Our Environment

1. What this chapter is about

You hear the word "environment" everywhere — on TV, in newspapers, at global summits. This chapter studies how the different components of the environment interact with one another, and how our own activities change that environment. Two human-made problems are looked at closely: depletion of the ozone layer and management of waste.

The environment is not just "nature out there". You are part of it. Changes in the environment affect you, and your actions change the environment. Keeping that two-way link in mind is the heart of the chapter.

2. Ecosystem and its components

All organisms — plants, animals, microorganisms and humans — together with the physical (non-living) surroundings interact and maintain a balance in nature. All the interacting organisms in an area, taken together with the non-living constituents, form an ecosystem.

So an ecosystem has two kinds of components:

• Biotic components — the living organisms: plants, animals, microorganisms.
• Abiotic components — the physical factors: temperature, rainfall, wind, soil and minerals (and sunlight, water, air).

Example — a garden: it has grasses and trees, flower-bearing plants like rose, jasmine and sunflower, and animals like frogs, insects and birds. Every organism interacts with the others, and its growth and reproduction are affected by the abiotic factors. So a garden is an ecosystem.

Two types of ecosystems:

• Natural ecosystems — forests, ponds, lakes (form on their own).
• Human-made (artificial) ecosystems — gardens, crop-fields, an aquarium.

3. Producers, consumers and decomposers

The biotic part of every ecosystem is grouped by how organisms get their food:

• Producers (autotrophs): organisms that make their own organic food (sugar, starch) from inorganic substances using the Sun's radiant energy in the presence of chlorophyll — i.e. by photosynthesis. All green plants and certain bacteria are producers. They are the entry point of energy into the ecosystem.
• Consumers (heterotrophs): organisms that depend on producers directly or indirectly for food. They consume food made by producers, either directly or by feeding on other consumers. Consumers are sub-classed as:
  • Herbivores — eat plants (deer, goat, grasshopper).
  • Carnivores — eat other animals (frog, snake, tiger).
  • Omnivores — eat both (humans, crow).
  • Parasites — live on/in a host and draw food from it (lice, ticks, Cuscuta).
• Decomposers: microorganisms — bacteria and fungi — that break down the dead remains and waste products of organisms. They convert complex organic substances into simple inorganic substances that go back into the soil and are taken up again by plants.

4. Food chains and trophic levels

A food chain is a series of organisms feeding on one another, taking part at various biotic levels. Each step or level of the food chain is a trophic level.

Counting from the bottom:

• First trophic level — Producers (autotrophs): they fix solar energy and make it available to all others.
• Second trophic level — Primary consumers (herbivores).
• Third trophic level — Secondary consumers (small carnivores).
• Fourth trophic level — Tertiary consumers (larger carnivores).

Example food chains (NCERT Fig. 13.1):

The energy pyramid (Fig. 13.2) shows producers at the wide base, then primary, secondary and tertiary consumers narrowing towards the top — because both the amount of energy and the number of organisms decrease as you move up. Generally there are more individuals at lower trophic levels, and the greatest number is of the producers.

5. Food webs

In nature, a single straight-line chain is rare. Each organism is usually eaten by two or more kinds of organisms, which in turn are eaten by several others. So instead of one straight chain, the relationships branch into a network of many interconnected food chains — called a food web (Fig. 13.3).

Picture it in words: grass feeds grasshoppers, rabbits and deer; a grasshopper may be eaten by a frog or a bird; the frog by a snake; the snake by a hawk; the rabbit by a fox or a hawk; the deer by a tiger. Cross all these arrows and you get a web. A food web makes the ecosystem more stable: if one prey is scarce, a predator can switch to another, so the system does not collapse.

Length of chains: the length and complexity of food chains vary greatly. But because energy is lost at every step, chains generally consist of only three or four steps.

6. Flow of energy and the 10% Law

The interactions among the components of the environment involve a flow of energy. Autotrophs capture energy and convert it into chemical energy that supports all activities of the living world. From autotrophs the energy moves to heterotrophs and decomposers. As you learnt in "Sources of Energy", when one form of energy is changed to another, some is always lost to the environment in forms that cannot be used again.

Studying this flow carefully reveals a clear pattern:

• Green plants in a terrestrial ecosystem capture only about 1% of the Sun's energy falling on their leaves and convert it into food energy.
• When green plants are eaten by primary consumers, a great deal of energy is lost as heat, some goes into digestion and doing work, and the rest into growth and reproduction. On average only 10% of the food eaten is turned into the consumer's own body and made available to the next level.
• So 10% is taken as the average value of organic matter that reaches the next level — the Ten Percent Law (Lindeman).

Two key features of energy flow (from the energy-flow diagram, Fig. 13.4):

• Unidirectional: energy flows in one direction only — Sun → producers → herbivores → carnivores. Energy captured by autotrophs never goes back to the Sun, and energy passed to herbivores never returns to plants. Once it moves up a level, it is no longer available to the previous level.
• Progressively diminishing: the energy available decreases at each successive trophic level because of the loss at every transfer.

Because so little energy is left after a few steps, very little usable energy remains beyond the fourth trophic level — this is exactly why food chains are short (3-4 steps).

7. Biological magnification

Some harmful chemicals enter our bodies unknowingly through the food chain. To protect crops from pests and diseases, farmers use pesticides and other chemicals. These are washed into the soil or into water bodies. From the soil they are absorbed by plants along with water and minerals; from water bodies they are taken up by aquatic plants and animals. This is one way they enter the food chain.

Because these chemicals are not degradable (the body cannot break them down or excrete them easily), they get accumulated progressively at each trophic level. As human beings occupy the top level in most food chains, the maximum concentration of these chemicals collects in our bodies. This progressive increase in concentration of harmful, non-degradable chemicals at each successive trophic level is called biological magnification (biomagnification).

8. How our activities affect the environment

We are an integral part of the environment, so our activities change it — often for the worse. You have already studied pollution in Class IX. Here the chapter focuses on two specific problems in detail: depletion of the ozone layer and waste disposal.

9. The ozone layer and how it gets depleted

Ozone (O₃) is a molecule made of three atoms of oxygen. Ordinary oxygen, O₂, is essential for all aerobic life. Ozone at ground level is actually a deadly poison — but high up in the atmosphere it does an essential job: it shields the surface of the Earth from ultraviolet (UV) radiation from the Sun.

UV radiation is highly damaging to organisms — for example, it is known to cause skin cancer in human beings (and cataracts, and damage to plants).

How ozone forms: high in the atmosphere, UV radiation acts on oxygen (O₂) molecules. Higher-energy UV splits some molecular oxygen into free oxygen atoms (O). These atoms then combine with molecular oxygen to form ozone:

How it gets depleted: the amount of ozone began to drop sharply in the 1980s. This decrease has been linked to synthetic chemicals called chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. CFCs release chlorine high in the atmosphere, which destroys ozone.

Steps being taken: in 1987 the United Nations Environment Programme (UNEP) succeeded in forging an agreement (the Montreal Protocol) to freeze CFC production at 1986 levels. It is now mandatory for all manufacturing companies worldwide to make CFC-free refrigerators.

10. Managing the garbage we produce

Our daily activities generate a lot of waste material that we throw away. What happens after we throw it away depends on the kind of substance.

Why some substances break down and others don't: in the body, food is digested by enzymes, and enzymes are specific — a particular enzyme acts only on a particular substance. (That is why we cannot get energy from coal!) For the same reason, many human-made materials like plastics are not broken down by bacteria or other saprophytes. Such materials are acted upon only by physical processes like heat and pressure, so under ordinary conditions they persist for a very long time.

• Biodegradable substances: substances that are broken down by biological processes (by the action of microorganisms) — e.g. vegetable peels, paper, cotton cloth, food waste, wood, leather, jute.
• Non-biodegradable substances: substances that are not broken down by biological processes — e.g. plastics, glass, metals, DDT, polythene bags. They may be inert and simply persist for a long time, or they may actively harm members of the ecosystem.

Why the problem is growing: improvements in lifestyle generate far more waste; more things are now disposable; and changes in packaging have made much of our waste non-biodegradable. The "disposable cups in trains" story shows the trade-off: plastic cups replaced reusable glasses for hygiene, but created huge plastic waste; clay kulhads were tried but their large-scale making destroys fertile topsoil; now paper cups are used as a better compromise.

11. NCERT in-text "Questions" — answered

Page 212, Q1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.

Each step or level of a food chain at which transfer of energy takes place is a trophic level. Example: Grass → Grasshopper → Frog → Snake → Hawk. Here grass = first trophic level (producer); grasshopper = second (primary consumer/herbivore); frog = third (secondary consumer); snake = fourth (tertiary consumer); hawk = fifth (top carnivore/quaternary consumer).

Page 212, Q2. What is the role of decomposers in the ecosystem?

Decomposers (bacteria and fungi) break down the dead remains and wastes of organisms into simple inorganic substances. They (i) recycle nutrients back into the soil for plants to reuse, (ii) clean the environment by clearing dead matter, and (iii) help replenish soil fertility, keeping the nutrient cycles running.

Page 214, Q1. Why are some substances biodegradable and some non-biodegradable?

Biodegradable substances can be broken down by the enzymes of microorganisms because those enzymes are able to act on them. Enzymes are specific, so substances for which no enzyme exists (like plastics) cannot be broken down biologically — they are non-biodegradable and are changed only slowly by physical processes.

Page 214, Q2. Give any two ways in which biodegradable substances would affect the environment.

• While decomposing they release foul smell and gases, and act as breeding grounds for flies, mosquitoes and disease-causing microbes.
• Their decomposition can release gases like methane (a greenhouse gas) and, in excess, can cause water pollution / eutrophication — though ultimately they recycle nutrients and enrich the soil.

Page 214, Q3. Give any two ways in which non-biodegradable substances would affect the environment.

• They persist in the environment for a long time, causing soil and water pollution and choking drains and water bodies.
• Non-degradable chemicals (e.g. pesticides, plastics) enter food chains and undergo biological magnification, harming organisms; they also kill animals that swallow plastic and reduce soil fertility.

Page 216, Q1. What is ozone and how does it affect any ecosystem?

Ozone (O₃) is a molecule of three oxygen atoms. High in the atmosphere it forms a layer that absorbs harmful UV radiation from the Sun, protecting all living things on Earth. By blocking UV it prevents skin cancer, cataracts and damage to plants and aquatic life, so it protects the whole ecosystem. (At ground level, however, ozone is a harmful pollutant.)

Page 216, Q2. How can you help in reducing the problem of waste disposal? Give any two methods.

• Reduce, reuse and recycle — use cloth bags, refillable bottles and avoid disposable plastics.
• Segregate waste into biodegradable and non-biodegradable, and compost kitchen/garden waste so less goes to landfills.

12. NCERT Exercises (Q1-Q9) — fully solved

Q1. Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather  (b) Grass, wood and plastic  (c) Fruit-peels, cake and lime-juice  (d) Cake, wood and grass

Answer: (a), (c) and (d) — all their items are biodegradable. The accepted single best option is (c) (and (a), (d) are also fully biodegradable). Option (b) is wrong because plastic is non-biodegradable.

Q2. Which of the following constitute a food-chain?
(a) Grass, wheat and mango  (b) Grass, goat and human  (c) Goat, cow and elephant  (d) Grass, fish and goat

Answer: (b) Grass → Goat → Human — a producer, a herbivore and an omnivore in a proper eat-and-be-eaten sequence. (a) and (c) are all of one type; (d) makes no feeding sequence.

Q3. Which of the following are environment-friendly practices?
(a) Carrying cloth-bags for shopping  (b) Switching off unnecessary lights and fans  (c) Walking to school instead of using a scooter  (d) All of the above

Answer: (d) All of the above — each reduces plastic waste, saves electricity or cuts fuel emissions.

Q4. What will happen if we kill all the organisms in one trophic level?

The ecosystem becomes unbalanced. Organisms in the level below are no longer controlled and may overgrow; organisms in the level above lose their food and starve or migrate. Energy flow is broken and the whole food chain/web is disrupted.

Q5. Will the impact of removing all the organisms in a trophic level be different for different trophic levels? Can the organisms of any trophic level be removed without causing any damage to the ecosystem?

Yes, the impact differs. Removing producers is the most damaging — without them the whole ecosystem collapses, as they are the source of all energy and food. Removing top carnivores causes their prey (herbivores) to multiply, who then over-graze the producers. No — organisms of no trophic level can be removed without harming the ecosystem, because each level is linked to the others through energy flow; removing any one upsets the balance.

Q6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?

Biological magnification is the progressive increase in the concentration of harmful, non-degradable chemicals (like pesticides) in the bodies of organisms at each successive trophic level. Yes, the level differs: concentration is lowest in producers and increases at every step, reaching its maximum in the top consumers (such as humans).

Q7. What are the problems caused by the non-biodegradable wastes that we generate?

• They persist for a very long time and cause soil, water and air pollution.
• They choke drains and water bodies, killing aquatic life; plastics swallowed by animals kill them.
• Non-degradable chemicals enter food chains and undergo biological magnification, poisoning organisms including humans; they also reduce soil fertility.

Q8. If all the waste we generate is biodegradable, will this have no impact on the environment?

It would still have some impact. Even biodegradable waste, if produced in large amounts, would overload the decomposers, pile up, release foul smell and harmful gases (like methane), breed disease-carrying organisms, and pollute soil and water until fully decomposed. So reducing total waste still matters.

Q9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?

The ozone layer absorbs harmful UV radiation; if it is damaged, more UV reaches Earth, causing skin cancer, cataracts, weakened immunity, damage to plants and reduced crop yields, and harm to aquatic ecosystems. Steps: UNEP forged an agreement in 1987 to freeze CFC production at 1986 levels (Montreal Protocol); it is now mandatory to make CFC-free refrigerators and to phase out other ozone-depleting chemicals worldwide.

13. Common mistakes to avoid

• Confusing the direction of energy flow — it is one-way (unidirectional); it never cycles back like nutrients do.
• Saying 10% of energy is lost — wrong; ~10% is passed on and ~90% is lost.
• Mixing up biodegradable and non-biodegradable — leather, paper and wood ARE biodegradable; glass and metals are NOT.
• Thinking ground-level ozone is good — at ground level ozone is a poison; only the high-altitude layer is protective.
• Writing that biological magnification is highest in producers — it is highest in top consumers.
• Forgetting that a food web is more stable than a single food chain.
• Saying decomposers are producers or consumers — they are a separate group that breaks down dead matter.

14. Quick revision checklist

• Ecosystem = biotic (living) + abiotic (non-living) components, interacting.
• Producers (autotrophs) → consumers (herbivore/carnivore/omnivore/parasite) → decomposers (bacteria, fungi).
• Food chain = trophic levels; food web = interconnected chains (more stable).
• Energy flow is unidirectional and diminishes at each step — 10% Law.
• Chains have only 3-4 steps because of energy loss.
• Biological magnification: non-degradable chemicals concentrate up the chain, max in humans.
• Ozone (O₃) blocks UV; CFCs deplete it; UNEP 1987, CFC-free fridges.
• Waste: biodegradable (broken down by microbes) vs non-biodegradable (plastics, glass, metals).