Case Based Questions: How Nature Works in Harmony | Science Curiosity Class 8 - New NCERT PDF Download

Q1: Read the source below and answer the questions that follow:

On a fun school trip, students look at a small pond. They see fish swimming, frogs hopping on rocks, lotus plants on the water, and dragonflies flying in the sun. The water is clear, the sun shines in, and the muddy bottom helps plants grow. All these things help each other live well.

(a) Identify one biotic and one abiotic component in this pond habitat, and explain how they interact. (2 Marks)
(b) If this pond is an example of an aquatic ecosystem, what role do the lotus plants play? ​(1 Mark)​​​
(c) Predict what might happen if all the insects disappear from the pond. (1 Mark)

Ans:
(a) Biotic: Fish (living organism); Abiotic: Water (non-living). Fish interact with water by getting oxygen from it to breathe and using it as a space to swim and find food.
(b) Lotus plants are producers; they make their own food through photosynthesis and provide oxygen and shelter for other organisms like fish and frogs.
(c) Frogs might starve as they lose their food source (insects), leading to fewer frogs, and the balance in the pond could be disturbed with more small plants growing unchecked.

Q2: Read the source below and answer the questions that follow:

Students check out a big banyan tree in the park. It has long roots like a tree, squirrels jumping on branches, birds in nests, ants on the trunk, and mushrooms near the ground after rain. The tree gives cool shade, and wet soil has rotting leaves that bring bugs. One tree holds a whole tiny world of friends.

(a) Classify the squirrels and mushrooms as biotic components and explain their roles in this habitat. (2 Marks)
(b) How does the banyan tree demonstrate an overlap between individual, population, and community? ​(1 Mark)​​​
(c) If heavy rain washes away the soil around the tree, how might this affect the habitat? (1 Mark)

Ans:
(a) Squirrels are consumers (omnivores, eating fruits and insects); mushrooms are decomposers, breaking down dead leaves to return nutrients to the soil.
(b) The tree is an individual; multiple squirrels form a population; all organisms (squirrels, birds, ants, mushrooms) together form a community sharing the habitat.
(c) Soil erosion could expose roots, making the tree unstable, reducing moisture for mushrooms, and forcing animals like ants to move, disrupting the habitat's balance.

Q3: Read the source below and answer the questions that follow:

In a simple class test, students make two small garden spots (1m x 1m). Spot A has 15 grass plants, 5 ants, and 2 butterflies in sunny, wet ground that helps things grow; Spot B has 20 grass plants and 3 ants in shady, dry dirt with fewer visitors. After one week of counting, Spot A has more pretty flowers.

(a) What is the population of grass in each plot, and how does it relate to the community? (2 Marks)
(b) Identify an interaction between biotic components in Plot A. ​(1 Mark)​​​
(c) Suggest why Plot A might have more flowers, based on pollination. (1 Mark)

Ans:
(a) Plot A: Population of 15 grass plants; Plot B: 20. The community includes all populations (grass, ants, butterflies) interacting in the plot.
(b) Butterflies (biotic) interact with grass plants (biotic) by pollinating flowers, helping in reproduction.
(c) Butterflies in Plot A act as pollinators, carrying pollen between flowers, leading to more seed production and flowers, unlike Plot B.

Q4: Read the source below and answer the questions that follow:

People in the village hear fewer frog calls at night after using bug-killers on farms in rainy times. Students go see with jars and notes: dead bugs on top, fewer dragonflies over water, and green algae covering the sides, making a clean pond dirty. This breaks bug control, and farmers worry about the next crops.

(a) Explain how the decline in frogs shows a disruption in the food chain. (2 Marks)
(b) Classify frogs in the trophic levels of this pond ecosystem. ​(1 Mark)​​​
(c) What practical step could villagers take to restore balance? (1 Mark)

Ans:
(a) Frogs are consumers eating insects; fewer frogs mean more insects survive, leading to overgrowth of algae (as insects might eat less algae-eating organisms), disrupting the chain.
(b) Frogs are secondary consumers (carnivores) at the third trophic level, eating primary consumers like insects.
(c) Reduce pesticide use and promote natural predators like frogs by creating safe breeding areas to maintain pest control and ecosystem balance.

Q5: Read the source below and answer the questions that follow:

Students build a compost hole at school with leaves, fruit skins, veggie bits, and dirt, mixing it each week in the sun. After two weeks, they see worms digging, white fungus lines, and tiny germs turning junk to black dirt that smells nice and fresh. This fun way shows how nature turns waste into plant food.

(a) Identify the decomposers and explain their role in nutrient recycling. (2 Marks)
(b) How does this process link biotic and abiotic components? ​(1 Mark)​​​
(c) Predict what happens if decomposers are absent from the pit. (1 Mark)

Ans:
(a) Earthworms, fungi, and bacteria are decomposers; they break down dead matter into simpler nutrients for reuse by plants.
(b) Decomposers (biotic) interact with dead leaves (from biotic sources) and soil (abiotic) to release nutrients, enriching the soil for plant growth.
(c) Waste wouldn't break down, leading to the accumulation of dead matter, nutrient loss, and poor soil for plants, disrupting the cycle.

Q6: Read the source below and answer the questions that follow:

On a park walk with a guide, students watch an elephant family use a safe path between green forests to get water, staying away from village homes. They see stepped-on grass, poop balls with seeds to grow new plants, and birds eating bugs jumped up by big feet. This path lets animals travel freely without fights with people.

(a) Explain how the corridor helps maintain harmony in the ecosystem. (2 Marks)
(b) Identify a mutualism interaction involving elephants. ​(1 Mark)​​​
(c) What might happen if the corridor is blocked by a road? (1 Mark)

Ans:
(a) The corridor connects habitats, allowing elephants to move safely for food and water, reducing conflicts with humans and preserving forest balance.
(b) Elephants disperse seeds through dung, benefiting plants (mutualism: elephants get food, plants get spread).
(c) Elephants might enter villages, damaging crops and causing conflicts, leading to habitat loss and imbalance in wildlife populations.

Q7: Read the source below and answer the questions that follow:

Students draw a bright grassland on a trip, with yellow grass in the wind, jumping grasshoppers, calling frogs in puddles, sliding snakes, and flying hawks above. They add lines for eating: grasshoppers chew grass, frogs catch grasshoppers, snakes grab frogs, hawks hunt snakes or frogs. This drawing shows how all connect to keep the grass happy.

(a) Differentiate between a food chain and a food web in this sketch. (2 Marks)
(b) What trophic level does the hawk occupy, and why? ​(1 Mark)​​​
(c) If a disease kills all frogs, how does the food web change? (1 Mark)

Ans:
(a) A food chain is a single path (e.g., grass → grasshopper → frog → snake); a food web is an interconnected chain showing multiple feeding relationships.
(b) Hawk is a top carnivore at the fourth or fifth trophic level, as it eats secondary or tertiary consumers like snakes.
(c) Grasshoppers increase (fewer predators), snakes decrease (less food), and hawks might eat more grasshoppers or starve, shifting the balance.

Q8: Read the source below and answer the questions that follow:

Students ride a boat in the Sundarbans tree home, where twisty roots hide crabs in the mud and fish from hunters, with tall birds in branches. Guides say these trees catch big storm waves to save beach villages from a water rush. But people cut trees for firewood, hurting this strong wall home.

(a) How do mangroves demonstrate the benefits of an ecosystem? (2 Marks)
(b) Identify a commensalism interaction in the mangroves. (1 Mark)
(c) Suggest an exciting way students can help protect this ecosystem. (1 Mark)

Ans:
(a) Mangroves provide protection from storms (absorb waves), clean air (absorb CO2), and habitat for animals, benefiting humans and nature.
(b) Birds nesting in trees (benefit from shelter) while trees are unaffected (commensalism).
(c) Plant mangrove saplings in a school project or create awareness posters showing how mangroves prevent floods, encouraging community involvement.

Q9: Read the source below and answer the questions that follow:

On sunny farm day, students see two lands: one with home-made dirt food full of worms digging and ladybugs eating bad bugs on tomatoes, the other with chemical sprays having few bugs, hard, dry ground, and sad plants. Home-made land plants grow strong and full, giving more food without hurting the earth. Farmers say this way keeps dirt happy long time.

(a) Explain how earthworms contribute to sustainable farming. ​(1 Mark)​​​
(b) What type of interaction do ladybugs show with pests? (1 Mark)
(c) Predict long-term effects of chemical use on the ecosystem. (2 Marks)

Ans:
(a) Earthworms are decomposers; they break down organic waste into nutrient-rich soil, improving fertility without chemicals.
(b) Predation: Ladybugs (predators) eat pests, controlling their population naturally.
(c) Chemicals kill beneficial organisms, reduce soil fertility, increase pest resistance, and pollute water, harming the overall farm ecosystem.

Q10: Read the source below and answer the questions that follow:

During a chilly winter dawn, students spot migratory birds like graceful cranes landing in a misty wetland, their wings folding as they wade through shallow waters foraging for insects and roots. They observe the birds not only eating but also shaking off water droplets that scatter seeds across the marsh and brushing pollen onto nearby reeds while feeding.

(a) How do migratory birds link different ecosystems? ​(1 Mark)​​​
(b) Classify the birds' role in the food web. (1 Mark)
(c) What observational activity could students do to help conserve these birds? (2 Marks)

Ans:
(a) Birds migrate between habitats, carrying seeds and controlling pests, connecting ecosystems like wetlands and forests.
(b) Birds are consumers (omnivores or carnivores), acting as pollinators, seed dispersers, and predators in the web.
(c) Set up a bird-watching journal, count birds weekly, note changes, and report pollution to authorities to promote conservation.