Our Environment: Class 10 Science · Chapter 13

Every year on 5 June, World Environment Day, newspapers fill with photos of tree-planting drives and beach clean-ups. But what exactly is the environment? For any living thing it is everything around it: the other plants, animals and microbes, and the air, water, soil, light and temperature, all acting on one another.

In this chapter you will see how the living and non-living parts of nature fit together and how food and energy pass from one organism to the next. You will also study two human-made problems that upset this balance: a thinning ozone layer and growing heaps of garbage.

13.1 Eco-system — What are its Components?

Walk along the raised bund of a paddy field in July. Rice plants stand in ankle-deep water, snails cling to their stems, a frog sits on the bund and an egret stalks the shallows. Hidden in the mud, bacteria and fungi are breaking down dead leaves. Sunlight, rain, warm air and the soil set the conditions for all of them. Let the field dry out and almost every organism in it is affected.

Living things never live alone: they act on one another and on the non-living things around them. All the organisms of one place plus those surroundings, working as a single unit, make up an ecosystem (पारितंत्र). Its living part is the biotic component: plants, animals and microorganisms. Its non-living part is the abiotic component: the soil and its minerals, water, sunlight, wind and how hot or cold the place is (Fig. 13.1).

Ecosystems come in every size. A forest, a grassland, a pond, a lake or a stretch of sea runs on its own: these are natural ecosystems. A crop field, a park, a school garden or an aquarium is set up and looked after by people: these are human-made (artificial) ecosystems. The paddy field is human-made, because farmers choose the crop, flood the field and pull out weeds.

Producers, consumers and decomposers

In earlier classes you sorted organisms by how they get their food. Green plants, algae and some bacteria use the energy of sunlight, trapped by chlorophyll, to turn carbon dioxide and water into sugars, which they can store as starch. Every other organism lives on the food they make, so they are the producers (उत्पादक).

All other organisms are consumers (उपभोक्ता). They cannot make food, so they take it ready-made: a goat straight from the grass, a leopard second-hand by eating the goat. By diet they fall into four groups. Herbivores eat plants (a goat, a grasshopper) and carnivores eat other animals (a kingfisher, a leopard). Omnivores eat both, like a crow, a bear or most people. Parasites live on or in another living thing and feed from it, like a tapeworm or the leafless yellow amarbel (Cuscuta) that twines over hedges.

What happens to a fallen leaf or a dead beetle? Decomposers (अपघटक), mainly bacteria and fungi, pour enzymes onto dead remains and wastes, digest them from the outside and absorb the products. Large organic molecules end up as small inorganic ones: carbon dioxide, water and mineral salts. These seep into the soil and water, and roots take them up again (Fig. 13.2).

Without decomposers, dead bodies, fallen leaves and dung would pile up, and the minerals locked inside them would never return to the soil. Plants would slowly run short of nutrients, and so would every animal that depends on plants. Producers bring energy into an ecosystem; decomposers keep its materials going round.

13.1.1 Food Chains and Webs

List who eats whom in the paddy field and a sequence appears. Rice plants are eaten by rats, rats by cobras, and cobras by a crested serpent eagle. A sequence like this, in which each organism is eaten by the next, is a food chain (आहार श्रृंखला). Every food chain begins with a producer. A few more from around India:

• Pond: algae → water fleas → small fish → kingfisher
• Kitchen garden: cabbage → caterpillar → bulbul → shikra (a small hawk)
• Sea: tiny floating algae → krill → blue whale, a short chain that ends in the largest animal ever

Number the steps of a chain from the bottom and each number is a trophic level (पोषी स्तर). Level I always belongs to the producers (autotrophs), because every bit of food energy in the chain enters through them. Plant-eaters come next at level II; they are the primary consumers. An animal that eats a plant-eater is a secondary consumer (level III), and one that eats secondary consumers is a tertiary consumer (level IV). Table 13.4 sorts two chains this way.

How energy flows

Food is fuel. The energy stored in a chapati lets you walk, think and stay warm. So when one organism eats another, energy moves along the chain. It all starts with sunlight: producers change light energy into chemical energy stored in food. From them it passes to herbivores and carnivores and, whenever anything dies, to decomposers.

Every transfer is wasteful. As you have learnt, whenever energy changes from one form to another, part of it ends up as heat that cannot be used again. Measurements in real ecosystems show how big the losses are:

• Of all the sunlight that lands on a leaf, only about 1 % ends up stored in food. The rest is reflected, passes through or simply warms the leaf.
• Think of a goat grazing. Part of the grass's energy leaves undigested in its dung. Most of what the goat absorbs is burnt to keep it warm, walking and breathing, and escapes as heat. Only about 10 % ends up as new goat flesh, the part a leopard could eat.
• The same happens at every later step, so roughly a tenth of the energy at one level reaches the next. This rule of thumb is the 10 per cent law.

Look back at Example 13.1: the snakes get 5 kJ out of the 5000 kJ the grass stored. A fifth level would get half a kilojoule, far too little to keep a population of predators alive. So most food chains stop after three or four steps, and top carnivores such as tigers and eagles are rare and need huge territories.

The same losses explain numbers. A meadow holds millions of grass plants, fewer grasshoppers, fewer frogs still and only a handful of snakes. In most ecosystems the producers are the most numerous, and the number of individuals falls as you go up the trophic levels.

Put these losses together and you get the energy-flow picture in Fig. 13.5. It makes two key points. Energy moves in one direction only, like water down a staircase. Nothing a goat eats ever flows back into the grass, and nothing the grass traps goes back to the Sun. And the amount dwindles at each step, since every level spends most of what it gets and loses it as heat. Materials are different: decomposers hand carbon, nitrogen and minerals back to the soil, so materials go round in cycles while energy passes through once.

Real feeding relationships are rarely a single line. Grasshoppers are eaten by frogs and also by mynas; snakes eat frogs as well as rats; an eagle takes snakes, rats, hares and birds. Join all these chains and you get a criss-crossing network, a food web (आहार जाल) (Fig. 13.6). One organism can even sit at different levels: a snake eating a rat is a secondary consumer, but a snake eating a frog is a tertiary consumer.

Webs make an ecosystem steadier. If disease wipes out the rats in a field, snakes and eagles can switch to frogs, birds and hares. An animal that depends on a single food has no such back-up.

Harmful chemicals in the food chain

Food chains can carry poisons as well as energy. Farmers spray pesticides to kill insects, fungi and weeds, but much of the spray never touches a pest. It drips onto the ground or drifts in the wind, and the next rain carries it into the soil, ditches and village ponds. Roots soak it up with soil water, and algae and water plants absorb it straight from the pond. Once it is inside a producer, it is on the menu for everything above.

Many such chemicals, like the insecticide DDT, are hard to break down and dissolve in fat rather than in water. An animal can neither digest nor excrete them, so they stay in its body fat. A small fish eats thousands of plankton over its life and keeps the chemical from all of them; a heron eats hundreds of such fish. So the concentration rises at every step up the chain (Fig. 13.7). This build-up is biological magnification (जैव आवर्धन). We humans eat from the top of many chains (fish, meat, milk), so our own bodies can end up with some of the highest doses.

That is why food-testing laboratories regularly find traces of pesticides in cereals, vegetables, fruit, milk and fish. A rinse under the tap removes residue sitting on the skin of a tomato, but not the chemical the plant has already drawn into its tissues.

13.2 How do our Activities affect the Environment?

No other species changes its surroundings as much as we do. We clear forests, dam rivers, burn fuels and bury land under rubbish, and those changes come back to affect us. Two such problems get a close look here: the thinning of the ozone layer and the garbage we throw away. (Air, water and soil pollution were the topic of Class 9.)

13.2.1 Ozone Layer and How it is Getting Depleted

About 15–35 km above you, in the stratosphere, is a thin spread of a gas that keeps you alive: the ozone layer (ओज़ोन परत). It absorbs most of the Sun's ultraviolet (UV) radiation before it reaches the ground. The gas is ozone, a form of oxygen whose molecule has three atoms, $\mathrm{O}{\phantom{X}}_3$, instead of the two in the oxygen we breathe, $\mathrm{O}{\phantom{X}}_2$. Down at street level the same gas is a pollutant: it forms in city smog and harms the lungs.

UV radiation carries enough energy to damage DNA. In people it causes sunburn, skin cancer and cataract (clouding of the eye lens), and it weakens the body's defences. It also stunts crop plants and harms the floating algae at the base of ocean food chains. So damage to the ozone layer affects whole ecosystems, not just us.

Curiously, the shield is built by the very radiation it blocks. When high-energy UV strikes an oxygen molecule, it can break the bond between its two atoms (13.1). A lone oxygen atom is too reactive to stay single for long: it latches on to an intact oxygen molecule, and the three-atom result is ozone (13.2).

By the mid-1980s, instruments on the ground and on satellites were recording a steep loss of ozone. It was worst over Antarctica, where a seasonal ozone hole opens every September and October. The main culprits were synthetic chlorofluorocarbons (CFCs). They were once widely used as coolants in refrigerators and air conditioners, in spray cans and for making foam. Related bromine compounds called halons, used in fire extinguishers, did similar damage. CFCs are so unreactive that they last for decades and slowly drift up to the stratosphere. There UV breaks them apart and frees chlorine atoms, which destroy ozone.

Once scientists had shown the link, countries acted together. In 1987 they signed the Montreal Protocol, arranged by the United Nations Environment Programme (UNEP). It first froze CFC production at 1986 levels. Later revisions phased out CFCs and most other ozone-destroying chemicals altogether, so fridges, air conditioners and sprays everywhere switched to ozone-safe gases. India stopped producing and using CFCs on 1 August 2008, ahead of the deadline for developing countries.

It is working. The 2022 UN scientific assessment, published in January 2023, expects the ozone layer to return to its 1980 level by about 2040 over most of the world. Over the Arctic it should recover by about 2045, and over Antarctica by about 2066. NASA and NOAA ranked the 2025 Antarctic ozone hole the fifth smallest since 1992. There is a twist: some replacement gases, the HFCs, do not harm ozone but trap heat strongly, so the 2016 Kigali Amendment now phases them down too. India ratified it in 2021.

13.2.2 Managing the Garbage we Produce

Think of everything that left your home in yesterday's dustbin: vegetable peels, a milk packet, a biscuit wrapper, perhaps a broken pen. Once the bin is emptied, most of us never think about it again. Where does it end up, and does it ever disappear? A two-month experiment answers the second question.

Why do some things vanish while others last? Decomposers break dead matter down with enzymes, and as you saw in Life Processes, enzymes are specific: each acts only on particular substances. Your gut has enzymes for the starch in a roti but none for the cellulose in its bran, so the bran passes through undigested. Over millions of years, bacteria and fungi have evolved enzymes for natural materials such as cellulose (in paper, cotton and wood), proteins and fats.

Plastics, synthetic fibres and metals such as aluminium are new, human-made materials, and almost no microbe has enzymes that can attack them. Sunlight and weathering slowly make plastics brittle, so they crumble into fragments. But the fragments are still plastic, and in soil or water they linger for decades or even centuries.

A glass bottle lying in a field does little harm: it is inert, just very long-lived. Many other non-biodegradable things are harmful. A cow that swallows polythene bags with food scraps can die of a blocked stomach, and metals leaking from old batteries poison soil and water.

How much waste, and where does it go?

The problem is easy to see from any bus window: overflowing community bins, plastic snagged on fences, a smouldering heap at the edge of town. According to Central Pollution Control Board (CPCB) data for 2021–22, India's towns and cities produce about 1.7 lakh tonnes of solid waste a day. Only a little over half of it is processed, and much of the rest ends up in open dumps and landfills.

Compare a grocery trip today with one your grandparents made. They carried a cloth bag, the grocer wrapped dal in newspaper and curd went home in a steel tiffin. Today the same items come in plastic pouches, many made of bonded layers of plastic and foil that are very hard to recycle. We also buy more, and more of it is designed to be used once: paper plates, plastic spoons, shampoo sachets, single-serve snacks, delivery boxes and bubble wrap. So the heap keeps growing, and a bigger share of it will never rot.

What we can do

Cleaner surroundings start at home. The most useful habit is segregation at source: keeping different kinds of waste apart from the moment they are thrown away, so that each can be treated properly. India's Solid Waste Management Rules, 2026, in force from 1 April 2026, require every household to separate its waste into four streams (Fig. 13.10):

• Wet waste: kitchen waste, peels, leftover food, flowers. It goes for composting or biogas.
• Dry waste: plastic, paper, metal, glass, wood, rubber. It goes to material recovery facilities for sorting and recycling.
• Sanitary waste: used diapers and sanitary pads. It is wrapped securely and kept apart.
• Special care waste: paint cans, bulbs, mercury thermometers, old medicines. Authorised agencies collect it, or you drop it at a collection centre.

Beyond sorting, follow the three Rs in order of value. Reduce: buy less, refuse single-use items, carry a cloth bag and a water bottle. Reuse: refill jars, pass on clothes and books, repair before replacing. Recycle: send paper, metal, glass and clean plastic to be made into new goods. Wet waste can become compost in a garden pit or a khamba (a stack of clay pots), or biogas for cooking. India has also banned many single-use plastic items, such as plastic cutlery, straws and plastic-stick earbuds, since 1 July 2022.