NCERT Solutions Class 8 Science Chapter 12 How Nature Works in Harmony

Question 1. Refer to the given diagram (Fig. 12.19) 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.
Answer: The incorrect statement is (iii). An ecosystem is part of a community is wrong because the relationship works the opposite way. A community forms part of an ecosystem, not the other way around. When you combine a community (all the living things) with the non-living parts of the environment (like soil, air, water, and sunlight), you create an ecosystem. So the correct relationship is: Ecosystem = Community (all living organisms) + Abiotic components (non-living factors).
In simple words: Statement (iii) is wrong. A community is inside an ecosystem, not the other way around. When you add living things to soil, water, and air, you get an ecosystem.

Exam Tip: Always remember the order from smallest to largest: Population - Community - Ecosystem. This helps you answer questions about which contains what.

Question 2. A population is part of a community. If all decomposers suddenly disappear from a forest ecosystem, what changes do you think would occur? Explain why decomposers are essential.
Answer: If decomposers vanish from a forest ecosystem, several serious changes would take place. Dead plants and animals would pile up on the forest floor instead of being broken down. The important nutrients trapped in these dead organisms would stay locked away and not return to the soil. As soil fertility drops sharply, plants would struggle to find the nutrients they need to grow. This breakdown in the nutrient cycle would eventually disturb the entire food chain and throw the ecosystem balance into chaos. Decomposers matter so much because they break down dead material and put nutrients back into the earth, keeping nature's recycling system working smoothly.
In simple words: Without decomposers, dead leaves and animals pile up and do not turn back into soil. Plants cannot get food from the earth, so nothing can grow, and the whole forest falls apart.

Exam Tip: When answering about decomposers, mention both nutrient recycling AND the buildup of dead matter - examiners look for both aspects to award full marks.

Question 3. Selvam from Cuddalore district, Tamil Nadu, shared that his village was less affected by the 2004 Tsunami compared to nearby villages due to the presence of mangrove forests. This surprised Sarita, Shabnam, and Shijo. They wondered if mangroves were protecting the village. Can you help them understand this?
Answer: Yes, mangroves were indeed protecting the village from the tsunami's impact. Mangrove trees work as a natural shield between the ocean and the land. When powerful storms and tsunamis hit, these forests slow down the force of fierce winds and massive waves before they reach human settlements. The dense network of roots beneath the mangroves grips the soil tightly and stops the ground from washing away during these natural disasters. By acting as both a barrier and an anchor, mangroves keep coastal villages safer from the destructive power of water-related catastrophes and natural calamities.
In simple words: Mangrove forests are like a wall that stops big waves and stops the ground from washing away. They keep villages safe from tsunamis and storms.

Exam Tip: Mention three specific roles - barrier against waves, slowing wind force, and root anchoring of soil - to show complete understanding of how mangroves protect coastal areas.

Question 4. Look at this food chain: Grass → Grasshopper → Frog → Snake. If frogs disappear from this ecosystem, what will happen to the population of grasshoppers and snakes? Why?
Answer: If frogs vanish from this ecosystem, the grasshopper population will grow rapidly because nothing is eating them anymore. At the same time, the snake population will drop sharply because snakes lose their primary food supply and go hungry. This sudden imbalance in the food chain breaks down the natural order and upsets the entire ecosystem. The removal of even one species in the middle of the food chain creates a ripple effect that harms other organisms connected to it, showing how delicate and interconnected food chains really are.
In simple words: Without frogs, grasshoppers multiply because no one eats them. Snakes starve because they lose their food. The food chain falls apart.

Exam Tip: Always explain BOTH the predator and prey effects when a middle species is removed - this shows you understand food chain connections.

Question 5. In a school garden, students noticed fewer butterflies the previous season. What could be the possible reasons? What steps can students take to have more butterflies on campus?
Answer: Several factors could explain why the butterfly population dropped. The school garden might have fewer flowering plants now, which means butterflies have less nectar to drink. Chemical pesticides used on the grounds kill insects directly or poison the plants butterflies depend on. Changes in weather patterns or loss of suitable living spaces can also drive butterflies away. To bring more butterflies back to campus, students should plant more colorful flowers and flowering shrubs that butterflies love to visit. Stop using chemical sprays and switch to natural, insect-safe methods instead. Keeping the garden clean, green, and natural - with plenty of shade and shelter - creates a welcoming home where butterflies want to live and lay their eggs.
In simple words: Butterflies need flowers for food and a safe place to live. Plant more flowers, stop using poison sprays, and keep the garden natural.

Exam Tip: Provide both reasons (why butterflies are missing) and solutions (how to attract them) in equal detail - questions asking "Why?" and "What steps?" expect both parts answered fully.

Question 6. Why is it not possible to have an ecosystem with only producers and no consumers or decomposers?
Answer: An ecosystem made up of only producers and nothing else cannot survive or function properly. Producers like plants create food through photosynthesis, yet without consumers around to eat that food, all the plant material simply sits there unused and wasted. Even more critically, without decomposers present, dead plant matter would heap up in piles and never break down. The nutrients locked in these dead organisms would remain stuck and never cycle back into the soil for new plants to use. Therefore, all three groups - producers, consumers, and decomposers - are absolutely needed together to keep an ecosystem balanced, healthy, and running smoothly.
In simple words: Plants make food but animals need to eat it. Dead plants must rot and turn into soil, or nothing new can grow. You need all three: plants, animals, and decomposers.

Exam Tip: Explain the role of each group separately (producers make food, consumers use it, decomposers recycle it) - this three-part structure is what marks look for.

Question 7. 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?
Answer: When you observe a park, you will find living organisms such as large trees, grass, birds, insects, and squirrels. The non-living parts include soil, water bodies, sunlight, and fresh air. In contrast, a roadside ecosystem contains far fewer plants - just some hardy weeds - along with stray dogs, crows, and ants. The non-living elements here are mostly concrete, dust clouds, passing vehicles, and intense heat. The park ecosystem shows much greater biodiversity with more species thriving together, and the environment feels more balanced and natural. The roadside ecosystem has little greenery and experiences more pollution from vehicles and human activity, making it less suitable for diverse life forms. These differences show how human activity shapes the health and variety of life in different places.
In simple words: A park has many trees, animals, and water. A roadside has concrete, dust, and cars. Parks are healthier with more living things. Roads are dry and polluted.

Exam Tip: When comparing two ecosystems, make a clear list for each one separately, then write out the differences - this organized format earns more marks than mixing the information together.

Question 8. 'Human-made ecosystems like agricultural fields are necessary, but they must be made sustainable.' Comment on the statement.
Answer: This statement correctly points out a real tension in how we farm. Human-made ecosystems like farms and orchards are absolutely vital because they produce the food and raw materials we need to survive. However, many traditional farming methods rely on excessive chemical sprays, grow only one crop repeatedly, and pull too much water from the earth - all of which damage the soil, harm people's health, and hurt the natural environment. To build sustainable farms that will last for many years without destroying nature, farmers must shift toward organic and natural methods instead of chemicals. They should rotate different crops through their fields each season to keep soil healthy. They must also protect water by using irrigation wisely rather than wasting it. Making these changes ensures that farms keep producing food today while safeguarding the earth and water for future generations to farm and live on.
In simple words: We need farms to grow food, but old farming methods poison the soil and water. We should farm in ways that keep the land healthy forever.

Exam Tip: Address both parts - why farms are needed AND what makes them unsustainable AND what solutions exist - to fully answer a "comment on the statement" question.

Question 9. If the Indian hare population (Fig. 12.20) drops because of a disease, how would it affect the number of other organisms?
Answer: A sudden drop in the hare population due to disease would ripple through the entire food web and cause dramatic changes in other species. Grass and plants would increase sharply because fewer hares are there to graze and eat them down. The predators that hunt hares - such as foxes and eagles - would face a severe food shortage and their numbers would fall as they go hungry and struggle to survive. Other prey animals in the ecosystem, particularly deer and mice, would face increased pressure from these hungry predators shifting their diet toward them as an alternative food source. This example demonstrates clearly that the loss of even one species sends shockwaves throughout the entire food web, affecting almost every other organism that shares the same ecosystem through direct and indirect connections.
In simple words: If hares die, plants grow more because nothing eats them. Foxes and eagles starve because they lose their food. Deer and mice get hunted more. Everything in the food web is connected.

Exam Tip: When describing food web effects, trace the chain upward (predators suffer), downward (prey increase), and sideways (other species face new pressure) - showing you understand all the connections, not just one direction.