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

F R E E S T U D Y M AT E R I A L F O R E V E R Y S T U D E N T

CLASS 8 · SCIENCE

NCERT Solutions

Chapter 12: How Nature Works
in Harmony

NCERT Textbook — Curiosity

BOOK PAGES SECTIONS QUESTIONS MEDIUM

190 – 209 22 60 English

Solutions, notes, sample papers & more at 65 pages

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

CLASS 8 · SCIENCE · CURIOSITY

NCERT Solutions — Chapter 12: How Nature Works in
Harmony
Elephants walking out of a shrinking forest into a sugarcane field are not behaving strangely — they are
following food and water. This chapter follows that thread from a single habitat to a whole ecosystem, and
shows how energy and nutrients travel through a food web, and why pulling out one species is felt all the way
along it.

TEXTBOOK BOOK PAGES

Curiosity (Class 8) 190 – 209

SECTIONS QUESTIONS

22 60

MEDIUM

English

Probe and ponder — Page 190
Chapter opener — Elephant Corridor

PROBE AND PONDER

Q1 How might the loss of forest cover and changes in rainfall patterns lead to
elephants to enter human farms and villages?

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:

Rainfall and temperature change → vegetation is affected

Trees cut for roads and buildings → forests dry and shrink

Waterholes dry up → loss of habitat

Animals move into human habitats → crop damage, and at times harm to people and

domestic animals

An elephant is a very large herbivore, so it needs a very large amount of fodder and water every
single day. A farm or plantation offers exactly what a thinning forest no longer does — bananas
and sugarcane standing close together, easy to reach. So the elephants are not straying at

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

random; they are following the food.

Why it happens: elephants are adapted to forest life. Adaptation is slow — it builds
up over many generations. Felling and a shifted monsoon change the forest in a few
years, far faster than any adaptation can keep up with, so the animals must move
instead. This is why wildlife ecologists identify and mark corridors: strips that
connect one forest patch to the next, so elephants can travel between large forest
areas without coming into contact with human settlements.

Q2 Imagine you are a tree in a dense forest. What kind of relationships would you have
with water, sunlight, other animals, and other components of the forest?

As a tree I am a producer, so almost every relationship in the forest either feeds me or feeds off
me.

With sunlight, water and air (abiotic → biotic): I use sunlight, carbon dioxide and water to
make my own food by photosynthesis, and release oxygen. Soil gives me a medium to stand
in and the mineral nutrients I need.
With the soil and the air (biotic → abiotic): my roots hold the soil in place and prevent
erosion, my shade and fallen leaves keep the soil moist, and the water I release helps cool
the air around me. So I change the non-living parts as much as they change me.
With animals: deer and hares browse my leaves and seedlings; insects, birds and bats visit
my flowers and pollinate them; birds and squirrels eat my fruits and drop the seeds far away,
which is how my seedlings reach new ground; birds nest in my branches.
With other plants: I compete with neighbouring trees for sunlight, water and space — that
competition is why forest trees grow tall and straight. An orchid may sit on my branch and
take only support (commensalism), while a parasitic climber takes food from me and harms
me (parasitism).
With microorganisms: when my leaves fall and when I finally die, fungi and bacteria break
me down and return my nutrients to the soil, where the next tree will use them.

Tip: notice that the tree sits at the first trophic level. Every animal relationship above
is really about energy that the tree first captured from the Sun.

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

Q3 Do you think the Earth can thrive without humans? Can humans survive without
the Earth?

The Earth can thrive without humans; humans cannot survive without the Earth.
Look at where the energy in any ecosystem comes from. It enters only at the producers, from
sunlight, and then moves upward through consumers and back through decomposers into the
soil. Humans are consumers. We take from that flow — we do not add anything to it. Remove us
and the flow still closes on itself.
Now reverse the question. Everything we need is a benefit produced by ecosystems: forests give
us fresh air, fertile soil, food, fibres, timber and medicines; aquatic ecosystems give us water and
food; and ecosystems also give us places of beauty and recreation. None of these can be
manufactured from nothing.

Why it happens: we are a part of the web, not outside it. That is exactly why the
chapter warns that when we overuse or misuse natural resources, we disturb the
balance in nature — the damage travels back to us through the same connections
that supply us.

Q4 If two kinds of birds compete for the same fruit, how might their way of living
change over time?

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.
The reasoning is about supply and demand. One fruit crop can support only so many birds. If
both kinds want the same fruit at the same time and place, each gets less; a bird that gets less
food raises fewer young, so its numbers fall. The pressure eases only for a bird that finds a way
of feeding the other is not using. Possible changes:

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.

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Class 8 Science Chapter 12 How Nature Works in Harmony
a g l AglaSem · NCERT Solutions

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Why it happens: the chapter says that competition for shared resources helps

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Q5 Can human actions cause natural disasters?

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Human actions can turn an ordinary natural event into a disaster, and can setemoff some disasters
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less affected by the 2004 Tsunami than nearby villages because it still had its mangrove forest.
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Where the mangroves have been cut for fuelwood and farming, that shield is gone — the same
wave now reaches houses and fields.
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The same logic runs through thegrest

Cut the trees on a slope and the roots no longer hold the soil in place, so ordinary rain now
brings landslides and heavy silt into rivers.
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Overuse synthetic fertilisers and the humus that binds soil particles falls;

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Harvest frogs on a large scale and agricultural pests rise — a crop failure caused entirely by
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Did you know? The event (storm, wave, rain) is natural. Whether it becomes a
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disaster often depends on whether the ecosystem that used to absorb it is still there.

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agl Frame questions that can actually be checked — by an observation, a count, or a survey. Some
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How wide does an elephant corridor have to be, and who decides where it runs?

a s em in villages near a forest, and in which month?
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Which crops are damaged most
If a corridor is protected,
Which animals other than elephants use the same corridor?

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

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?

Try This: write each question on a slip, and next to it write how you would find the
answer — a count, a map, an interview, or a library search. A question you cannot
check is only an opinion.

Activity 12.1: Let us explore — Page 191
12.1 How Do We Experience and Interpret Our Surroundings?

ACTIVITY

Q1 Identify two habitats in your surroundings. These could be any two of the following:
a pond, a forest, an agricultural farm, or even a large tree like banyan, mango, or
pilkhan (white fig) tree.

Pick two habitats that are clearly different in their non-living conditions — that is what makes the
comparison worth doing. A village pond and a mango tree, or a school garden and a roadside,
work well.
Sample answer: our group chose (i) the village pond and (ii) the old banyan tree beside the
school wall.
Before you go, decide what you will look at, so that two groups record comparable things:

the surface and the edges, not just the middle;
under stones, under bark and under leaf litter — most small organisms are hidden;
things you can only feel or measure: how warm it is, how moist the soil is, how much
sunlight reaches the ground.

Caution: explore the habitat in groups with your teacher. Do not enter deep water,
and do not disturb nests, burrows or the animals themselves — put every stone back
the way you found it.

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

Q2 List the living beings and non-living things that you observe in these habitats.
Record your observations in Table 12.1.

Table 12.1: Different components of two habitats

POND FOREST

LIVING NON-LIVING LIVING NON-LIVING
BEINGS THINGS BEINGS THINGS

Fish Water Grass Soil

Trees

Birds

Table 12.1, page 192 — reproduced as printed; the blank cells are for your own
observations.

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

POND FOREST

LIVING BEINGS NON-LIVING LIVING BEINGS NON-LIVING
THINGS THINGS

Fish Water Grass Soil

Duck Sunlight Trees Sunlight

Turtle, frog Air (dissolved oxygen) Birds (vulture, shikra) Air

Dragonfly, snail Mud and stones at the Deer, hare, fox, mouse, Rocks and stones
bottom squirrel

Lotus, water lily, reeds, Temperature Mushrooms on the dead Temperature
algae stump

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

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

Why it happens: both lists have two columns for a reason. The living beings are the
biotic components of the habitat and the non-living things are the abiotic
components. Neither list makes sense alone: the lotus needs the water and the
sunlight, and the water in turn is kept cool and clear partly by the plants growing in
it.

In-text Questions — Page 192
12.1 How Do We Experience and Interpret Our Surroundings?

Q1 What common characteristics do you observe in the two habitats in Activity 12.1?

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.

POND FOREST

Same in both Biotic components + abiotic components, interacting

Biotic components Fish, frogs, algae, lotus Trees, grass, deer, fox, mushroom

Abiotic Water is the medium; oxygen dissolved in Air is the medium; soil holds the
components it water

Why it happens: every habitat must supply the same four things — food, oxygen,
shelter and space to grow. What differs is how it supplies them. A fish takes its
oxygen from water, a deer from air. So the same requirement is met in two different
ways, and that is why the two habitats hold two different sets of organisms.

Q2 Have you wondered why some organisms live on land while others live in water?

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

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

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.

Why it happens: organisms adapt to survive in the surrounding conditions they live in.
Adaptation is a fit between a body and a set of conditions. Change the conditions
and the fit is lost — which is exactly what happens to elephants when a forest dries
out.

Q3 How do fish survive in a pond?

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

Biotic needs — food → from small plants and animals in the pond

Abiotic needs — oxygen → from the water (oxygen dissolved in it)

Shelter → weeds, reeds and the lotus leaves

Space → the volume of the pond itself

Notice how the two kinds of need are met by two different kinds of component. That is the
whole point of the activity: a fish does not survive because of the water alone, or because of the
algae alone, but because the biotic and abiotic components of the pond work together.

Check it yourself: ask what would happen if the pond's plants died. Less oxygen
would be produced in the water — and the fish would be in trouble even though the
water is still there. You will meet exactly this chain again in Fig. 12.13.

Activity 12.2: Let us record — Page 193

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Class 8 Science Chapter 12 How Nature Works in Harmony
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12.2 Who All Live Together in Nature?
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ACTIVITY

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a s em an area of 1 m × 1 m in your school garden. Identify four organisms in this
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area, and count their numbers. Record the number of the organisms in Table 12.2.

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Table 12.2: Number of particular
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NAME OF ORGANISM agPOPULATION (NUMBER OF INDIVIDUAL ORGANISMS)

Plant 1 : 20
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Plant 2 : 05

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Table 12.2, page 193 — reproduced as printed; fill in the names and the two missing
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Animal 1: Ant 32

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

Why it happens: the size and the time are both fixed on purpose. A count only
means something if you say how much area and when. Two groups counting
different-sized patches, or counting ants at noon and at dusk, cannot compare their
numbers at all. That is why a population is defined as a group of the same kind of
organisms in a habitat at a given time.

Q2 In the given example, there is a population of 20 ______ plants and is only 5 ______
plants in the same 1 × 1 m2 area.

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:

There is a population of 20 grass plants and only 5 marigold plants in the same 1 × 1 m2
area.

Two different numbers, two different populations, one habitat.

Why it happens: the two plants are not equally common because they do not have
equally good conditions. Grass spreads by runners and tolerates trampling and
shade, so it fills the square; marigold has to grow from seed and needs more light
and space, so far fewer of it fit in. Comparing populations in the same square is how
you begin to see which conditions each kind of organism actually needs.

In-text Questions — Page 194
12.2 Who All Live Together in Nature?

Q1 Can a habitat have only one type of living organism? What might happen then?

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.

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

Worse, the jobs that other organisms do would not be done:

if the one kind were a plant, there would be no pollinators, so few fruits and seeds would
form;
if it were an animal, there would be no producer to feed it at all;
with no decomposers, the dead bodies would pile up and their nutrients would never return
to the soil.

Why it happens: a community works because its different populations need different
things and do different jobs. One kind of organism cannot be producer, consumer and
decomposer at once, so the flow of energy and the recycling of matter both stop.

Q2 What else do you think could happen?

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.

Did you know? The soil itself would change. With no root variety and no
decomposers, the soil would lose its humus, hold less water, and slowly stop
supporting even the one species that remained.

Activity 12.3: Let us read — Pages 194–195

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

12.3 Does Every Organism in a Community Matter?

ACTIVITY

Q1 Researchers conducted a study to see how fish in ponds affect seed production in
the plants nearby. They observed two ponds — A with fish and large number of
flowering plants around it; B without fish and fewer flowering plants around it (Fig.
12.3). Think of a reason for these observations.

Pond A Pond B
fish present · many flowering plants no fish · many dragonflies · few flowering plants

Fig. 12.3, page 194 — redrawn sketch: Pond A with fish and Pond B without fish.

A likely reason: the fish are not affecting the plants directly at all — they are affecting the
insects that pollinate them.
Trace the links. Fish eat dragonfly larvae, which live in the water. Dragonflies (the adults) eat
flies, bees and butterflies, which live in the air above the pond and visit the flowers around it. So:

Pond A: fish present → fewer dragonfly larvae → fewer dragonflies → more bees, flies and

butterflies → more pollination → more seeds

Pond B: no fish → dragonfly larvae survive → more dragonflies → fewer bees and

butterflies → less pollination → fewer seeds

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

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

Tip: this is what Fig. 12.4 shows. The red arrows are direct effects (fish on larvae,
dragonfly on bee); the green arrows are indirect effects (fish on bees, fish on the
plant). An indirect effect works through a third organism.

Q2 Compare the number of dragonflies, bees, and butterflies in both the ponds. Do you
find any relationship between the number of dragonflies and bees/butterflies? We
observed that in Pond A (with fish) the number of dragonflies were less as
compared to Pond B. Why?

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 (WITH FISH) POND B (WITHOUT FISH)

Dragonflies Fewer More

Bees, flies, butterflies More Fewer

Seeds set by nearby flowers More Fewer

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.

Why it happens: the dragonfly is a predator of bees and butterflies. Take away a
predator and its prey increases — so bees, flies and butterflies become more
common in pond A. This is the same rule you will use again in Question 4 of Keep the
curiosity alive, where frogs disappear from a food chain.

Q3 What does this study show? How does the population of fish in a pond affect the
seed production in nearby plants?

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.
A fish never touches a flower. Yet the number of fish in the pond decides the number of seeds
the flowers set, through a chain of three steps:

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Class 8 Science Chapter 12 How Nature Works in Harmony
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More fish → fewer dragonfly larvae
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→ fewer adult dragonflies
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→ more bees, flies and butterflies
a→ more pollen carried from flower to flower

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→ more seeds produced

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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

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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.
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Didemyou know? An effect that passes through another organism, like the fish's effect
a gl on the flowers, is called an indirect effect. Indirect effects are often stronger than
the direct ones, and they are the reason removing a single species can change an
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ecosystem in ways nobody expected.

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Similarly, can overfishing by humans change this balance? How do you think it may
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affect the living and non-living parts of the habitat?
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Fewer fish → more dragonfly larvae survive → more adult dragonflies

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→ less pollination → fewer seeds and fruits on the plants around the pond
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→ fewer plants in the next season → less food and shelter for other pond animals
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Effect on the non-living parts: a thick mat of algae cuts off sunlight to the plants below. Those
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plants die, and dying plants mean less oxygen produced in the water. Decomposing the dead
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Class 8 Science Chapter 12 How Nature Works in Harmony AglaSem · NCERT Solutions

Why it happens: the balance of an ecosystem is dynamic, not fixed. It is held by
the interactions among organisms, not by any one organism. Taking out a whole
population at once removes several interactions together, and the ecosystem settles
into a new and usually poorer state.

Activity 12.4: Let us relate and identify — Page 196

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

12.4 What Are the Different Types of Interactions Among Organisms and their Surroundings?

ACTIVITY

Q1 Based on the given criteria, identify and describe the interactions between biotic
and abiotic components shown in Fig. 12.5. Criterion 1 — Interactions between
abiotic and biotic components. These may influence life processes like nutrition,
respiration, and reproduction in biotic components. Criterion 2 — Interaction
between two abiotic components these may influence the physical characteristics
of a habitat. Criterion 3 — Interaction among the biotic components. These may
influence the availability of resources needed for life processes like nutrition,
respiration, and reproduction.

Kite (bird of prey) Sun

Water vapour

Kingfisher
Reeds

Microbes Insect

Duck

Fish

Aquatic plants Water snake
Frog
Earthworm in moist soil

Fig. 12.5, page 195 — redrawn sketch of the pond scene showing biotic and abiotic
interactions.

Fig. 12.5 shows a pond with its bank: sunlight and hills above, water with a duck, fish, frogs on
lily pads, a turtle, a dragonfly, a kingfisher on a rock, a water snake and water plants, and moist
soil with an earthworm below. Sorting what happens there into the three criteria gives a
completed Table 12.3:

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

CRITERION 1: CRITERION 2: CRITERION 3:
INTERACTIONS BETWEEN INTERACTION BETWEEN INTERACTION AMONG THE
BIOTIC AND ABIOTIC TWO ABIOTIC BIOTIC COMPONENTS
COMPONENTS COMPONENTS

Earthworms live in moist soil. The day temperature is high A frog eats insects.
due to the bright sunlight.

Many microbes are present in the Water is evaporating fast due A water snake eats fish.
pond. to the sunlight.

A fish lays eggs in water. Air current is blowing slowly Frogs and fish may compete
on the water surface creating for small insect larvae.
gentle waves.

The soil near the pond is moist. A fish lays eggs in water near
vegetation to protect them
from other fish or frogs.

Water plants use sunlight, water and Sunlight warms the upper layer The kingfisher dives and catches a
carbon dioxide to make food. of water more than the deeper fish.
water.

The fish takes in oxygen dissolved in Wind and warmth together make The duck feeds on water plants
the water. the pond lose water, so it becomes and small animals.
shallower.

The frog sits on a floating leaf, out of Air passing over the water The dragonfly hunts small flying
the water, to breathe air. surface helps oxygen dissolve into insects above the water.
it.

(The first four rows of each column are the book's own examples; the orange rows are further
ones you can read off Fig. 12.5.)

Why it happens: the three criteria are really three different questions. Criterion 1
asks how does a non-living thing let a living process happen? — sunlight allows
photosynthesis, dissolved oxygen allows respiration, water allows the eggs to
develop. Criterion 2 asks how do two non-living things change the habitat itself? —
sunlight plus wind change temperature, water level and how much oxygen the water
holds. Criterion 3 asks how do organisms change what is available to each other? — by
eating, by competing, or by using a place for shelter. Together these three are what
an ecosystem is.

In-text Questions — Page 197

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

12.4 What Are the Different Types of Interactions Among Organisms and their Surroundings?

Q1 Can you find overlapping ecosystems in Fig. 12.6?

Mountains

Houses

Forest

Cattle

Farmland
Students observing with their teacher

Grassland River (aquatic ecosystem)

Fig. 12.6, page 197 — redrawn sketch: a small river (an aquatic ecosystem) running past
mountains, forest, grassland and farmland (terrestrial ecosystems).

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.

Why it happens: ecosystems have no walls. Water, nutrients and organisms cross
from one to the next: rain falling on the mountain becomes the river; soil washed off
the farm ends up in the river; a bird that nests in the forest feeds over the grassland.
That is also why ecosystems can be very large or very small — a whole forest is one,
and so is a single banyan tree inside it.

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Class 8 Science Chapter 12 How Nature Works in Harmony
a g l AglaSem · NCERT Solutions

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Activity 12.5: Let us classify — Pages 197–198
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Class 8 Science Chapter 12 How Nature Works in Harmony AglaSem · NCERT Solutions

12.4 What Are the Different Types of Interactions Among Organisms and their Surroundings?

ACTIVITY

Q1 Study the picture carefully and spot the organisms listed in Table 12.4. Using the
internet or your school library, find out what do these organisms eat. Record your
observations in Table 12.4 by identifying whether each organism feeds only on
plants and plant products, only on animals, or on both.

Vulture

Bird (Shikra)

Tree

Deer

Bengal fox

Hare
Squirrel
Mouse
Mushrooms on a dead stump

Fig. 12.1(b), page 192 — redrawn sketch of the forest habitat: the nine organisms of
Table 12.4 are all in this picture.

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

Table 12.4: Eating habits of different organisms

NAME OF PERFORMS FEEDS ON FEEDS ON FEEDS ON
THE PHOTOSYNTHESIS PLANTS ANIMALS BOTH
ORGANISM AND PLANT PLANTS
PRODUCTS AND
ANIMALS

Deer No Yes — Grass No No — Only
and leaves of on plants
plants

Hare

Vulture

Bengal Fox

Bird (Shikra)

Squirrel

Mouse

Mushroom

Tree Yes

Table 12.4, page 198 — reproduced as printed; only the Deer row and the ‘Tree — Yes’ cell
are filled in the book.

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.

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

NAME OF PERFORMS FEEDS ON FEEDS ON FEEDS ON
THE PHOTOSYNTHESIS PLANTS AND ANIMALS BOTH PLANTS
ORGANISM PLANT AND
PRODUCTS ANIMALS

Deer No Yes — grass and No No — only on
leaves of plants plants

Hare No Yes — grasses, No No — only on
tender shoots, bark plants

Vulture No No Yes — flesh of No — only on
dead animals animals

Bengal Fox No Yes — fruits Yes — Yes — omnivore
rodents,
insects, birds

Bird (Shikra) No No Yes — lizards, No — only on
small birds, animals
insects

Squirrel No Yes — nuts, Sometimes Yes — mainly
seeds, fruits insects and plants
eggs

Mouse No Yes — grains and Yes — insects Yes — omnivore
seeds

Mushroom No Neither — it grows on dead plants and animal matter and
breaks it down (a decomposer)

Tree Yes No No No — it makes
its own food

Why it happens: the table sorts the forest into three roles. The tree is a producer —
it alone brings energy into the forest, from sunlight. The deer, hare, fox, shikra,
squirrel, mouse and vulture are consumers — they pass that energy along. The
mushroom is a decomposer — it takes the energy and nutrients left in dead bodies
and returns the nutrients to the soil. Every organism in a forest fits one of these
three roles.

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

Q2 List the heterotrophs from Table 12.4.

Table 12.4: Eating habits of different organisms

NAME OF PERFORMS FEEDS ON FEEDS ON FEEDS ON
THE PHOTOSYNTHESIS PLANTS ANIMALS BOTH
ORGANISM AND PLANT PLANTS
PRODUCTS AND
ANIMALS

Deer No Yes — Grass No No — Only
and leaves of on plants
plants

Hare

Vulture

Bengal Fox

Bird (Shikra)

Squirrel

Mouse

Mushroom

Tree Yes

Table 12.4, page 198 — reproduced as printed; only the Deer row and the ‘Tree — Yes’ cell
are filled in the book.

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

Page 23 of 65

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Class 8 Science Chapter 12 How Nature Works in Harmony
a g l AglaSem · NCERT Solutions

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Why it happens: hetero = other, troph = food. A heterotroph is any organism that

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In-text Questions — Page 198
12.5 Who Eats Whom?
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Q1 How do plants get their food?

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

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Sunlight + carbon dioxide + water → m
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food made in the green leaf, oxygen released
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This is the single most important fact in the whole chapter. Every animal in the forest, and every
decomposer too, is living on energy that a green plant captured from sunlight first.
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Why it happens: energy cannot be made by any organism. It has to come in froml
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and the only doorway into an ecosystem is photosynthesis. That is why

a gl producers always occupy the first trophic level — nothing can be below them.
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Q2 How can we make linkages with the feeding relationship among organisms in a
given ecosystem?

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By drawing an arrow from the organism that is eaten to the organism that eats it. A row of
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Grass → Grasshopper → Frog → Snake → Eagle
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s e mbetter, as "passes its food and energy to". That is why it points
Read the arrow as "is eaten by", or

a g la and not the other way round: the arrow shows the direction
from the grass to the grasshopper
in which food and energy travel.

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m .
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Class 8 Science Chapter 12 How Nature Works in Harmony AglaSem · NCERT Solutions

Check it yourself: if you find that an arrow in your diagram points from a hunter to
its prey, you have drawn it backwards. Ask "which way is the energy going?" — the
answer is always upward from the producer.

Activity 12.6: Let us link (relate) — Page 199

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

12.5 Who Eats Whom?

ACTIVITY

Q1 Draw the feeding relationships for the remaining organisms by adding arrows,
similar to those in Fig. 12.8.

Leopard

Hare

Grass

Fig. 12.8, page 198 — redrawn sketch of the food chain grass → hare → leopard. The
arrow points from the food to the eater.

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

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:

Grass → Grasshopper (the grasshopper is a herbivore, so it feeds on the producer)

Grasshopper → Frog (the frog is a small carnivore that catches insects)

Frog → Snake (the snake preys on frogs)

Snake → Eagle (the eagle is a large carnivore at the top)

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

Hare Leopard

Grass Eagle

Grasshopper Frog Snake

Each arrow means “passes its food and energy to”

Two food chains in the same grassland, both starting at the grass.

Why it happens: the grass appears in both chains because a producer supports
several different herbivores at once. This is the first hint of a food web — real
ecosystems are never a single line.

Q2 Which is another food chain that can be drawn for the organisms given in this
activity?

The one the book itself gives:

Grass → Grasshopper → Frog → Snake → Eagle

Reading it as trophic levels:

Page 27 of 65

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

TROPHIC LEVEL ORGANISM ROLE

First Grass Producer — makes food from sunlight

Second Grasshopper Herbivore

Third Frog Small carnivore

Fourth Snake Carnivore

Fifth Eagle Large carnivore

Tip: a chain always begins with a producer, because that is where the energy enters.
Any chain you draw that begins with an animal is incomplete.

Activity 12.7: Let us draw — Page 199

Page 28 of 65

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Class 8 Science Chapter 12 How Nature Works in Harmony
a g l AglaSem · NCERT Solutions

12.5 Who Eats Whom?
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ACTIVITY

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a s em the number of each type of organism in Fig. 12.10a. Make a table and set a
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Count
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number in the table against each of the organisms.

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Millet — 58 plants in all · Mouse — 15 in all · Eagle — 1
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Fig. 12.10(a), page 199 — redrawn sketch of the crop field: millet plants, mice and one

aeagle

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ORGANISM ROLE IN THE FOOD CHAIN NUMBER COUNTED IN FIG. 12.10(A)
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Millet plants Producer about 50

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Mouse Herbivore (consumer) 15

Eagle Carnivore (consumer) 1

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

The mice are easy to count exactly — 15, spread across the plots. The millet plants are drawn as
clumps and a few are hidden behind the tree, so different students will get slightly different
totals, somewhere near 50. Count your own picture carefully and use your own figure; what
matters is not the exact number but the order: millets ≫ mice ≫ eagle.

Check it yourself: if your count gives more mice than millet plants, count again. A
field cannot feed more herbivores than it has plants.

Q2 Arrange the numbers in the ascending order, consider the highest number at the
base and the lowest at the top. Place the mouse, millet, and eagle appropriately in
Fig. 12.10b.

Fig. 12.10(b), page 200 — redrawn sketch of the empty pyramid as printed. The mouse is
already on the middle level; place the millet and the eagle on the other two.

Ascending order of the counts: 1 (eagle) < 15 (mice) < about 50 (millets). Put the largest
number at the base and the smallest at the top:

Page 30 of 65

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

3rd level
Eagle — 1

Mouse — 15 2nd level

Millet plants — about 50 1st
level

Producers at the base, the top carnivore at the tip

Fig. 12.10(b) completed: the millet–mouse–eagle food chain drawn to scale.

Millet (producer, 1st trophic level) → Mouse (2nd) → Eagle (3rd)

Q3 What figure do you get?

A pyramid — broad at the base and narrow at the top.
It comes out that way because of how energy moves along the chain. A mouse does not turn all
the millet it eats into mouse. Most of that food is used up in moving, breathing, keeping warm
and growing, and is finally lost as heat. Only a small part is left stored in the mouse's body for
the eagle to get.

Energy stored in about 50 millet plants → supports 15 mice

Energy stored in 15 mice → supports 1 eagle

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

Why it happens: at every step from one trophic level to the next, most of the energy
is spent and lost. So each level can support far fewer individuals than the one below
it, and the chain narrows as you go up. This is also why food chains are short — after
four or five steps there is simply not enough energy left to support another level.
Energy flows through an ecosystem one way and leaves as heat; only matter is
recycled.

Activity 12.8: Let us trace and link — Page 200

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

12.5 Who Eats Whom?

ACTIVITY

Q1 Look at Fig. 12.11. Put more arrows for the missing relationship of ‘who eats whom’.

Hawk Snake

Fox

Owl

Bird

Frog

Hare Mouse Grasshopper

Grasses

Fig. 12.11, page 200 — redrawn sketch of the food web with the arrows exactly as
printed. Some ‘who eats whom’ links are missing on purpose.

Fig. 12.11 already prints these arrows: Grasses → Hare, Grasses → Mouse, Hare → Hawk, Mouse
→ Fox, Grasshopper → Frog, Grasshopper → Bird, Frog → Owl, Bird → Snake and Snake →
Hawk.
The links still missing, which you should add, are:

Page 33 of 65

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Class 8 Science Chapter 12 How Nature Works in Harmony
a g l AglaSem · NCERT Solutions

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ARROW TO ADD WHY

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Grasses → Grasshopper
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The grasshopper is a herbivore; the grass is its producer.

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Hare → Fox

Mouse → Owl The owl hunts small rodents at night.

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Mouse → Hawk The hawk also takes mice from open ground.

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Frog → Snake
agprey on frogs, not only on birds.
Snakes

Bird → Hawk The hawk hunts small birds.
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Snake → Owl Owls take small snakes too.

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Fox Owl Snake

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Frog Bird

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already printed arrows you should add

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Fig. 12.11 completed. A few further links (for example hawk → nothing, since it is a top predator) are

l a se left out to keep the picture readable.

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

Q2 How many other organisms might be connected to one organism through a feeding
relationship in an ecosystem?

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

One organism → several kinds of food below it

One organism → several kinds of predator above it
So one organism is usually tied to four, five or more others directly, and to the whole

ecosystem indirectly

Why it happens: no animal eats only one thing, and no animal is eaten by only one
thing. Being tied to several food sources is what lets an animal survive a bad season
for any one of them. This is exactly why food chains do not stay separate — they
cross and interlink to form a food web.

Q3 Are these food chains interlinked?

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

Grasses → Hare → Hawk

Grasses → Mouse → Fox

Grasses → Grasshopper → Frog → Owl
Grasses → Grasshopper → Bird → Snake → Hawk

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.

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

Why it happens: a web is far more stable than a single chain. If the grasshoppers
fail one year, the frog can still be fed from another route, and the owl still has mice.
But the same connections also carry damage: harm one organism and the effect can
travel along several chains at once, which is the theme of the next section.

In-text Questions — Page 201
12.6 What Happens to Waste in Nature?

Q1 Does nature really waste anything?

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).

Producers Consumers

Decomposers Dead matter,
(fungi, bacteria) dung, litter

Nutrients released back into the soil are taken up again by
the producers
Matter goes round the loop; energy does not — it leaves as heat
at every step

Decomposers close the loop, which is why nothing in nature is finally wasted.

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

Why it happens: the nutrients an ecosystem uses are the same atoms over and over
again. Plants grow in soil, and many of the nutrients in soil come from the
decomposition process. If the loop were ever broken, the nutrients would stay
locked inside dead bodies and the producers would starve. Energy is different — it
enters once from the Sun and leaves as heat, so it must be resupplied every day,
while matter simply circulates.

Ever heard of … — Page 201
12.6 What Happens to Waste in Nature?

EVER HEARD OF …

Q1 Do you know which birds in your area are seen only during winter?

This one you must answer from your own place — so here is the method, and then a model
answer.
Method: keep a notebook from October to March. Each week, write the date, the place, the bird
and how many. A bird is a winter visitor if it appears in that window and is gone by summer.
Compare your list with an older person in the village or with a local bird checklist.
Sample answer: in our area we saw these only in the winter months —

BIRD WHERE WE SAW IT MONTHS

Demoiselle Crane Village water body (as at Khichan, Jodhpur district) November – February

Northern Pintail, Common Teal Village pond and canal November – March

Rosy Starling Flocks over the fields October – March

Greater Flamingo Shallow lake / salt pan December – March

Why it happens: these birds fly thousands of miles to avoid harsh climate
elsewhere and to search for food. While they are here they act as pollinators and
seed dispersers, so they link two habitats that are thousands of kilometres apart.
Many are also predators of insect pests, and so help farmers control pest
populations and indirectly help healthy crop growth.

Page 37 of 65

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

Try This: collect the postal stamps and covers of migratory birds released by the
Indian Postal Department, find out where each bird comes from and why it migrates,
and display them in your science laboratory or school library.

In-text Questions — Page 202
12.7 How Does One Change Lead to Another? / 12.8 How Do Interactions Maintain Balance in Ecosystems?

Q1 What happens when we intervene in nature?

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.

1980s: large-scale export of frog legs (Indian bullfrog, Hoplobatrachus tigerinus)
→ decline in frog populations

→ frogs eat insects, so agricultural pests rose

→ farmers forced to use more synthetic pesticides

→ harm to environment, soil and water quality

→ effects on overall environmental and human health

The Government of India banned the export of frog legs to prevent further ecological damage.
Fig. 12.13 shows the same shape of chain in a pond: plants die because of pollution → less
oxygen is produced in the water → the fish population drops → with fewer consumers, insects
increase → the insects spread to nearby farmland → farmers are compelled to use pesticides,
which again affect the environment.

Why it happens: an ecosystem stays in balance when interactions among
organisms and their environment keep populations and resources stable. That
balance is dynamic, not fixed. Removing one population removes several
interactions at once, so the numbers above and below it move — and each of those
movements starts a further one. The frogs were being taken for a reason that had
nothing to do with farming, yet the cost landed on farmers.

Page 38 of 65

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Class 8 Science Chapter 12 How Nature Works in Harmony
a g l AglaSem · NCERT Solutions

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Based on the example given in Fig. 12.16, what do you observe?
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Q2

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Flower gets pollinated ✓
g l as Bee gets nectar ✓
(Benefitted)
a (Benefitted)

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m l as
m.co a g
l a se
a g Orchid gets physical
✓
support (Benefitted)
Tree branch is
unaffected ✓
m a s
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✓ .com
The dog gets skin The tick feeds on dog
✗
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irritation (Harmed) blood (Benefitted)

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Fig. 12.16, page 203 — redrawn sketch: mutualism (flower and bee), commensalism

agl
(orchid and tree branch) and parasitism (tick and dog).

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

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RELATIONSHIP ORGANISM ORGANISM EXAMPLE IN FIG. 12.16
m l
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1 2

em Mutualism a
a s
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Benefitted Benefitted Bee gets nectar; flower gets pollinated
a c
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Commensalism Benefitted Unaffected Orchid gets physical support; the tree branch

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is unaffected

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Parasitism
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Benefitted
l a The tick feeds on dog blood; the dog gets

a skin irritation

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

Why it happens: notice that in all three, at least one partner gains — that is why the
relationship persists. What differs is the cost to the other. A mutualism can spread
because both partners do better; a parasite must not harm its host so badly that the
host dies, or it loses its own home. All of these are part of the complex web of life
in an ecosystem, along with the competition described just before.

In-text Questions — Page 204
12.9 What Are the Benefits of an Ecosystem?

Q1 How can we stop damaging forests, rivers, and wetlands? Think about what actions
you and your community can take to protect these important places.

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.

THE DAMAGE WHAT A COMMUNITY CAN DO

Trees cut for fuelwood Use fuel-efficient stoves and biogas; plant a woodlot of fast-growing local trees
so the natural forest is not cut

Untreated sewage and industrial Insist that village and town waste water be treated before it reaches the river;
waste in rivers keep solid waste out of drains

Wetlands filled in or built over Keep village ponds and tanks de-silted and in use — a pond that a village
needs is a pond a village protects

Overuse of forest resources and Support protected areas — national parks, wildlife sanctuaries, biosphere
illegal hunting reserves and community conserved areas

Invasive species Remove them from ponds and roadsides and plant native species instead

Litter and plastic Segregate waste, refuse single-use plastic, hold clean-up days

Why it happens: most of these places are damaged not by one big act but by many
small ones, so they are also repaired by many small ones. And it is worth doing:
these ecosystems give us fresh air, fertile soil, food, fibres, timber, medicines and
water, and they protect us in storms — benefits that are crucial for human survival
and well-being.

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

In-text Questions — Page 205
12.9.1 Human-made ecosystems / 12.9.2 How do healthy ecosystems serve our farms?

Q1 Can you name any human made ecosystem in your area?

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.

Why it happens: when well designed, these ecosystems can help reduce pollution,
support biodiversity and provide recreational spaces. But unlike natural ecosystems,
they need human care and management — because we choose what grows there,
we have also taken on the job of keeping the balance that a natural ecosystem keeps
by itself.

Q2 How do these practices harm both the environment and human health?

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.

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

PRACTICE WHAT IT DOES TO THE ENVIRONMENT WHAT IT DOES TO
HEALTH

Overuse of synthetic Reduces soil fertility by decreasing friendly Excess chemicals run off
fertilisers microorganisms and lowering organic matter (humus); into ponds and wells and
without humus the soil becomes prone to erosion enter drinking water

Overuse of Kills natural predators as well as pests, so pest Residues reach us through
pesticides populations finally increase; some pests develop food; farmers are exposed
resistance and become hard to control while spraying

Excessive Water table falls; wells and ponds dry, affecting the Less safe water for drinking
groundwater whole neighbourhood and for animals
extraction

Monoculture — one Reduces biodiversity, affects pollinators, and degrades Less varied food, and a
crop, repeatedly the soil crop failure hits everything
at once

Heavy irrigation and Disturbs soil organisms like earthworms and snails that Poorer soil means poorer
repeated ploughing maintain ecological balance harvests over time

Why it happens: a farm is an ecosystem too. These practices work by removing
organisms — microbes, earthworms, predatory beetles, pollinators — that were
doing useful work for free. Once they are gone, the farmer has to do their work with
machines and chemicals, which costs more and removes still more organisms. That
is the loop that makes the practice unsustainable.

Activity 12.10: Let us survey — Page 205
12.9.2 How do healthy ecosystems serve our farms?

ACTIVITY

Q1 Prepare a list of questions for farmers to find out the pesticides and other farm
inputs they use, and whether they reuse or recycle materials to improve their crops.

Keep the questions short, open, and about what the farmer actually does — not about what he
or she thinks is correct. A good list to carry:

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?

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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?

Tip: ask "how many times" and "how much" rather than "do you use a lot". Numbers
can be compared between farmers; opinions cannot.

Q2 How have your farming practices changed over time? And why?

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."
Reasons a farmer usually gives:

higher yield and a better price for a single marketable crop;
fewer cattle in the household, so less farmyard manure available;
new seed varieties that respond to fertiliser;
labour shortage, which pushed the shift to machines and chemicals.

Why it happens: this is the local version of the change described in the chapter.
Between 1950 and 1965 India faced a food crisis due to low crop production; from
the mid-20th century tractors, machines, synthetic fertilisers and pesticides helped
raise output, and this period is known as the Green Revolution. It genuinely made
India food secure. What has since become clear is that the same methods, used
heavily for decades, wear the soil out.

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co m
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What effects do you notice when using synthetic fertilisers and pesticides?
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Q3

m l as
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a gl answer given by a farmer: "The crop greens up quickly after urea and the yield is good
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in the first years. But now I have to put more to get the same result. After spraying, the pest

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goes for a fortnight and comes back stronger, and the ladybird beetles and spiders that used to

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be in the crop are gone. There are fewer earthworms when I plough."

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Effects to look for, and the reason behind each:

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WHAT THE FARMER WHAT IS GOING ON
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NOTICES

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More fertiliser needed for the Soil fertility is falling as friendly microorganisms and humus decrease

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same yield

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Did you know? Fig. 12.18 shows the alternative at work — a beetle feeding on pests.
g farmer nothing.
.c is natural control of pests by predators, and it costs the
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Q4 Have you seen any changes in soil health after using these synthetic fertilisers and
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pesticides?

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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 crumblym
s e and full of earthworms;

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nowoafter
. a gla
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Soil becomes hard and pale — the organic matter (humus) that helps bind soil particles has

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fallen.

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Topsoil washes away — without enough humus, soil becomes prone to erosion.

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Fewer earthworms and snailsse — heavy irrigation and repeated ploughing disturb these soil
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organisms, which areaimportant for maintaining ecological balance.
Water drains away or stands too long — a soil low in humus holds water badly.

com
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Why it happens: soil is not just ground-up rock; it is a living system. Overuse of
synthetic fertilisers reduces the friendly microorganisms in it and lowers its organic
matter, so the part of the soil that is alive — and that makes the mineral part usable
— is what is lost first. That is why yields fall even when the chemicals continue.

Q5 Interact with farmers based on these questions. Based on your findings, prepare a
report. What inference do you draw from your interactions with farmers?

Structure the report in four parts: (i) where and when you went and how many farmers you
spoke to; (ii) a table of their answers; (iii) what most of them said in common; (iv) your inference.
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.

Check it yourself: a report is stronger if it separates what the farmer said from what
you concluded. Keep them in different sections so a reader can check your reasoning.

In-text Questions — Page 206
12.9.2 How do healthy ecosystems serve our farms?

Q1 Based on your learning, what practices do you think can help farmers protect the
soil, the environment, and our food security for the future?

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

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PRACTICE HOW IT HELPS

Compost, farmyard manure, green Restore humus and the friendly microorganisms in soil, so the soil binds
manure together and holds water

Crop rotation and mixed cropping Different crops draw different nutrients and support different insects, so
instead of monoculture biodiversity and pollinators recover and pests do not build up

Leaving field borders and hedges Shelters the natural predators — beetles, spiders, birds — that control pests
uncut for free (Fig. 12.18)

Spraying only when pests actually Fewer sprays means fewer resistant pests and more surviving predators
cross a damaging level

Drip and sprinkler irrigation, Reduces excessive groundwater extraction and keeps the water table from
rainwater harvesting falling

Less ploughing; returning crop Protects earthworms and snails and keeps the topsoil in place
residue to the field

Organic and natural farming Aim to reduce synthetic fertilisers and support sustainable farming with
methods minimal interference in natural ecosystems

Did you know? The ancient text Vrikshayurveda already emphasised soil health and
nourishment. It advocates feeding the soil continuously with organic manure such
as Kunapa Jala — a liquid fertiliser made from animal and plant waste by
fermentation, a process that breaks complex substances into simpler ones, exactly
as decomposers do in nature.

Keep the curiosity alive — Pages 207–208

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End-of-chapter questions

KEEP THE CURIOSITY ALIVE

Q1 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.

Community

Population

Ecosystem

Fig. 12.19, page 207 — redrawn: three circles, one inside the next — population inside
community inside ecosystem.

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:

Population ⊂ Community ⊂ Ecosystem

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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.

Why it happens: each step outward adds something. Going from population to
community adds the other kinds of organism; going from community to ecosystem
adds the non-living surroundings. A community is therefore always contained in an
ecosystem, and can never contain it.

Q2 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.

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

Decomposers gone → dead leaves, fallen trees, dung and dead animals stop breaking down

→ litter piles up on the forest floor

→ nutrients stay locked inside the dead bodies and never reach the soil

→ soil loses its nutrients and its humus; it becomes hard and erodes

→ producers grow poorly

→ less food for herbivores → fewer carnivores → the whole web thins out

Why they are essential: decomposers are the only organisms that carry matter backwards in
the ecosystem. Every other arrow in a food web points upward — grass to grasshopper to frog.
Decomposers alone return material to the start of the chain. Microorganisms like fungi and
bacteria break down the complex substances in dead plants and animals into simpler ones, and
this process returns important nutrients to the soil. Plants grow in soil, and many of the
nutrients in soil come from decomposition.

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co m
m.
Why it happens: an ecosystem has a fixed stock of nutrients. If they are not

m l a se
recycled, they cannot be replaced — there is no outside supply, the way sunlight is
o g an untidiness
.csupply of energy. So the loss of decomposers is not just
a
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an outside

l a se it cuts the nutrient supply of the producers, and therefore of everything
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problem;
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Q3 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

m
mangrove forests. This surprised Sarita, Shabnam, and Shijo. They wondered if
co
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mangroves were protecting the village. Can you help them understand this?

o m l a se
ANSWER .c a g
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se is right — the mangroves acted as a living wall between the sea and the village.
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Selvam
A mangrove forest sits in the shallow water at the coast. Its trees stand on dense, arching stilt

om a s
through all of them. Each root takes a little.c agl
roots, and thousands of small roots also stick up out of the mud. A wave coming in has to push

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of the wave's energy, so by the time the water
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reaches the village behind the forest

Wave energy meets thousands of stilt roots → energy spread out and absorbed
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→ water arrives slower and shallower behind the belt

→ .less a g l
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This is exactly what the chapter says about the Sundarbans: they protect us by slowing down
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strong winds and waves during storms and floods. Villages that had cleared their mangroves

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for fuelwood or ponds had nothing to slow the same wave, so they were hit harder.

Why it happens: this is an ecosystem benefit — something the ecosystem does for
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us that we would otherwise have to build. A concrete sea wall could do part of the
m
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job, but the mangrove also holds the soil, absorbs carbon dioxide and releases

s e m oxygen, and shelters fish and crabs that the village eats. That is why the Sundarbans
agla were declared a World Heritage Site by UNESCO in 1987.
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Q4 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?

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

Grass Grasshopper Frog Snake
▲ increases ▼ decreases

Below the break: predator removed. Above the break: food supply removed.
Grass will also be eaten down as grasshopper numbers rise.

Removing one link changes the levels below and above it in opposite directions.

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. The energy the snake used to get came up
through the frog, and that route is now closed. Some snakes may switch to other prey such as
mice or birds if the ecosystem has them; where it does not, the snake population falls.

Why it happens: every organism in a chain has two roles at once — it is food for the
level above and a check on the level below. Take it out and both roles vanish
together, which is why the effect runs in both directions and not just one.

Q5 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?

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

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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

STEP WHY IT WORKS

Plant nectar plants — marigold, zinnia, ixora, pentas, Feeds the adult butterflies and brings them in
tulsi

Plant host plants — curry leaf and lemon (for the Gives the caterpillars the one plant each species can eat,
common mormon), calotropis (for the plain tiger), fennel, so butterflies breed on campus instead of only visiting
cassia

Stop spraying pesticide in the garden Pesticide kills caterpillars and pupae, which is where the
next season's butterflies come from

Leave one corner unmown and uncleared Weeds there are host plants; the litter shelters pupae
through the dry months

Keep a shallow dish of water with wet sand or mud Butterflies take water and minerals from damp soil

Plant in a sunny, wind-sheltered spot and keep it Butterflies are active in sunshine and need nectar
flowering across seasons available all year, not for one month

Why it happens: butterflies are pollinators, so they are not just decoration. The
reasoning here is the same as in Activity 12.3 — change one thing (the plants, or the
spray) and the population of an insect that depends on it moves with it.

Q6 Why is it not possible to have an ecosystem with only producers and no consumers
or decomposers?

Because energy could still enter such a system, but matter could never move — and an
ecosystem needs both.
Follow what happens. The producers photosynthesise and grow. They take nutrients out of the
soil to do it. Eventually each plant dies. Now:

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there are no decomposers, so the dead plants do not break down. The nutrients they took
from the soil stay locked inside them for ever;
the soil therefore runs out of nutrients, and the next generation of plants cannot grow;
dead plant material piles up over everything, blocking light and space from any seedling.

Consumers matter too, though in a different way. Many plants depend on animals to pollinate
their flowers and to disperse their seeds. With no animals, most flowering plants would set few
seeds and could not spread to new ground.

Sunlight → producers → dead plant matter → nowhere

Nutrients: soil → plant → dead plant → never back to soil

Why it happens: in nature, energy flows and matter cycles. Energy is resupplied every
day by the Sun, so the producers alone can capture it. Matter is not resupplied — the
same nutrients must be used again and again, and only decomposers can send
them back to the start. An ecosystem with only producers has a working supply of
energy and a broken supply of matter, so it stops within a few generations.

Q7 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?

Method: stand in each place for ten minutes at the same time of day. Write down every living
thing you can see or hear, and every non-living thing that affects them. Then compare.
Sample answer — a park and a roadside near our school:

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PARK ROADSIDE

Living (biotic) Grass, neem and gulmohar trees, hedges, A few hardy trees, dusty grass and
flowering shrubs, butterflies, bees, ants, weeds in the cracks, crows, pigeons,
earthworms, mynas, sparrows, squirrels, a houseflies, ants, a stray dog
stray dog

Non-living Soft moist soil, shade, water in a tap and a Concrete and tar, compacted soil, no
(abiotic) birdbath, cooler air, leaf litter shade for much of the day, dust,
hotter air, drain water

How many Many — several trees, shrubs, insects and birds Few — a handful of tough species
kinds of
organism

Food chains Several, interlinked into a small web Very short, and often fed by human
litter rather than by producers

How they differ: the difference in the living things follows from the difference in the non-living
ones. Soft moist soil and shade let roots, earthworms and leaf litter build up, so the park has
producers, several kinds of consumer, and decomposers working in the soil. Tar and compacted
soil block roots and water, and heat and dust make conditions harsh, so only a few tough
species survive at the roadside. The roadside is also a place where humans keep interfering —
sweeping, cutting, dumping — so the community there never gets time to build up.

Why it happens: the abiotic components decide which organisms can live in a place
at all, and the number of organisms then decides how many links a food web can
have. Change the soil and the shade and you have changed the whole community.

Q8 ‘Human-made ecosystems like agricultural fields are necessary, but they must be
made sustainable.’ Comment on the statement

The statement is correct on both counts, and the two halves are connected.
They are necessary. Humans have been practising farming for thousands of years to grow
food, and as the population grew, our dependence on agriculture increased. Between 1950 and
1965 India faced a food crisis due to low crop production; tractors, machines, synthetic
fertilisers and pesticides in the mid-20th century — the Green Revolution — helped raise food
production and made the country food secure. No natural ecosystem could feed a population of
this size.
They must be made sustainable. Those same methods are now considered unsustainable
because of the overuse of synthetic chemicals, excessive groundwater extraction, and growing
only one type of crop for commercial gain. The result is soil degradation: less humus, fewer

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Class 8 Science Chapter 12 How Nature Works in Harmony
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friendly microorganisms, erosion, a falling water table, pests that have developed resistance,
co m
and fewer pollinators.
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A field is an ecosystem we manage
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Manage it badly → we must replace their work with chemicals → cost rises, soil falls
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What "sustainable" means here: using the field in a way that leaves the soil as good next year
as it is this year. Compost and manure, crop rotation and mixed cropping instead of
monoculture, efficient irrigation, spraying only when needed, and organic and natural farming
co m
methods that keep interference with natural ecosystems to a minimum.
se m.
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g human care and
mit. happens: unlike a forest, a human-made ecosystem needs
a
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Why

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management — we removed most of its natural checks when we made it. So the
choice is not between farming and not farming; it is between farming that renews

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the soil and farming that spends it.

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Q9 If the Indian hare population (Fig. 12.20) drops because of a disease, how would it
affect the number of other organisms?

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Sun

Grass and plants

Hare Deer

Fox
Eagle

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

Fig. 12.20, page 208 — redrawn sketch of parts of a food web: sun → grass and plants →
hare and deer → fox and eagle.

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.

Sun

Grass and plants ▲

Hare ▼ Deer ▲

Fox ▼ Eagle ▲

▼ falls ▲ rises — the fall in hares travels down one branch and up the
other

Effect of a disease that reduces the hare population in the web of Fig. 12.20.

ORGANISM CHANGE REASON

Fox Decreases The hare is its food in this web. With fewer hares the foxes get less to
eat, raise fewer young, and their numbers fall. Some may hunt other
prey instead — which then shifts the pressure onto those animals.

Grass and Increase at One of the two herbivores grazing them has become scarce, so less
plants first grass is eaten.

Deer Increase More grass is available and there is less competition for it, so more deer
survive and breed.

Eagle Increases The deer are its food in this web, so more deer means more eagles.

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Why it happens: the hare has two roles — it consumes grass and it feeds the fox.
Losing it therefore releases the grass (numbers rise) and starves the fox (numbers
fall). Because the grass is shared with the deer, the release does not stop there: it
travels along the second branch of the web and lifts the deer and then the eagle.
This is why a change in one population never stays in one place — a food web
carries it in every direction the arrows go.

Check it yourself: the rise in the deer will not go on for ever. As deer numbers climb
they eat down the grass, and competition among them brings the numbers back —
the balance is dynamic, settling at a new level rather than growing without limit.

Discover, design, and debate — Pages 208–209
End-of-chapter projects

DISCOVER, DESIGN, AND DEBATE

Q1 Plan a clean-up day at school or a nearby park. Wearing gloves and using bags,
collect the litter you find. Discuss the kinds of waste you found. Which was the most
common? How can we reduce such waste?

How to run it: divide the ground into blocks, one group per block, and carry three bags —
plastic, paper, and other. Wear gloves throughout, do not pick up glass or anything sharp by
hand, and wash your hands afterwards. Weigh or count what each group collects so the class
has data, not just impressions.
Sample record from a one-hour clean-up of a school ground and the park beside it:

KIND OF WASTE NUMBER OF PIECES WHERE MOST OF IT CAME FROM

Plastic wrappers and pouches 146 Snack packets near the gate and the canteen

Plastic bottles and cups 38 Around the benches

Paper and cardboard 27 Near the classrooms

Glass and metal 9 Along the boundary wall

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.

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

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the kitchen and

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actually

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Why it happens: in nature, nothing is wasted, because every kind of dead matter
co
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has a decomposer that can use it. Plastic is the exception: it has no decomposer. That ag
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is precisely why it accumulates, and why reducing it matters more than clearing it
up. a

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In Arunachal Pradesh, the Nyishi and Mishmi tribes treat the Tiger as sacred. In
em
Q2

a s Chhattisgarh, the Baiga tribe worships the Bagesur Dev and believes the Tiger is

a gl the protector of the forest. Find out about other Indian tribes that have a special
connection with animals.

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s

a
gl library and in state tourism or tribal-welfare publications;
a
How to find out: look in your school
ask an elder in your own community which animals, trees or groves are treated as sacred locally;
and note the reason given, not just the name.
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Sample answer — communities across India that protect particular animals:

c o m g l as
m . a
a s e
COMMUNITY WHERE THE CONNECTION

agl Bishnoi Rajasthan, Haryana Protect the blackbuck and the chinkara, and refuse to cut
green trees, as part of their religious code
se m
com g l a
m . Keep fish in the flooded paddy fields and maintain sacred a
e
as groves
Apatani Ziro valley,

gl
Arunachal Pradesh

Khasi and Jaintia
a
Meghalaya Maintain sacred groves (law kyntang) where no plant or
animal may be harmed
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Bhil and Gond Central India Clan totems tied to particular animals and trees, which

. a g
m
members of that clan will not harm

ase
agl Sacred buffalo herds and sacred groves in the shola forests
Toda Nilgiris, Tamil Nadu

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