Solved Examples
A dedicated bank of worked conceptual problems. Key ideas to recall:
- Ecosystem = biotic + abiotic components. Producers (autotrophs) → consumers (heterotrophs) → decomposers (recyclers).
- Food chain = straight sequence; food web = interconnected chains. Trophic levels: producer(1) → primary(2) → secondary(3) → tertiary(4).
- Ten per cent law: ~10% energy passes to the next level; flow is unidirectional.
- Biomagnification: non-degradable chemicals increase up the chain (max in humans).
- Ozone (O₃) shields UV; CFCs deplete it. Waste = biodegradable / non-biodegradable.
Example 1: Trophic levels (NCERT) What are trophic levels? Give an example food chain with its levels. Solution: Each feeding step is a trophic level. Grass → grasshopper → frog → snake: grass = producer (1), grasshopper = primary consumer (2), frog = secondary (3), snake = tertiary (4).
Example 2: Role of decomposers (NCERT) What is the role of decomposers in an ecosystem? Solution: They break down dead remains and waste into simple inorganic substances that return to the soil for producers to reuse - recycling nutrients and cleaning the ecosystem.
Example 3: Ten per cent law State the ten per cent law and calculate energy at each level if producers hold 10000 J. Solution: Only ~10% passes to the next level. Producers 10000 J → primary 1000 J → secondary 100 J → tertiary 10 J.
Example 4: Biological magnification (NCERT) What is biological magnification? Is it the same at all trophic levels? Solution: It is the progressive increase in concentration of non-degradable chemicals up a food chain. It is not the same - it increases at each higher level, being highest at the top (humans).
Example 5: Ozone and its effect (NCERT) What is ozone and how does it affect an ecosystem? Solution: Ozone (O₃) high in the atmosphere absorbs the Sun's harmful UV radiation, protecting living organisms. Its depletion lets more UV through, causing skin cancer and ecosystem damage.
Example 6: Biodegradable vs non-biodegradable (NCERT) Why are some substances biodegradable and some not? Solution: Decomposing enzymes are specific; microbes have enzymes for natural materials but not for man-made materials like plastics, which stay non-biodegradable.
Example 7: Group with only biodegradable items (NCERT Ex Q1) Which group has only biodegradable items - (a) grass, flowers, leather; (b) grass, wood, plastic; (c) fruit-peels, cake, lime-juice; (d) cake, wood, grass? Solution: (c) and (d) are fully biodegradable. Options with plastic/leather include non-biodegradable items. (The standard answer key marks the group without plastic/synthetic items.)
Example 8: Which is a food chain (NCERT Ex Q2) Which constitutes a food chain - (a) grass, wheat, mango; (b) grass, goat, human; (c) goat, cow, elephant; (d) grass, fish, goat? Solution: (b) grass → goat → human. In (a) and (c) the organisms do not eat one another; (d) is not a valid feeding order.
Example 9: Environment-friendly practices (NCERT Ex Q3) Which are environment-friendly - (a) cloth bags, (b) switching off unnecessary lights/fans, (c) walking instead of using a vehicle, (d) all of the above? Solution: (d) all of the above. Each reduces waste or saves energy.
Example 10: Killing one trophic level (NCERT Ex Q4) What happens if we kill all the organisms in one trophic level? Solution: The food chain/web is disrupted: the level below (its food) overpopulates, and the level above starves for lack of food. The whole ecosystem's balance is disturbed.
Example 11: Removing different trophic levels (NCERT Ex Q5) Is the impact of removing a trophic level the same for all levels? Can any level be removed without harm? Solution: The impact differs. Removing producers is most damaging (the whole ecosystem collapses). No trophic level can be removed without disturbing the ecosystem, because all levels are interdependent.
Example 12: Problems from non-biodegradable waste (NCERT Ex Q7) What problems are caused by the non-biodegradable wastes we generate? Solution: They persist and accumulate, polluting soil and water, harming animals that ingest them, choking drains, and entering food chains where they biomagnify to harmful levels.
Example 13: All-biodegradable waste (NCERT Ex Q8) If all our waste were biodegradable, would it have no impact? Solution: No - it would still have an impact. Large amounts of rotting biodegradable waste cause foul smell, flies, disease and can pollute water (using up dissolved oxygen). So it must still be managed.
Example 14: Ozone damage - concern and steps (NCERT Ex Q9) Why is ozone-layer damage a concern, and what steps limit it? Solution: Ozone absorbs harmful UV; its damage lets more UV reach Earth, causing skin cancer, cataracts and ecosystem harm. Steps: the 1987 UNEP/Montreal agreement to freeze CFC production and make CFC-free products.
Example 15: Ozone formation reactions Write the reactions by which ozone forms in the upper atmosphere. Solution: ; . UV splits O₂ into atoms, which combine with O₂ to form ozone.
Example 16: Producers as base Why are producers essential to every ecosystem? Solution: Producers capture solar energy and convert it to food (chemical energy), making it available to all consumers and decomposers. Without producers there would be no energy input and the ecosystem could not function.
Example 17: Aquarium cleaning Why must an aquarium be cleaned regularly but a pond need not be? Solution: A pond is a natural, balanced ecosystem with decomposers that recycle waste. An aquarium is artificial and lacks a complete set of decomposers/organisms, so waste accumulates and it must be cleaned.
Example 18: Autotroph vs heterotroph Distinguish autotrophs and heterotrophs with examples. Solution: Autotrophs make their own food (green plants, some bacteria) - producers. Heterotrophs depend on others for food (all animals) - consumers and decomposers.
Example 19: Energy pyramid Why is the energy pyramid always upright (narrowing at the top)? Solution: Because energy decreases at each higher trophic level (only ~10% passes up), the energy available is largest at the producer base and smallest at the top - so the pyramid always narrows upward.
Example 20: Direction of energy flow Is the flow of energy in an ecosystem cyclic or unidirectional? Explain. Solution: It is unidirectional - energy flows Sun → producers → consumers → decomposers and does not return. (Nutrients, in contrast, are cycled back by decomposers.)
Example 21: Classify consumers Classify: grasshopper, frog, human, tapeworm, lion. Solution: Grasshopper - herbivore (primary consumer); frog - carnivore (secondary consumer); human - omnivore; tapeworm - parasite; lion - carnivore.
Example 22: Food web stability Why is an ecosystem with a complex food web more stable? Solution: A complex food web gives organisms alternative food sources. If one species declines, others can use different pathways, so the ecosystem resists disturbance and is more stable.
Example 23: Pesticides and washing Why can pesticide residues not always be washed off food? Solution: Because pesticides are absorbed inside tissues (not just on the surface) and are non-degradable, surface washing cannot remove the residues stored within the food.
Example 24: Ground-level ozone Is ozone always beneficial? Explain. Solution: No. High in the atmosphere ozone is protective (absorbs UV). At ground level ozone is a harmful pollutant (a poison). So ozone is beneficial only in the upper atmosphere.
Example 25: Nutrient cycling vs energy flow How does nutrient flow differ from energy flow in an ecosystem? Solution: Nutrients are cycled (recycled by decomposers back to producers), whereas energy flows unidirectionally and is lost as heat at each level and never reused.
Example 26: Number of individuals per level At which trophic level is the number of individuals usually greatest? Solution: At the lowest trophic level - the producers. The number generally decreases as we move up to higher trophic levels.
Example 27: CFC alternatives Why must CFCs be replaced, and by what? Solution: CFCs deplete the ozone layer, so they must be replaced by CFC-free (ozone-friendly) refrigerants such as HFCs/newer coolants. It is now mandatory to make CFC-free refrigerators.
Example 28: Waste segregation benefit Why should biodegradable and non-biodegradable waste be collected separately? Solution: Separate collection lets biodegradable waste be composted/recycled into manure and non-biodegradable waste be recycled or safely disposed, reducing pollution and making treatment efficient.
Example 29: Example food chains in three ecosystems Give a food chain for a forest, a grassland and a pond. Solution: Forest: plants → deer → tiger. Grassland: grass → grasshopper → frog → snake → hawk. Pond: algae → small fish → big fish.
Example 30: Why humans should limit pesticide use Relate biomagnification to human health. Solution: Since humans are at the top of food chains, non-degradable pesticides biomagnify to their highest concentration in human bodies, posing health risks. Hence pesticide use should be limited and safer alternatives used.
Example 31: Disposable cups What is an advantage of disposable paper cups over disposable plastic cups? Solution: Paper cups are biodegradable (broken down by microbes), while plastic cups are non-biodegradable and persist as pollution. So paper cups cause less long-term environmental harm.
Example 32: Interdependence Give one line showing why all components of an ecosystem are interdependent. Solution: Producers feed consumers; consumers and producers die and are broken down by decomposers; decomposers return nutrients to the soil for producers - so each group depends on the others (and on abiotic factors).