Our Environment
CBSE Class 10 · Science · Notes and practice questions
Our Environment explains how living organisms interact with one another and with their physical surroundings through ecosystems, food chains and food webs, and how human activities such as waste disposal and ozone depletion affect these natural systems.
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An ecosystem is a functional unit where living organisms interact with each other and with the non-living physical environment. The non-living, or abiotic, components include sunlight, temperature, water, soil and air; they supply the energy and nutrients for life. The living, or biotic, components are grouped by how they get food: producers make their own food, consumers depend on others for nutrition, and decomposers recycle nutrients from dead waste. Together these components form a self-sustaining system that maintains balance in nature.
Energy enters an ecosystem almost entirely from sunlight, captured by green plants. When a herbivore eats a plant, energy passes to the herbivore; when a carnivore eats the herbivore, it passes further. This linear sequence, each step forming a trophic level, is called a food chain, and many such chains interlink to form a food web. At each transfer, about 90% of the energy is lost as heat through respiration and unassimilated food, so only about 10% becomes available to the next level. This ten per cent law explains why food chains seldom have more than four or five trophic levels.
Not all materials move along a food chain harmlessly. Chemicals such as the pesticide DDT are not broken down in organisms and cannot be excreted easily; they get stored in body fat. When a consumer eats many organisms from the level below, the chemical is concentrated further, so its amount increases at each successive trophic level. This rise in concentration of a non-biodegradable substance along a food chain is called biological magnification. As a result, organisms at the top of food chains, including human beings, suffer the highest accumulation of such toxic chemicals.
Human activities also damage the atmosphere. The ozone layer in the stratosphere protects Earth from harmful ultraviolet (UV) radiation. Chemicals like chlorofluorocarbons (CFCs), once used in refrigerators and spray cans, break down ozone molecules and thin this layer. Thinner ozone lets more UV radiation reach the surface, increasing the risk of skin cancer and eye problems. Another challenge is waste disposal. Biodegradable waste, such as food peels and paper, can be decomposed by microorganisms and should be allowed to rot, but non-biodegradable waste, like plastic and metal, persists in the environment and must be recycled or disposed of safely.
Key terms
- Ecosystem
- A self-sustaining unit in which living organisms interact with one another and with their non-living surroundings. It consists of two parts: the biotic component (producers, consumers and decomposers) and the abiotic component (sunlight, soil, air, water, temperature).
- Producers (autotrophs)
- Green plants and certain bacteria that can make their own food using sunlight through photosynthesis. They are the first trophic level and the source of energy for all other organisms in an ecosystem.
- Consumers (heterotrophs)
- Organisms that cannot synthesise their own food and depend directly or indirectly on producers. They include herbivores (primary consumers), carnivores that eat herbivores (secondary consumers), and higher-level consumers.
- Decomposers (saprophytes)
- Organisms such as bacteria and fungi that break down dead plants, animals and their waste products into simple inorganic substances. They recycle nutrients back to the soil and keep the ecosystem clean.
- Food chain and food web
- A food chain shows a single, straight sequence of who eats whom, for example grass → grasshopper → frog → snake. A food web is a network of many interconnected food chains existing in a community, and it gives a more realistic picture of feeding relationships. Each step in a chain is a trophic level.
- Ten per cent law
- The rule put forward by R. Lindeman stating that only about 10% of the energy stored in a trophic level is transferred to the next trophic level as biomass. The remaining energy is lost as heat during respiration and in unused or unabsorbed food.
- Biological magnification
- The progressive increase in the concentration of a non-biodegradable, toxic chemical such as DDT at each successive trophic level of a food chain. Because the chemical is not broken down, it accumulates in the body fat of organisms, so top consumers receive the highest dose.
- Ozone layer depletion
- The thinning of the ozone layer found in the stratosphere, which shields the Earth from ultraviolet radiation. It is caused mainly by the release of chlorofluorocarbons (CFCs) and other ozone-depleting substances, resulting in more UV radiation reaching the ground.
- Biodegradable and non-biodegradable substances
- Biodegradable substances are materials that can be broken down by decomposers, e.g. food waste, paper. Non-biodegradable substances cannot be decomposed by microorganisms, e.g. plastics, glass, metals, and therefore accumulate in the environment, causing pollution.
Practice questions with answers
1. What is an ecosystem? List its main components.
An ecosystem is a functional unit in which living organisms interact with one another and with their physical surroundings. Its main components are the biotic components, which include producers, consumers and decomposers, and the abiotic components, which include air, water, soil, temperature and sunlight.
2. Why are decomposers indispensable in an ecosystem?
Decomposers break down dead plants, animals and animal wastes into simple inorganic substances such as carbon dioxide, water and minerals. These substances return to the soil, making nutrients available to producers. Without decomposers, nutrients would remain locked up in dead bodies and the ecosystem would run out of raw material.
3. What is a food chain? Construct a four-step food chain from the organisms grass, grasshopper, frog and snake.
A food chain is the sequence of organisms in a community in which each one is eaten by the next, so that energy passes from one trophic level to the level above it. A four-step chain from these organisms is: grass → grasshopper → frog → snake. Grass is the producer, the grasshopper is the primary consumer, the frog is the secondary consumer and the snake is the tertiary consumer.
4. Justify that energy flow in an ecosystem is unidirectional.
Energy enters an ecosystem from sunlight and passes from producers to consumers and finally to decomposers. At each step, most of the energy is lost as heat during respiration and can never be reused by the plants. Since the flow always follows the direction producers → consumers → decomposers and is not recycled back, the flow of energy is said to be unidirectional.
5. In a given food chain, the amount of energy available at the producer level is 20,000 J. How much energy will be available to the tertiary consumer? (Assume 10% law)
According to the ten per cent law, only 10% of the energy is transferred at each trophic level. Producer: 20,000 J; primary consumer: 2,000 J; secondary consumer: 200 J; tertiary consumer: 20 J. So 20 J of energy will be available to the tertiary consumer.
6. What is biological magnification? Explain with reference to DDT.
Biological magnification is the progressive increase in the concentration of a non-biodegradable chemical at higher trophic levels of a food chain. DDT is a pesticide that is not broken down in organisms and remains in their body fat. When a predator eats many prey organisms, the DDT from all the prey collects in the predator, so its concentration rises at each trophic level and is greatest in top consumers.
7. How does the depletion of the ozone layer affect human health?
The ozone layer absorbs most of the sun's ultraviolet (UV) radiation. When it depletes, more UV radiation reaches the Earth's surface. This increased UV exposure can cause skin cancer, cataracts in the eyes, and weakens the immune system in humans, and it also harms crops and marine organisms.
8. Differentiate between biodegradable and non-biodegradable substances, giving one example of each.
Biodegradable substances can be decomposed by microorganisms into harmless simple substances, for example vegetable peels. Non-biodegradable substances cannot be broken down by these organisms and accumulate in the environment, for example plastic. Non-biodegradable substances must therefore be recycled or given special treatment.
9. Why do food chains usually not have more than 4-5 trophic levels?
Only about 10% of energy is passed on at each trophic level, the rest being lost as heat. After a few steps, the amount of usable energy becomes too small to support another trophic level, so chains are limited to about four or five links.
10. What are trophic levels? Name the first, second and third trophic levels with examples.
Trophic levels are the steps in a food chain at which organisms obtain their food and energy. The first trophic level is that of producers, e.g. green plants. The second trophic level is primary consumers, e.g. herbivores like deer; the third trophic level is secondary consumers, e.g. carnivores that eat herbivores like a frog or a fox.
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