishna and Godavari kill fish and make water unfit to drink. Smoke and gases from vehicles and factories cause acid rain that damages leaves and acidifies lakes. Plastic waste blocks drains, chokes cattle and marine turtles, and breaks into microplastics that enter food chains. Fertiliser run-off causes eutrophication of tanks and coastal waters.

Over-exploitation is taking more from an ecosystem than it can renew. Fishing with fine-meshed nets during the breeding season removes young fish before they reproduce, so catches fall year after year. Over-grazing by too many goats and cattle turns grassland into bare earth. Pumping ground water faster than rain refills it dries wells and lets sea water seep into coastal aquifers, as has happened along the coast near Nellore and Prakasam.

Introducing alien species also upsets ecosystems. Water hyacinth, brought from South America as an ornamental plant, has no natural enemies here and covers lakes and canals. Parthenium (congress grass) and lantana spread over pastures and forest floors, crowding out native plants and the animals that ate them. African catfish released into ponds eat the native fish.

The combined result of these activities is the loss of biodiversity: species become rare, then endangered, then extinct. The Great Indian Bustard, once common on the grasslands of Rayalaseema and the Deccan, now survives in a few hundred individuals; the Jerdon's courser of Kadapa district is one of the rarest birds in the world. When a species disappears, the links it held in the food web are broken, and the ecosystem becomes simpler and less stable. Once a species is extinct it can never return, so protection must come before, not after, the loss.

Ecosystems give us far more than we usually count. The goods and benefits that we receive free from nature are called ecosystem services. They include the food, fibre, timber, fuel and medicines we take from plants and animals; the pollination of crops by bees, butterflies, birds and bats, without which mango, cotton, sunflower and most vegetables would not set fruit; the purification of water by wetlands and soil; the control of floods by forests and lakes; the formation and holding of soil by roots and earthworms; the regulation of climate by forests and oceans absorbing carbon dioxide; the natural control of pests by predators; and the beauty, recreation and cultural value of rivers, hills and forests. Economists have tried to put a price on these services and find that they are worth many times the world's entire economy. Yet because they are free, they are usually ignored until they are lost.

Communities that have lived within forests for generations understand these services in practice. The tribal peoples of the Eastern Ghats in Andhra Pradesh, such as the Chenchus of the Nallamala forest, the Koyas and Kondareddis of the Godavari valley, and the Savaras and Jatapus of Srikakulam, take from the forest only what they need: honey, gum, tubers, fruit, bamboo, medicinal plants, small game. Their customs often protect nature. They do not cut fruit-bearing trees; they leave part of a honeycomb so the bees return; they do not fish in the breeding season; and certain groves, hills and pools are held sacred and left untouched, which makes them refuges for rare plants and animals. Sacred groves of this kind exist across India and are among the last patches of original forest in many districts.

The Chenchus hunt with bows and gather in the forest without destroying it, and the Nagarjunasagar-Srisailam Tiger Reserve in which they live still holds tigers largely because the Chenchu way of life left the forest intact. Their knowledge of hundreds of plants and their uses is itself an ecosystem service, a library that could be lost if the community is displaced.

The lesson is not that we should all return to the forest, but that the principle of taking within limits, which these communities practise, is the principle of a sustainable ecosystem. Modern conservation increasingly involves such communities as partners: Vana Samrakshana Samithis (joint forest management committees) in Andhra Pradesh give villages a share in protecting and benefiting from the forest, and tiger reserves employ tribal people as guides and guards. When people who live in an ecosystem are given responsibility for it, the ecosystem is usually better protected.

Since human activity is the chief cause of damage to ecosystems, human action can also reduce it. Protection works at three levels: the individual, the community and the government.

As individuals, we can reduce what we take and what we throw away. Refuse single-use plastic bags and bottles and carry a cloth bag; reuse containers; separate kitchen waste for composting and dry waste for recycling; save water and electricity, which reduces the demand on rivers and coal-fired power plants; walk, cycle or use buses; plant and care for native trees; and avoid buying products made from wild animals. When buying vegetables, prefer those grown with fewer pesticides. Never release aquarium fish or plants into natural water bodies. Each of these is small, but the environment is the sum of millions of small actions.

As a community, a village or school can protect its local tank, plant trees on the bund, stop sewage entering it, remove water hyacinth, and prevent dumping of garbage. Farmers' groups can adopt integrated pest management, organic manure and crop rotation, reducing the chemicals that enter food chains. Fishermen's cooperatives can enforce a closed season during breeding. The Vana Samrakshana Samithis mentioned earlier protect village forests, and Eco-clubs in schools run awareness drives and clean-up campaigns.

The government makes and enforces laws. India has the Wildlife (Protection) Act 1972, the Water (Prevention and Control of Pollution) Act 1974, the Forest (Conservation) Act 1980, the Air Act 1981, the Environment (Protection) Act 1986 and the Biological Diversity Act 2002. National parks, wildlife sanctuaries, biosphere reserves and tiger reserves protect whole ecosystems; in Andhra Pradesh these include the Nagarjunasagar-Srisailam Tiger Reserve, Papikonda National Park, the Coringa mangrove sanctuary, Kolleru and Pulicat bird sanctuaries and the Sri Venkateswara National Park. Pollution control boards set limits for effluents. International agreements such as the Ramsar Convention on wetlands and the Convention on Biological Diversity commit countries to protect ecosystems together.

Above all, protection needs knowledge. A person who understands that the frog in the field eats the pests, that the wetland holds the flood, that the pesticide sprayed today will be in the milk next year, will act differently from one who does not. That is the purpose of this chapter. The environment is not something outside us; we are one of the consumers in its food web, we breathe the oxygen its producers make and drink the water its wetlands clean, and its health is our health.