How Nature Works in Harmony Class 8: Ecosystems, Food Chains, Food Webs and Balance

What How Nature Works in Harmony is about

Key concepts

Habitat, biotic and abiotic components

NCERT section 12.1 How Do We Experience and Interpret Our Surroundings?

A habitat is the place where an organism lives, which can be as large as a forest or as small as the bark of a tree.

Activity 12.1: list living and non-living things in two habitats (for example a pond and a forest).

• Biotic components: the living beings (fish, frogs, turtles, ducks, algae, lotus, grass, trees, birds, microbes).
• Abiotic components: the non-living things and physical conditions (water, soil, air, sunlight, temperature).

Both habitats have both kinds of components, but the types differ, so each habitat offers different living conditions. A pond gives fish food (small plants and animals, a biotic need) and oxygen dissolved in water (an abiotic need), plus shelter and space.

Organisms sharing a habitat can use it differently: a snake active at night and a rodent active by day face different conditions in the same forest. This is one way organisms coexist in harmony.

Worked example

Sort these into biotic and abiotic components of a pond: duckweed, sunlight, snail, dissolved oxygen, bacteria, mud, dragonfly, temperature.

Try it first, then show the solutionSolution

1. Biotic (living): duckweed, snail, bacteria, dragonfly.
2. Abiotic (non-living): sunlight, dissolved oxygen, mud, temperature.

Answer: Biotic: duckweed, snail, bacteria, dragonfly. Abiotic: sunlight, oxygen, mud, temperature.

Population, community and ecosystem

NCERT section 12.2 Who All Live Together in Nature?

Levels of organisation, from small to large: individual → population → community → ecosystem.

• Population: a group of organisms of the same kind living in a habitat at a given time (for example all the fish of one kind in a pond).
• Community: all the different populations (plants, animals, microorganisms) sharing a habitat and depending on each other.
• Ecosystem: the community (biotic) together with the abiotic components, all interacting.

Activity 12.2: mark a 1 m × 1 m square in the school garden and count individuals of a few organisms, for example 20 of one plant and 5 of another. Comparing groups' counts gives an idea of population sizes.

If a habitat had only one type of organism, all would need the same food, water and space, leading to strong competition and shortage.

Pollination: stamens release pollen; wind, water, insects, bats and birds carry it to carpels; this is needed to form fruits and seeds.

Worked example

Arrange from smallest to largest and name each level: (a) all the living things in a lake; (b) one rohu fish; (c) the lake with its water, mud, sunlight and all its living things; (d) all the rohu fish in the lake.

Try it first, then show the solutionSolution

1. (b) One rohu fish: an individual.
2. (d) All the rohu in the lake: a population (one kind).
3. (a) All the living things together: a community; (c) the community with its non-living surroundings: an ecosystem.

Answer: (b) individual, (d) population, (a) community, (c) ecosystem.

Every organism matters: fish, dragonflies and flowers

NCERT section 12.3 Does Every Organism in a Community Matter?

Activity 12.3 describes a study of two ponds: Pond A with fish had many flowering plants around it; Pond B without fish had fewer.

The chain of effects:
1. Fish eat dragonfly larvae, so Pond A has fewer dragonflies.
2. Dragonflies eat flies, bees and butterflies, so fewer dragonflies means more pollinators.
3. More pollinators carry pollen between flowers, so plants near Pond A produce more seeds.

So fish, which never touch the flowers, indirectly affect seed production. Biotic components (fish, dragonflies, pollinators, plants) and abiotic components (water, temperature, nutrients) are all linked. Overfishing could therefore lower seed production around a pond.

Worked example

Explain how removing all the fish from a pond might reduce the number of seeds produced by plants nearby.

Try it first, then show the solutionSolution

1. Without fish, more dragonfly larvae survive, so dragonflies increase.
2. More dragonflies eat more bees and butterflies, so pollinators decrease.
3. Fewer pollinators mean less pollination, so plants produce fewer seeds.

Answer: Fish removal → more dragonflies → fewer pollinators → fewer seeds.

Interactions and types of ecosystem

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

Activity 12.4 sorts interactions into three groups:
1. Biotic with abiotic (affect nutrition, respiration, reproduction): earthworms live in moist soil; a fish lays eggs in water.
2. Abiotic with abiotic (affect physical conditions): bright sunlight raises day temperature; sunlight speeds evaporation; wind makes waves.
3. Biotic with biotic (affect resources): a frog eats insects; frogs and fish compete for insect larvae.

The biotic and abiotic components of an area interacting together form an ecosystem.

• Aquatic ecosystems: ponds, rivers, lakes, seas.
• Terrestrial ecosystems: forests, grasslands, farms, even a single large tree (banyan, mango, pilkhan). A farmland is a human-made ecosystem. Ecosystems can be large or small and they overlap.

Two-way dependence: plants need sunlight, carbon dioxide, water and soil nutrients; all organisms need water and oxygen. In turn, plants release oxygen, roots hold soil and prevent erosion, and plants keep soil moist and cool the air.

Worked example

Classify each interaction: (a) sunlight makes pond water evaporate, (b) a water snake eats fish, (c) roots hold soil on a riverbank.

Try it first, then show the solutionSolution

1. (a) Sunlight and water are both non-living: abiotic with abiotic.
2. (b) Snake and fish are both living: biotic with biotic.
3. (c) Roots (living) with soil (non-living): biotic with abiotic.

Answer: (a) abiotic–abiotic (b) biotic–biotic (c) biotic–abiotic.

Producers, consumers and food chains

NCERT section 12.5 Who Eats Whom?

Producers (autotrophs) make their own food by photosynthesis (*auto* = self, *troph* = food): green plants.

Consumers (heterotrophs) depend on other organisms (*hetero* = other):

• herbivores eat only plants (deer, hare);
• carnivores eat only animals (leopard);
• omnivores eat both (crow, fox, mouse).

Activity 12.5: classify forest organisms by diet (deer, hare, vulture, Bengal fox, shikra, squirrel, mouse, mushroom, tree).

A food chain is a simple sequence showing who eats whom; arrows point from the food to the eater:

• grass → hare → leopard
• grass → grasshopper → frog → snake → eagle

Trophic levels (Activity 12.7): producers at the first level, herbivores at the second, small carnivores (frogs) at the third, large carnivores (tigers, vultures) at the next. Counting millets, mice and eagles in a crop field and stacking the numbers (largest at the bottom) gives a pyramid: many producers, fewer herbivores, very few top carnivores.

Worked example

In the chain grass → grasshopper → frog → snake → eagle, give the trophic level and type of consumer for the grasshopper, frog and eagle.

Try it first, then show the solutionSolution

1. Grass is the producer (first level).
2. Grasshopper: second level, herbivore (primary consumer).
3. Frog: third level, small carnivore.
4. Eagle: fifth level here, a top carnivore.

Answer: Grasshopper 2nd (herbivore); frog 3rd (carnivore); eagle 5th (top carnivore).

Food webs and decomposers

NCERT section 12.6 What Happens to Waste in Nature?

Activity 12.8: in a grassland, grass is eaten by grasshoppers, hares and mice; grasshoppers by birds and frogs; frogs and mice by snakes; snakes, birds, hares and mice by hawks. Most organisms are eaten by two or more others, so food chains interlink into a food web. A web is more stable than a single chain: if one food is scarce, eaters can switch.

Decomposition (12.6): fungi (such as mushrooms on dead wood) and bacteria split the complex materials of dead organisms into simple substances, returning nutrients to the soil. Beetles and flies help break down dung. These organisms are decomposers or saprotrophs (*sapro* = rotten). Plants take up the recycled nutrients, so in nature nothing is wasted.

Migratory birds (such as Demoiselle Cranes at Khichan, Jodhpur, in winter) link habitats as pollinators and seed dispersers and eat insect pests, helping farmers.

Worked example

A heap of fallen leaves in a moist, shady corner of a garden turns into dark, crumbly soil within a few months, but a heap of leaves on a dry concrete roof hardly changes. Explain.

Try it first, then show the solutionSolution

1. In the garden, fungi and bacteria in the moist soil decompose the leaves, splitting them into simple substances and returning nutrients to the soil.
2. On the dry roof there is no soil with decomposers and little moisture, so decomposition is very slow.
3. So decomposers, and conditions that suit them, are needed for nature to recycle dead matter.

Answer: Decomposers in moist soil break the leaves down; on a dry roof they are missing, so little changes.

Chain reactions, balance and interactions

NCERT section 12.8 How Do Interactions Maintain Balance in Ecosystems?

One change leads to another (12.7): pollution kills pond plants → less oxygen → fish die → fewer consumers, so insects increase → insects spread to farms → farmers use more pesticides → more harm to the environment.

Activity 12.9: in the 1980s India exported large numbers of Indian bullfrog legs. Frog numbers fell, so insect pests rose, farmers used more synthetic pesticides, damaging soil, water and health. The Government banned frog-leg exports.

Balance (12.8): an ecosystem is balanced when interactions keep populations and resources stable. The balance is dynamic, not fixed.

• Competition for food, water, space or sunlight keeps any one species from multiplying too much.
• Mutualism: both benefit (honeybee gets nectar; flower gets pollinated).
• Commensalism: one benefits, the other unaffected (orchid gets support on a tree branch).
• Parasitism: one benefits, the other is harmed (tick feeds on a dog's blood; the dog gets skin irritation).

A.J.T. Johnsingh, a wildlife biologist, showed at Bandipur that tigers and leopards depend on healthy prey populations such as deer and wild boar.

Worked example

Name the interaction and say who benefits: (a) cattle egrets eating insects stirred up by grazing cattle, (b) a mosquito biting a person, (c) a bee visiting a mustard flower.

Try it first, then show the solutionSolution

1. (a) Egret benefits, cattle unaffected: commensalism.
2. (b) Mosquito benefits, person harmed: parasitism.
3. (c) Bee gets nectar and flower gets pollinated: mutualism.

Answer: (a) commensalism (b) parasitism (c) mutualism.

Benefits of ecosystems and sustainable farming

NCERT section 12.9.2 How do healthy ecosystems serve our farms?

Benefits (12.9): forests give fresh air, fertile soil, food, fibre, timber and medicines; aquatic ecosystems give water and food; ecosystems also give beauty and recreation.

Sundarbans: the world's largest mangrove forest, where the Ganges and Brahmaputra meet (India and Bangladesh). Mangroves slow storm winds and waves, absorb carbon dioxide and release oxygen. UNESCO declared it a World Heritage Site in 1987. Threats: cutting mangroves for fuelwood and farming, illegal hunting, overuse, industrial waste and sewage.

Protected areas: national parks, wildlife sanctuaries, biosphere reserves and community conserved areas, such as Jim Corbett (Uttarakhand), Manas (Assam), Nilgiri Biosphere Reserve, Chilika Lake (Odisha), Keibul Lamjao (Manipur), Hemis (Leh).

Human-made ecosystems (12.9.1): fish ponds, farms and parks; they need human care.

Farms (12.9.2): after food shortages (1950 to 1965), the Green Revolution used tractors, synthetic fertilisers and pesticides to raise production. But overuse harms soil: fewer friendly microbes, less humus (so more erosion), fewer natural predators of pests, pest resistance; heavy irrigation and ploughing disturb earthworms; monoculture reduces biodiversity and pollinators. Organic and natural farming reduce chemicals. The ancient *Vrikshayurveda* recommends nourishing soil with organic manure such as Kunapa Jala, a fermented liquid fertiliser.

Worked example

Give three ways overuse of synthetic fertilisers and pesticides can harm a farm in the long run.

Try it first, then show the solutionSolution

1. Fewer friendly soil microbes and less humus, so soil loses fertility and erodes more easily.
2. Natural predators of pests (such as beetles) are killed, so pests increase.
3. Some pests become resistant to pesticides and are harder to control.

Answer: Poorer, erodible soil; loss of natural predators; resistant pests.

Science in real life: How Nature Works in Harmony

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Food Chains, Food Webs, Decomposers and Ecological Balance
• Habitats, Populations, Communities and Ecosystems

How Nature Works in Harmony: formulas and key facts

Levels of organisation

$\text{individual}\subset \text{population}\subset \text{community}\subset \text{ecosystem}$

Ecosystem

$\text{ecosystem}=\text{biotic (community)}+\text{abiotic components, interacting}$

Food chain example

$\text{grass}\to \text{grasshopper}\to \text{frog}\to \text{snake}\to \text{eagle}$

Trophic levels

$\text{producer (1st)}\to \text{herbivore (2nd)}\to \text{small carnivore (3rd)}\to \text{large carnivore}$

Interactions

$\text{mutualism }(+,+);\text{commensalism }(+,0);\text{parasitism }(+,-)$

How Nature Works in Harmony: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks How Nature Works in Harmony

The question patterns to expect, and the method that works for each.

Tricks to remember How Nature Works in Harmony

Short memory aids. Say each one aloud once, then use it on a question the same day.

• I Pick Cool Eggs

  Individual, Population, Community, Ecosystem: small to large.

  Use it when: Levels of organisation.

• Arrow = 'eaten by'

  Read every arrow in a food chain as 'is eaten by'.

  Use it when: Drawing food chains.

• Plus, zero, minus

  Mutualism (+,+), commensalism (+,0), parasitism (+,−).

  Use it when: Types of interaction.

• Auto, hetero, sapro

  Self-feeders, other-feeders, rot-feeders.

  Use it when: Producers, consumers, decomposers.

• Domino nature

  Push one domino (remove one species) and many fall.

  Use it when: Cascade questions.

How Nature Works in Harmony: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in How Nature Works in Harmony

The slips examiners see most often, and what to write instead.

• ✗Mistake: Drawing food chain arrows backwards

  Arrow read as 'eats'.

  Wrong working

  eagle → snake → frog → grasshopper → grass

  Right working

  grass → grasshopper → frog → snake → eagle (arrow from food to eater).

  ✓Do this: Arrows follow the food.

  Usually costs: 1 mark.

• ✗Mistake: Confusing population and community

  Both are groups.

  Wrong working

  All the frogs, fish and plants in a pond form a population.

  Right working

  That is a community; all the frogs of one kind form a population.

  ✓Do this: Population = one kind; community = many kinds.

  Usually costs: 1 mark.

• ✗Mistake: Leaving abiotic components out of an ecosystem

  Ecosystem treated as only living things.

  Wrong working

  An ecosystem is all the organisms in an area.

  Right working

  An ecosystem is the community plus the abiotic components, all interacting.

  ✓Do this: Ecosystem = biotic + abiotic.

  Usually costs: 1 mark.

• ✗Mistake: Giving a one-step answer to cascade questions

  Only the direct effect is noticed.

  Wrong working

  If frogs disappear, snakes decrease.

  Right working

  Snakes decrease (less food) and grasshoppers increase (fewer predators), which may damage grass.

  ✓Do this: Trace effects both up and down the chain.

  Usually costs: 1-2 marks.

• ✗Mistake: Calling orchids parasites

  They grow on trees.

  Wrong working

  Orchids harm the tree they grow on: parasitism.

  Right working

  Orchids only get support; the tree is unaffected: commensalism.

  ✓Do this: Check whether the host is harmed.

  Usually costs: 1 mark.

NCERT exercise guide: How Nature Works in Harmony

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on How Nature Works in Harmony

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   A group of individuals of the same kind living in a habitat is a
   (a) community
   (b) population
   (c) ecosystem
   (d) food web

   Hint

   Same kind.

   Answer

   (b) population

2. Q2 · 1 mark · MCQExam level

   Ticks on a dog are an example of
   (a) mutualism
   (b) commensalism
   (c) parasitism
   (d) competition

   Hint

   Dog is harmed.

   Answer

   (c) parasitism

3. Q3 · 1 mark · Very short answerExam level

   What does 'saprotroph' mean?

   Hint

   Sapro = rotten.

   Answer

   An organism that feeds on dead and decaying matter (a decomposer).

4. Q4 · 2 marks · Very short answerExam level

   Give two examples each of aquatic and terrestrial ecosystems.

   Hint

   Water and land.

   Answer

   Aquatic: pond, river (also lake). Terrestrial: forest, grassland (also farm, a large banyan tree).

5. Q5 · 2 marks · Very short answerHarder exam level

   Name the trophic level of each in grass → hare → leopard.

   Hint

   Producer first.

   Answer

   Grass: first (producer); hare: second (herbivore); leopard: third (carnivore).

6. Q6 · 2 marks · Very short answerHarder exam level

   Give two ways biotic components change abiotic components.

   Hint

   Plants.

   Answer

   Plants release oxygen into the air during photosynthesis; roots hold soil and prevent erosion (also keep soil moist and cool the air).

7. Q7 · 3 marks · Short answerHarder exam level

   Why is a food web more stable than a single food chain?

   Hint

   Alternatives.

   Answer

   In a web, most animals have several food sources and several predators. If one population falls, eaters can switch to other foods, so the effects are spread out and the ecosystem is less likely to collapse than a single chain where losing one link breaks it.

8. Q8 · 3 marks · Short answerHarder exam level

   In the 1 m × 1 m count, a group found 20 plants of one kind and 5 of another. How many times as many are there of the first? Estimate each population in a 12 m² flowerbed with similar conditions.

   Hint

   Multiply by area.

   Answer

   $\frac{20}{5}=4$ times as many. Estimates: $20\times 12=240$ and $5\times 12=60$ plants (assuming even spread).

9. Q9 · 3 marks · Short answerHarder exam level

   Name the threats to the Sundarbans and suggest two ways to protect it.

   Hint

   Human activities.

   Answer

   Threats: cutting mangroves for fuelwood and farming, illegal hunting, overuse of forest resources, industrial waste and untreated sewage. Protection: enforce bans on cutting and hunting, treat waste before it enters rivers, and involve local communities in replanting and guarding mangroves.

10. Q10 · 5 marks · Long answerHarder exam level

    Explain the three types of interactions in Activity 12.4 and then the three relationships mutualism, commensalism and parasitism, with one example each.

    Hint

    Two lists.

    Answer

    Biotic–abiotic: earthworms living in moist soil. Abiotic–abiotic: sunlight evaporating water. Biotic–biotic: a frog eating insects. Mutualism (both benefit): bee and flower. Commensalism (one benefits, other unaffected): orchid on a tree. Parasitism (one benefits, other harmed): tick on a dog.

11. Q11 · 3 marks · Short answerHOTS / competency

    Migratory birds visit a wetland every winter. Give two ways they help the ecosystem and farmers, and predict one effect if the wetland is drained.

    Hint

    Pollinators, pest control.

    Answer

    They pollinate flowers and disperse seeds along their route, and eat insect pests, helping crops. If the wetland is drained, the birds lose a resting and feeding place and may stop coming, so pests could increase in nearby farms.

12. Q12 · 2 marks · Very short answerHOTS / competency

    A farmer grows only sugarcane year after year and sprays pesticides heavily. Predict two long-term effects on the farm ecosystem.

    Hint

    Monoculture.

    Answer

    Biodiversity and pollinators decline, and soil loses nutrients, humus and friendly organisms; pests may become resistant and natural predators disappear, so yields eventually fall.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.What is a habitat?Show answer

   The place where an organism lives.

2. 2.Give two abiotic components of a forest.Show answer

   Soil and sunlight (also air, water, temperature).

3. 3.What is an ecosystem?Show answer

   The biotic and abiotic components of an area interacting together.

4. 4.Why are plants called autotrophs?Show answer

   They make their own food by photosynthesis.

5. 5.Give an example of an omnivore.Show answer

   Crow, fox or mouse.

6. 6.What is a food web?Show answer

   A network of interlinked food chains in an ecosystem.

7. 7.What do decomposers do?Show answer

   Break down dead matter into simpler substances, returning nutrients to the soil.

8. 8.What is commensalism?Show answer

   An interaction in which one organism benefits and the other is unaffected, like orchids on trees.

9. 9.When did UNESCO declare the Sundarbans a World Heritage Site?Show answer

   1987.

10. 10.What is monoculture?Show answer

    Growing the same crop repeatedly on the same land.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you list biotic and abiotic components of a pond?
• ☐Can you order individual, population, community, ecosystem?
• ☐Can you explain how fish affect seed production near a pond?
• ☐Can you give an example of each of the three kinds of interaction?
• ☐Can you write two food chains and mark trophic levels?
• ☐Can you draw a food web with at least six organisms?
• ☐Can you explain why decomposers are essential?
• ☐Can you trace a cascade when one species is removed?
• ☐Can you tell mutualism, commensalism and parasitism apart?
• ☐Can you give three harms of overusing farm chemicals?

How Nature Works in Harmony: frequently asked questions