Keep the Curiosity Alive — Chapter Exercises — "How Nature Works in Harmony"

1. 12 marks

   Refer to Fig. 12.19 (population, community, ecosystem) and select the wrong statement: (i) A community is larger than a population. (ii) A community is smaller than an ecosystem. (iii) An ecosystem is part of a community.

   Hint. Check each statement against the chapter's own stated hierarchy: Individual → Population → Community → Ecosystem.

   ✦ Statement (iii), 'An ecosystem is part of a community,' is the wrong one. The chapter's hierarchy (Fig. 12.7) runs Individual → Population → Community → Ecosystem, meaning the ecosystem is the LARGEST unit, containing the community (plus abiotic components) — not the other way round. Statements (i) and (ii) are both correct, since a community is indeed larger than a population and smaller than an ecosystem.

2. 24 marks

   A population is part of a community. If all decomposers suddenly disappeared from a forest ecosystem, what changes would occur? Explain why decomposers are essential.

   Hint. Trace what decomposers normally do, then remove that function entirely and follow the consequences.

   ✦ Without decomposers, dead plants and animals would no longer be broken down into simpler substances, so dead matter would accumulate rather than being recycled. Since decomposers return important nutrients to the soil, their disappearance would mean plants stop receiving these recycled nutrients, likely reducing soil fertility and plant growth over time — which would then affect every organism further up the food chain, since producers form its base. Decomposers are essential because, as the chapter states, 'in nature, nothing is wasted — everything is reused,' and it is decomposers that make this recycling of nutrients possible.

3. 33 marks

   Selvam's village in Cuddalore district, Tamil Nadu, was less affected by the 2004 Tsunami than nearby villages, because of nearby mangrove forests. Explain why mangroves protected the village.

   Hint. The chapter states directly what mangrove forests physically do during storms.

   ✦ The chapter states that mangrove forests like the Sundarbans 'protect us by slowing down strong winds and waves during storms and floods.' A tsunami is an extreme version of large, powerful waves reaching the coast, and the dense root systems and vegetation of mangrove forests would have physically absorbed and slowed some of that wave energy before it reached the village, which is why the force and impact were reduced compared to villages without this natural buffer.

4. 43 marks

   In the food chain Grass → Grasshopper → Frog → Snake, if frogs disappear from this ecosystem, what will happen to the population of grasshoppers and snakes? Why?

   Hint. Removing the frog breaks the chain at one link — think about what that link normally does to the population below it and above it.

   ✦ If frogs disappear, the grasshopper population would likely increase, because frogs (which eat grasshoppers) are no longer controlling their numbers. The snake population, however, would likely decrease, since snakes eat frogs and would lose this direct food source — unless snakes in this ecosystem have another food source not shown in this simplified chain, they would face reduced food availability.

5. 53 marks

   In a school garden, students noticed fewer butterflies the previous season. What could be the possible reasons, and what steps could students take to have more butterflies on campus?

   Hint. Think about what butterflies need to survive and reproduce, drawing on what this chapter says pollinators depend on.

   ✦ Possible reasons include: fewer flowering plants providing nectar (reducing food availability), increased use of pesticides (which can harm butterflies directly, similar to how pesticide overuse harms pollinators generally), loss of host plants that butterfly caterpillars specifically feed on, or an increase in predators/competitors. Steps students could take: planting more nectar-rich and native flowering plants, avoiding pesticide use in the garden, and since undisturbed areas are where caterpillars can develop into butterflies without being cleared away.

6. 63 marks

   Why is it not possible to have an ecosystem with only producers and no consumers or decomposers?

   Hint. Consider what would happen to the producers themselves, and to their nutrients, without either group present.

   ✦ Without consumers, producers would have no organisms to transfer energy and matter to further up a food chain, and populations could grow unchecked with nothing regulating them (unlike the balance competition and predation normally provide). Without decomposers, dead plant matter would never be broken down, so nutrients would stay locked in dead tissue instead of being recycled back into the soil — meaning producers would eventually run short of the very nutrients they need to keep growing. An ecosystem needs all three roles working together to keep both energy flow and nutrient recycling functioning.

7. 73 marks

   Observe two different places near your home or school (e.g., a park and a roadside). List the living and non-living components you see. How are the two ecosystems different?

   Hint. Apply Activity 12.1's exact method — biotic list, abiotic list, then compare.

   ✦ A park might show biotic components like grass, trees, birds, squirrels and insects, and abiotic components like soil, sunlight and a water source (if present); a roadside might show fewer plant species (perhaps only hardy weeds), fewer animals due to traffic disturbance, and abiotic components like exposed soil, more heat from asphalt, and higher pollution levels. The two ecosystems would differ mainly in the variety and number of biotic components present, and in specific abiotic conditions like heat, pollution and disturbance — since both are shaped heavily by how much human activity and construction surrounds each space.

8. 84 marks

   'Human-made ecosystems like agricultural fields are necessary, but they must be made sustainable.' Comment on this statement, using the chapter's own reasoning.

   Hint. The chapter directly addresses both halves of this statement — why farms are necessary, and what makes current practices unsustainable.

   ✦ Agricultural fields are necessary because farming has fed a growing human population for thousands of years, and practices like the Green Revolution were a direct, successful response to a real food crisis (1950-1965) in India. However, the chapter also explains why current large-scale practices are considered unsustainable: overuse of synthetic chemicals, excessive groundwater extraction, and monoculture degrade soil fertility, reduce natural pest control, and harm pollinators. This supports the statement fully — agricultural fields remain essential for food security, but the chapter's own examples (soil degradation, reduced biodiversity, pesticide dependency) show why sustainable, more natural farming approaches (as explored in Activity 12.10) need to replace practices that damage the very ecosystem farms depend on.

9. 93 marks

   If the Indian hare population (Fig. 12.20) drops because of a disease, how would it affect the number of other organisms?

   Hint. Fig. 12.20 shows a PARTIAL food web: Sun → Grass and plants → (Hare, Deer); Hare → Fox; Deer → Eagle. Follow only what this specific diagram actually connects.

   ✦ In Fig. 12.20's food web, hares are eaten only by foxes, so a hare population drop from disease would directly reduce the food available to foxes, likely causing the fox population to decline too (or forcing foxes to rely more heavily on other food sources not shown in this partial diagram). Since grass and plants are eaten by both hares and deer, slightly less grazing pressure from fewer hares might allow grass and plants to increase a little. Because the deer-eagle branch of this particular food web is not directly connected to the hare-fox branch, the deer and eagle populations would not be directly affected by the hare decline in this simplified diagram — though a real ecosystem, with more interconnections than a partial food web shows, could well have additional links not pictured here.

Solutions written by the tuition.in editorial team and checked against NCERT Curiosity — Textbook of Science for Grade 8, Chapter 12 (hecu112.pdf), Reprint 2026-27, pages 190-209. Questions are referenced from the NCERT textbook for identification.