Class 10 › Science › Chapter 13
Our Environment
Components of an ecosystem, food chains, trophic levels and energy flow, biological magnification, ozone layer depletion, and the problem of waste disposal.
1. Ecosystem and Its Components
Ecosystem: all the interacting organisms in an area together with the non-living constituents of the environment.
Biotic
Living components
Plants, animals, microorganisms and human beings.
Abiotic
Non-living components
Physical factors such as temperature, rainfall, wind, soil and minerals.
| Type | Examples |
|---|---|
| Natural ecosystem | Forest, pond, lake |
| Human-made (artificial) ecosystem | Garden, crop-field, aquarium |
- Biotic and abiotic components interact and maintain a balance in nature; the growth, reproduction and activities of organisms are affected by the abiotic factors.
- Aquarium as a model: it needs space, water, oxygen (aerator) and food. Adding aquatic plants and animals makes it a self-sustaining system; it still needs occasional cleaning because wastes and dead remains build up (large natural ponds and lakes balance themselves).
- Do not put an animal that eats others into an aquarium; it would eat the other organisms and upset the balance.
2. Producers, Consumers and Decomposers
| Group | How they get food | Examples |
|---|---|---|
| Producers (autotrophs) | Make sugar and starch from inorganic substances using sunlight and chlorophyll (photosynthesis) | All green plants, some bacteria |
| Consumers (heterotrophs) | Eat food produced by producers, directly or indirectly | Herbivores, carnivores, omnivores, parasites |
| Decomposers | Break down dead remains and wastes of organisms | Bacteria and fungi |
- Types of consumers: herbivores (cow, deer), carnivores (tiger, frog), omnivores (humans, crow) and parasites (tapeworm, lice).
- Role of decomposers: break complex organic substances into simple inorganic substances that go into the soil and are used again by plants. They clean up the environment and replenish the soil; without them, dead matter and garbage would pile up and nutrients would not return to the soil.
- Every group depends on the others, so no one group can be called the only important one.
3. Food Chains and Trophic Levels
Food chain: a series of organisms, each feeding on the one before it, through which food (energy) passes. Trophic level: each step or level of a food chain.
A grassland food chain with its four trophic levels.
| Trophic level | Organisms |
|---|---|
| First | Producers (autotrophs) |
| Second | Herbivores (primary consumers) |
| Third | Small carnivores (secondary consumers) |
| Fourth | Larger carnivores (tertiary consumers) |
- Other examples: Plants → Deer → Tiger (forest); Algae → Small fish → Large fish → Bird (pond).
- A food chain has generally only 3 or 4 steps (reason in Section 4).
- Decomposers act on the dead remains at every level.
4. Flow of Energy and the 10% Law
Energy available falls by about 90% at every trophic level.
Key facts on energy
1. Green plants of a terrestrial ecosystem capture only about 1% of the sunlight falling on their leaves and convert it to food energy.
2. When a herbivore eats a plant, much energy is lost as heat, some goes into digestion and work, and the rest into growth and reproduction. Only about 10% of the food eaten becomes the body of the consumer and is available to the next level.
3. So the 10% law: on average, 10% of the energy of one trophic level reaches the next.
- Energy flow is unidirectional: energy captured by producers never returns to the Sun, and energy passed to herbivores never returns to producers.
- Energy decreases at each level because of loss at every step.
- Why food chains are short (3–4 steps): so little usable energy remains after four trophic levels that a fifth level cannot be supported.
- Number of individuals is greatest at the lowest level (producers) and falls at higher levels.
- Producers make solar energy available to the rest of the ecosystem.
5. Food Web
Food web: a network of many interconnected food chains. Since each organism is usually eaten by two or more kinds of organisms, the feeding relationship is a set of branching lines, not a straight line.
- A food web gives a more realistic picture of feeding relationships than a single food chain.
- If one organism is removed, others have alternative food sources, so a food web is more stable than a single chain.
- Removing a whole trophic level upsets the ecosystem: removing producers starves everyone above; removing herbivores leaves carnivores without food and lets plants grow unchecked; removing carnivores lets herbivore numbers rise and overgraze. No trophic level can be removed without damage.
6. Biological Magnification
Biological magnification: the progressive increase in the concentration of non-degradable harmful chemicals (like pesticides) at each trophic level of a food chain.
How chemicals enter and build up
1. Pesticides and other chemicals used on crops are washed into the soil or water bodies.
2. Plants absorb them from the soil along with water and minerals; aquatic plants take them from water. This is how they enter the food chain.
3. The chemicals are not degradable, so they accumulate more at each higher level.
4. Humans are at the top, so the maximum concentration collects in our bodies.
- Hence wheat, rice, vegetables, fruits and even meat contain pesticide residues, which washing cannot always remove.
- Magnification is different at different levels: lowest in producers, highest in the top consumers.
- Reducing intake: wash and peel produce, prefer organic or low-pesticide farming, avoid over-use of chemicals.
7. How Our Activities Affect the Environment
- Humans are an integral part of the environment: changes in the environment affect us, and our activities change the environment.
- This chapter focuses on two problems: depletion of the ozone layer and waste disposal.
- Environment-friendly practices: carrying cloth bags, switching off unnecessary lights and fans, walking or cycling instead of using vehicles for short distances.
8. Ozone Layer and How It Is Getting Depleted
Oxygen, O₂
Essential for life
The oxygen we normally refer to; needed by all aerobic organisms.
Ozone, O₃
Poisonous at ground level
A molecule of three oxygen atoms; deadly poison, but in the upper atmosphere it protects life.
Formation of ozone in the upper atmosphere
1. High-energy UV splits molecular oxygen into free oxygen atoms: $$\mathrm{O_2 \xrightarrow{UV} O + O}$$
2. The free atoms combine with molecular oxygen to form ozone: $$\mathrm{O + O_2 \rightarrow O_3}$$
- Importance: the ozone layer shields the Earth's surface from harmful ultraviolet (UV) radiation, which can cause skin cancer in humans and damage other organisms.
- Depletion: ozone dropped sharply in the 1980s, linked to synthetic chemicals called chlorofluorocarbons (CFCs), used in refrigerators and fire extinguishers.
- Action taken: in 1987, the UNEP (United Nations Environment Programme) got an agreement to freeze CFC production at 1986 levels; manufacturers are now required to make CFC-free refrigerators.
9. Biodegradable and Non-biodegradable Substances
Biodegradable
Broken down by biological processes
Decomposed by bacteria, fungi and other saprophytes. Examples: kitchen waste, vegetable peels, paper, cotton and wool, dead plants and animals, cake, wood, grass.
Non-biodegradable
Not broken down biologically
Persist for a long time. Examples: plastics, polythene, glass, metals, DDT and other pesticides, synthetic fibres.
- Why the difference? Enzymes are specific in their action: a particular enzyme breaks down a particular substance (which is why we cannot get energy from coal). Many human-made materials such as plastics have no enzyme that can act on them, so decomposers cannot break them down.
- Heat and pressure can act on them, but under normal conditions they persist for a very long time.
| Effect on the environment | |
|---|---|
| Biodegradable | Decompose and return nutrients to the soil, but heaps of such waste can smell and breed flies and disease-causing organisms (and release methane if rotting without air), and cause water pollution if they enter water bodies. |
| Non-biodegradable | Persist for long, pollute soil and water, choke drains and harm animals that swallow them, and may enter and accumulate in food chains (biological magnification). |
- Even if all waste were biodegradable, it would still have an impact: large amounts need time and space to decompose, cause foul smell and spread disease, and cannot be handled fast enough in crowded places.
10. Managing the Garbage We Produce
- Why waste is increasing: better lifestyles, more disposable products, and packaging changes that make much of our waste non-biodegradable.
- Think it over (disposable cups in trains): plastic cups were hygienic but caused huge waste; clay kulhads would remove fertile top-soil if made on a large scale; paper cups are better than plastic as they are biodegradable and easier to dispose of safely.
- Sewage and industrial waste must be treated before release so that water bodies and soil are not polluted.
- Hazardous e-waste: electronic items contain toxic materials (such as lead and mercury) that need special disposal.
What we can do to reduce waste
Reduce: avoid disposable items, carry cloth bags, buy only what is needed.
Reuse: use containers, bags and bottles again instead of throwing them away.
Recycle: send paper, glass, metal and plastic for recycling (recycling has its own energy use and pollution).
Segregate: keep biodegradable and non-biodegradable waste separate; compost kitchen waste.
11. Quick Revision
| Concept | Key point |
|---|---|
| Ecosystem | Biotic (living) + abiotic (non-living) components |
| Producers | First trophic level; make food by photosynthesis |
| Consumers | Herbivores, carnivores, omnivores, parasites |
| Decomposers | Bacteria and fungi; return nutrients to soil |
| Trophic levels | Producers → herbivores → small carnivores → large carnivores |
| Energy flow | One-way; only about 10% passes to the next level |
| Food web | Network of interlinked food chains |
| Biological magnification | Non-degradable chemicals increase at each level; maximum in humans |
| Ozone | $\mathrm{O_3}$; absorbs UV; depleted by CFCs; CFC production frozen in 1987 (UNEP) |
| Waste | Biodegradable vs non-biodegradable |
Remember
Numbers to know
1% of sunlight captured by plants · 10% energy passed on · 3–4 steps in a food chain · CFC freeze in 1987 at 1986 levels.
Check
Valid food chain?
Each organism must eat the one before it, starting from a producer, e.g. Grass → Goat → Human. Grass, wheat and mango (all producers) do not form one.
Common mistakes:
• Drawing arrows from the eater to the eaten; arrows point to the one who eats (direction of energy).
• Saying energy flows in a cycle; it is one-way, while nutrients are recycled.
• Confusing 10% of energy with 10% of organisms.
• Calling ozone harmful everywhere; it protects in the upper atmosphere but is poisonous near the ground.
• Mixing up O₂ (oxygen) and O₃ (ozone).
• Placing decomposers inside a food chain as a trophic level.
• Thinking biodegradable waste causes no pollution at all.
• Drawing arrows from the eater to the eaten; arrows point to the one who eats (direction of energy).
• Saying energy flows in a cycle; it is one-way, while nutrients are recycled.
• Confusing 10% of energy with 10% of organisms.
• Calling ozone harmful everywhere; it protects in the upper atmosphere but is poisonous near the ground.
• Mixing up O₂ (oxygen) and O₃ (ozone).
• Placing decomposers inside a food chain as a trophic level.
• Thinking biodegradable waste causes no pollution at all.