NCERT Solutions for Class 8 Science Chapter 12 How Nature Works in Harmony (PDF) – 2027

Because a forest that has shrunk and dried can no longer feed and water the elephants living in it, so they walk out to where food is.

The chapter sets the events out as a chain:

As a tree I would be a producer, linked to almost everything around me:

• Sunlight, water, air and soil: I use sunlight, water and carbon dioxide to make my food by photosynthesis and release oxygen; the soil holds me up and gives me nutrients.
• I change them too: my roots hold the soil and prevent erosion, my shade and fallen leaves keep it moist, and I help cool the air.
• Animals eat my leaves and fruits, pollinate my flowers, carry my seeds away and nest in my branches.
• Other plants compete with me for sunlight, water and space; an orchid may grow on my branch.
• Decomposers break down my fallen leaves and return the nutrients to the soil.

The Earth can thrive without humans; humans cannot survive without the Earth.

Plants, animals and decomposers kept the Earth’s ecosystems working long before humans appeared, and would go on doing so without us. But everything humans need — air, water, fertile soil, food, fibres, timber and medicines — comes from the Earth’s ecosystems. So when we overuse or misuse natural resources, we harm ourselves.

Over time the two birds are likely to stop overlapping so completely — each shifts a little in what it eats, where it feeds, or when it feeds.

For example:

• one feeds high in the canopy and the other on the lower branches;
• one feeds early in the morning, the other later in the day;
• one takes the small unripe fruits its beak can handle, the other the large ripe ones;
• one moves to a different part of the habitat, or adds insects and nectar to its diet.

Human actions can turn an ordinary natural event into a disaster, and can set off some disasters directly.

The chapter’s own example makes this clear. The Sundarbans protect us by slowing down strong winds and waves during storms and floods. Selvam’s village in Cuddalore district was less affected by the 2004 Tsunami than nearby villages because it still had its mangrove forest. Where the mangroves have been cut for fuelwood and farming, that shield is gone — the same wave now reaches houses and fields.

Frame questions that can actually be checked — by an observation, a count, or a survey. Some you could ask after reading the opening page:

• How wide does an elephant corridor have to be, and who decides where it runs?
• Which crops are damaged most in villages near a forest, and in which month?
• If a corridor is protected, do fewer elephants enter farms in the years that follow?
• Which animals other than elephants use the same corridor?
• How much fodder and water does one adult elephant need in a day?
• Which trees in our own area are cut most often, and what replaces them?

our group chose (i) the village pond and (ii) the old banyan tree beside the school wall.

A completed Table 12.1, taking the pond and the forest shown in Fig. 12.1:

Fill your own table with what you actually saw — the names will differ, the pattern will not.

Both habitats have living beings as well as non-living things. That is the similarity. But the types of living beings are different, and the non-living things vary too.

Because every organism needs specific conditions to survive, and different habitats offer different living conditions.

An organism can live only where its body actually works. A fish’s body is built to take oxygen that is dissolved in water, its shape is built to move through water, and its eggs must be laid in water so they do not dry out. Put it on land and none of these work. A deer’s body is built the opposite way — it takes oxygen from air, its legs are built for running on soil, and its skin would lose water very fast if it stayed submerged.

The pond supplies everything a fish needs — food, oxygen, shelter, and space to grow.

(one group’s 1 m × 1 m plot in the school garden):

Then compile the data from all four or five groups, so that the class has counts from several squares.

The blanks are for the names of the two kinds of plant you identified — Plant 1 and Plant 2 of your own table. With the sample counts above:

Two different numbers, two different populations, one habitat.

Not for long. A habitat with only one kind of organism would break down quite quickly.

If all the organisms are the same, they all have the same requirements — food, water, space. There is no sharing out of the habitat at all: every individual wants exactly what every other individual wants, at the same time and in the same place. That leads straight to competition and to a possible scarcity of resources.

Two further things, both of which you can see in real ecosystems:

• Disease would spread very fast. Individuals of one kind are alike, so a disease that can attack one can attack all of them. Nothing in the habitat would stop it. This is the same weakness the chapter later points out in monoculture, growing the same crop repeatedly on the same land.
• Numbers would swing wildly. With no predator and no competitor to check it, the population would multiply until it ate out its own food supply, and would then crash. The chapter makes this point directly: without competition, one species could multiply too much causing an imbalance in the ecosystem.

A likely reason: the fish are not affecting the plants directly at all — they are affecting the insects that pollinate them.

That is why pond A has a large number of flowering plants around it and pond B has fewer.

Yes — the relationship is inverse. Where dragonflies are many, bees and butterflies are few; where dragonflies are few, bees and butterflies are many.

Pond A has fewer dragonflies because fish eat dragonfly larvae. The dragonfly spends its early life in the water as a larva, and that is exactly where the fish can reach it. Fewer larvae survive, so fewer adult dragonflies emerge into the air.

It shows that organisms in a community are connected even when they never meet — and that a change in one population can travel through the community to reach a completely different kind of organism.

The study also shows that biotic components (fish, dragonflies, pollinators, plants) and abiotic components (temperature, water, nutrients) interact with and affect each other. So the answer to the section’s title question — does every organism in a community matter? — is yes: each one is a link that something else depends on.

Yes. Overfishing removes the fish, so the whole chain in Fig. 12.4 runs the other way.

Effect on the living parts:

Effect on the non-living parts: with fewer fish, algae and insect larvae can multiply. A thick layer of algae blocks sunlight, water plants die, less oxygen is produced and decay uses up more, so the water becomes cloudy and low in oxygen.

Fig. 12.5 shows a pond and its bank: the Sun and hills, a bird of prey overhead, reeds and water plants, a duck, fish (one leaping at an insect), frogs on floating leaves catching insects, a kingfisher catching a fish, a water snake eating a fish, microbes in the water, earthworms in the moist soil and water vapour rising. Sorting what happens there into the three criteria gives a completed Table 12.3:

(The first four rows are the book’s own examples, placed as in the book; the orange rows are further ones you can read off Fig. 12.5.)

Yes. Fig. 12.6 shows an overlap of different terrestrial and aquatic ecosystems in one landscape:

• a small river — an aquatic ecosystem;
• mountains, forest, grassland and farmland — terrestrial ecosystems;
• the farmland is a human-made ecosystem.

They do not sit in separate boxes. They meet along their edges and interact with each other at any given point — the river runs down out of the mountains, through the forest, past the grassland and along the farmland.

All nine organisms of Table 12.4 can be found in the forest habitat of Fig. 12.1(b): the deer and the hare grazing, a Bengal fox in the open, a mouse and a squirrel near the tree, a vulture overhead, a shikra perched on a branch, mushrooms on a dead stump, and the trees themselves.

Deer, hare, vulture, Bengal fox, bird (shikra), squirrel, mouse and mushroom — eight of the nine. Only the tree is an autotroph.

Sorted by what they eat:

• Herbivores (eat only plants): deer, hare
• Carnivores (eat only animals): vulture, shikra
• Omnivores (eat both): Bengal fox, squirrel, mouse
• Saprotroph (feeds on dead matter): mushroom

Plants make their own food by the process of photosynthesis. Because of this they are called producers or autotrophs (auto = self + troph = food).

By drawing an arrow from the organism that is eaten to the organism that eats it. A row of such arrows is a food chain.

Fig. 12.8 already shows Grass → Hare → Leopard. The organisms still to be linked are the grasshopper, the frog, the snake and the eagle. Add these arrows:

Put together with what is already there, the grassland now has two chains starting from the same producer:

The one the book itself gives:

Reading it as trophic levels:

Counting plot by plot across the nine squares of the crop field in Fig. 12.10(a):

A few millet plants are partly hidden by the tree, so a count between 50 and 55 is fine. What matters is the order: millet plants are the most, mice fewer, and there is only one eagle.

Ascending order of the counts: 1 (eagle) < 15 (mice) < about 54 (millets). Put the largest number at the base and the smallest at the top:

A pyramid — broad at the base and narrow at the top.

The millet plants (producers) are the most numerous and form the broad base; the mice (herbivores) are fewer and form the middle level; the single eagle (carnivore) sits at the narrow top. The number of organisms becomes smaller at each higher level of the food chain.

Fig. 12.11 already prints these arrows: Grasses → Hare, Grasses → Mouse, Hare → Hawk, Mouse → Fox, Mouse → Owl, Grasshopper → Frog, Grasshopper → Bird, Frog → Owl, Bird → Snake and Snake → Hawk.

Links that are missing, which you can add:

Many more than two. Take the mouse in Fig. 12.11: it eats grasses and is eaten by the fox and the owl, and — once you add the missing arrows — by the hawk and the snake too: five direct links from one animal. And that is only the part drawn; a real mouse also eats grains and insects, and is taken by cats as well.

Yes. In Fig. 12.11 you can trace at least four chains, and they share organisms at every level:

The grasses appear in all four; the hawk appears in two. Because each of the organisms may be eaten by two or more types of organisms, the chains join up into a network — a food web.

No. In nature, nothing is wasted — everything is reused.

What we would call waste — fallen leaves, dead animals, dung — is food for something else. Fungi like mushrooms and bacteria break down the complex substances in dead plants and animals into simpler ones, and beetles and flies do the same job on animal droppings such as elephant dung. This is decomposition, and the organisms that carry it out are decomposers or saprotrophs (sapro = rotten + troph = food).

in our area we saw these only in the winter months —

One change sets off a chain of others, and the last link is usually back on us. Activity 12.9 gives a real Indian example.

The Government of India banned the export of frog legs to prevent further ecological damage.

That organisms live together in three clearly different ways, and the difference lies in who gains and who loses:

Start from what is doing the damage. The chapter names it: deforestation, overuse of natural resources, the spread of invasive species, unsustainable land use, and pollution. Each has a matching action.

Yes — humans have created artificial ecosystems like fish ponds, farms, and parks to meet their needs. Around most schools you can find several:

• the school garden and the kitchen garden;
• an agricultural field, an orchard or a plantation;
• a village fish pond or a tank used for irrigation;
• a city park, a roadside avenue of trees, an aquarium.

Sample answer: the paddy field behind our school is a human-made ecosystem. Its biotic components are the paddy plants, weeds, frogs, spiders, dragonflies, earthworms and birds; its abiotic components are the water we let in, the soil, sunlight and temperature.

The practices in question are the ones the chapter names as unsustainable: overuse of synthetic chemicals, excessive groundwater extraction, and growing only one type of crop for commercial gain. Each harms the farm itself before it harms anyone else.

1. Which crops do you grow, and in which season?
2. Do you grow the same crop on the same field every year, or do you rotate crops?
3. Which fertilisers do you use — farmyard manure, compost, synthetic fertiliser? In what quantity?
4. Which pesticides do you spray, how many times in a season, and how do you decide when to spray?
5. What protection do you wear while spraying?
6. Do you use crop residue, cattle dung or kitchen waste to make compost?
7. Where does your irrigation water come from — canal, pond, or borewell? Has the water level changed?
8. Which birds, insects and earthworms do you see in your field now, compared with earlier?
9. What is your seed source — saved seed or bought each year?

Sample answer given by a farmer: “When I was young we sowed a mixture — bajra with moth bean — and we manured with cattle dung from our own animals. From about the 1990s I moved to a single crop, hybrid seed, urea and DAP, and a borewell, because the yield per bigha was higher and the market wanted one crop in quantity. Now I spray at least three times a season, which we never did earlier.”

Sample answer given by a farmer: “The crop greens up quickly after urea and the yield is good in the first years. But now I have to put more to get the same result. After spraying, the pest goes for a fortnight and comes back stronger, and the ladybird beetles and spiders that used to be in the crop are gone. There are fewer earthworms when I plough.”

Sample answer given by a farmer: “The soil has become hard and pale. It takes more water than before and dries fast. Earlier the top layer was dark and crumbly and full of earthworms; now after rain the topsoil washes off into the drain.”

Model inference: Synthetic fertilisers and pesticides have played a vital role in improving crop production and helped countries like India become food secure. But every farmer we met described the same slow change — more input needed for the same yield, pests returning stronger, fewer earthworms and pollinators, and topsoil washing away. So the gain came first and the cost came later, and the cost falls on the soil itself.

The inference that follows is the chapter’s own: understanding ecosystems can help us adopt better and more sustainable farming practices. Some farmers we met are already trying organic and natural methods, which aim to reduce synthetic inputs and keep interference with natural ecosystems to a minimum.

Practices that put back the organisms a farm was losing, so that the ecosystem does part of the work again:

The wrong statement is (iii) An ecosystem is part of a community.

It is the other way round. Fig. 12.19 shows three circles one inside another:

• A population is one kind of organism in a habitat at a given time — say all the grass plants in a field.
• A community is all the different populations sharing that habitat — the grass, the grasshoppers, the frogs, the microbes. So a community is larger than a population: statement (i) is correct.
• An ecosystem is that community together with the abiotic components it interacts with — the soil, water, sunlight and temperature. So a community is smaller than an ecosystem: statement (ii) is correct.

The forest would first fill up with dead matter, and then slowly starve.

Decomposers are essential because they break down dead plants and animals into simpler substances and return nutrients to the soil. Plants need these nutrients to grow, so without decomposers the producers — and every organism that depends on them — would suffer.

Selvam is right — the mangroves acted as a living wall between the sea and the village.

This is exactly what the chapter says about the Sundarbans: they protect us by slowing down strong winds and waves during storms and floods. Villages that had cleared their mangroves for fuelwood or ponds had nothing to slow the same wave, so they were hit harder.

Grasshoppers will increase and snakes will decrease. The frog is the link that joins them, and removing it cuts the chain in the middle.

Grasshoppers increase because the frog was their predator. With nothing eating them, more grasshoppers survive and breed, and their numbers climb. They then eat far more grass, so the grass itself begins to suffer — the effect passes downward as well.

Snakes decrease because the frog was their food. With less food, fewer snakes survive and breed — unless they can find other prey.

A butterfly needs two quite different things — nectar plants for the adult and host plants on which the caterpillar can feed. Fewer butterflies usually means one of these has gone.

Possible reasons

• Flowering plants were removed, or the garden was replanted with plants that give little nectar.
• Weeds and shrubs at the edges were cleared — those were the caterpillars’ host plants.
• Insecticide or pesticide was sprayed in the garden or on a neighbouring field; it kills caterpillars along with pests.
• The grass and hedges were mown just when caterpillars were feeding on them.
• No water and no bare damp soil for the butterflies to drink from.
• Heavy dust, smoke or a very dry season.

Steps students can take

Because an ecosystem needs all three groups to keep working.

• Without decomposers, dead plants would not break down. The nutrients in them would never return to the soil, so the soil would run out of nutrients and new plants could not grow.
• Dead plant matter would pile up and block light and space for new seedlings.
• Without consumers, many plants would not be pollinated or have their seeds dispersed.

So the producers themselves would slowly die out.

— a park and a roadside near our school:

How they differ: the park has soft moist soil, shade and water, so it has many kinds of plants, insects and birds, several interlinked food chains, and decomposers at work in the leaf litter. The roadside has tar, dust, heat and little shade, so only a few hardy species live there, and people keep disturbing it.

I agree with the statement.

They are necessary: farms feed our large population. Between 1950 and 1965 India faced a food crisis; tractors, machines, synthetic fertilisers and pesticides (the Green Revolution) increased food production and helped India become food secure.

They must be sustainable: overuse of synthetic chemicals, excessive groundwater extraction and growing one crop again and again (monoculture) degrade the soil, reduce friendly microbes, earthworms and pollinators, and make pests resistant. Compost and manure, crop rotation and mixed cropping, careful irrigation and organic or natural farming keep the field productive for the future.

Fig. 12.20 shows this web: the Sun feeds the grass and plants, which feed the hare and the deer; the hare is eaten by the fox and the deer by the eagle. So a fall in the hares moves along both branches.

Sample record from a one-hour clean-up of a school ground and the park beside it:

Most common: plastic wrappers, by a wide margin. They are light, they blow about, they do not decompose, and no decomposer can break them down — so they stay in the ecosystem indefinitely.

How to reduce it: carry food in a steel or cloth container instead of buying wrapped snacks; keep a refillable water bottle; put segregated bins where the litter actually appears (near the gate and canteen, not only near the office); ask the canteen to stop single-use plastic; compost the kitchen and garden waste in a school pit so that the part of the waste that can decompose actually does.

— communities across India that protect particular animals:

Sample record — a neem tree beside the school gate, March:

What to conclude: the bees came when the flowers opened and left when they faded, and the fruits appeared only after the flowering. That is pollination happening in front of you — the insects carry pollen from the stamens to the carpels, and only then do fruits and seeds form. Upload the same table to http://www.seasonwatch.in, where it joins observations of the same species from all over India.

Part 1 — indigenous practices farmers commonly describe: mixed cropping and crop rotation (a cereal with a pulse); farmyard manure, compost and green manure; saving seed from the best plants; neem leaves or neem cake, cow-dung slurry and ash against pests; trees and uncut borders around fields.

Part 2 — the garden: choose a sunny patch and make a compost pit; plant easy local herbs such as tulsi, mint, ajwain, lemon grass and aloe in mixed beds with marigold along the edges; mulch with dry leaves; use no synthetic fertiliser or pesticide; keep a register of watering, planting and visitors.

What Fig. 12.21 shows: a farm that is doing several things at once, not one thing on all its land — a cattle shed with a cow and fodder (dairy), a mustard field, a farmer ploughing with a pair of bullocks, fenced tomato beds, goats in a fenced pen, hens being fed near a poultry coop, and a fruit tree that an elderly farmer and a girl are harvesting into a basket. This is mixed farming: crops, trees and livestock together on the same holding.

Suggestions to note down: