ScienceCBSE Class 8 · How Nature Works in Harmony

Food Chains, Food Webs, Decomposers and Ecological Balance: Class 8

Updated 17 October 2026 · Fact-checked

Producers make their own food, consumers (herbivores, carnivores, omnivores) feed on others, and decomposers break dead matter into nutrients. A food chain shows who eats whom, and linked chains form a food web. Each step is a trophic level. An ecosystem stays in dynamic balance, but one change, such as the loss of frogs, can cascade through the whole system, and human activity can disturb it.

Understand Food Chains, Food Webs, Decomposers and Ecological Balance

Producers and consumers (Activity 12.5). Green plants make food by photosynthesis, so they are producers or autotrophs (auto = self, troph = food). Organisms that cannot make their own food and depend on others are consumers or heterotrophs (hetero = other). Those that eat only plants are herbivores (deer, hare); those that eat only animals are carnivores (leopard); those that eat both are omnivores (crows, foxes, mice). Mushrooms do not perform photosynthesis.

Food chain (Activity 12.6). A food chain is a simple sequence showing who eats whom, drawn with arrows pointing from the food to the eater: grass to grasshopper to frog to snake to eagle; grass to hare to leopard. Each position is a trophic level: producers (green plants) first, herbivores second, small carnivores (frogs) third, larger carnivores (tigers, vultures) after that. In Activity 12.7, when the numbers of organisms (millet, mouse, eagle) are arranged with the highest number at the base, the figure is a pyramid: many producers, fewer herbivores, still fewer carnivores at the top.

Food web (Activity 12.8). Most organisms are eaten by two or more kinds, so food chains are interlinked into a network called a food web. In Fig. 12.11 the grasses are eaten by the hare and grasshopper, mice are eaten by the owl, snake and fox, and so on.

Decomposers. Mushrooms on dead wood, bacteria, fungi and small insects such as beetles and flies on dung break down the complex substances of dead plants and animals into simpler ones and return nutrients to the soil. This is decomposition, and the organisms are decomposers or saprotrophs (sapro = rotten). In nature nothing is wasted: everything is reused.

One change leads to another (Fig. 12.13, Activity 12.9). If pollution kills pond plants, oxygen in the water falls and fish die; with fewer fish (consumers) insects increase and spread to farms, so farmers use pesticides that harm the environment. The chapter's real case: in the 1980s large-scale export of frog legs of the Indian bullfrog reduced frog numbers; with fewer frogs eating insects, agricultural pests rose, so more synthetic pesticides were used, harming soil, water and health. The Government of India banned the export of frog legs. An ecosystem is in balance when interactions keep populations and resources stable; this balance is dynamic, not fixed, and can be disrupted by natural or human-made changes. Competition controls populations. Migratory birds link habitats, pollinate, disperse seeds and control insect pests.

Benefits and threats. Forests give fresh air, fertile soil, food, fibres, timber and medicines; aquatic ecosystems give water and food; ecosystems also give beauty and recreation. The Sundarbans, the world's largest mangrove forests, slow down winds and waves in storms and floods and absorb carbon dioxide while releasing oxygen. UNESCO declared them a World Heritage Site in 1987. Mangroves are cut for fuelwood and farming, wildlife is hunted illegally, and industrial waste and untreated sewage pollute the water. Other threats are deforestation, overuse of resources, invasive species, unsustainable land use and pollution. Protected areas (national parks, wildlife sanctuaries, biosphere reserves, community conserved areas) such as Jim Corbett and Manas conserve habitats. A.J.T. Johnsingh showed that healthy prey populations are key to predator survival.

Farming. Between 1950 and 1965 India faced a food crisis; tractors, machines, synthetic fertilisers and pesticides raised production in the Green Revolution, but overuse of chemicals, excessive groundwater extraction and growing a single crop are considered unsustainable and can degrade soil. Human-made ecosystems such as fish ponds, farms and parks need human care.

Habitats, populations and relationships among organisms are covered in the previous topic.

Food Chains, Food Webs, Decomposers and Ecological Balance: formulas and key facts

Food chain example: $G r a ss > G r a ss h o pp er > F r o g > S nak e > E a g l e$; Arrows point to the eater.
Trophic levels: 1 Producers
2 Herbivores
3 Small carnivores
4 Large carnivores; Number of organisms generally falls going up.
Types of consumers: Herbivore plants only
Carnivore animals only
Omnivore both; Deer; leopard; crow, fox, mouse.
Decomposers: Dead matter + fungi and bacteria > simple nutrients returned to soil; Also called saprotrophs.
Balance of an ecosystem: Stable populations and resources through interactions | Dynamic, not fixed; Natural or human changes can disrupt it.

Herbivore	plants only
Carnivore	animals only
Omnivore	both

How to solve Food Chains, Food Webs, Decomposers and Ecological Balance questions

Use this method for food chain and balance questions.

1Start with the producer and draw arrows towards each eater.
2Label trophic levels from 1 (producer) upwards.
3For a food web list all the chains that share organisms.
4To predict the effect of removing an organism, follow what it eats and what eats it.
5Name the decomposers acting on waste and what is returned to the soil.
6Link the change to a human cause and one remedy if asked.

Food Chains, Food Webs, Decomposers and Ecological Balance in real life

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.

Number pyramid

Suppose a millet field has 4000 millet plants, 120 mice and 3 eagles (invented numbers in the style of Activity 12.7).

(a) Write the food chain. (b) Give the trophic level of each. (c) How many millet plants are there per mouse, correct to the nearest whole number? (d) What happens to mouse numbers if the eagles are removed?

Try it first, then show the solutionSolution

Millet > mouse > eagle.
Millet: level 1; mouse: level 2; eagle: level 3.
4000 ÷ 120 = 33.3, about 33 plants per mouse.
Mice would increase (fewer predators) and eat more millet, until food runs short; this is a prediction from feeding relations.

Answer: (a) Millet > mouse > eagle; (b) 1, 2, 3; (c) about 33; (d) mice increase.

Memory hook: Many at the base, few at the top.

Frogs and pests

Imagine a district where frogs are caught in large numbers, falling from 900 to 360 over a few years while insect pests double from 5000 to 10000 per hectare (invented numbers).

(a) Find the percentage fall in frogs. (b) Find the percentage rise in pests. (c) Explain the link. (d) Name one human response and one effect of that response.

Try it first, then show the solutionSolution

(900 − 360) ÷ 900 × 100 = 540 ÷ 900 × 100 = 60%.
(10000 − 5000) ÷ 5000 × 100 = 100%.
Frogs eat insects, so fewer frogs let pests increase.
Farmers use more synthetic pesticides, which harm soil, water and health.

Answer: (a) 60%; (b) 100%; (c) fewer predators of insects; (d) more pesticides, harming the environment.

Memory hook: Remove a link and the chain wobbles.

Tricks to remember Food Chains, Food Webs, Decomposers and Ecological Balance

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

Arrow points to the eater
Grass → hare means the hare eats the grass.
Use it when: Drawing food chains.
Pro-Her-Car-Top
Producer, Herbivore, small Carnivore, Top carnivore: levels 1 to 4.
Use it when: Trophic levels.
Decomposers are the clean-up crew
They turn dead into nutrients: nothing is wasted.
Use it when: Role of decomposers.

Food Chains, Food Webs, Decomposers and Ecological Balance: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Write two food chains · 3 marks · Short answerBoard level

From grass, grasshopper, frog, snake, hare, eagle and leopard, write two food chains with at least three organisms, and label the producer.

Try it first, then show the solutionSolution

Grass (producer) > grasshopper > frog > snake > eagle. (1.5)
Grass (producer) > hare > leopard. (1.5)

Answer: Grass > grasshopper > frog > snake > eagle; grass > hare > leopard.

Example 2: Define decomposers · 1 mark · MCQBoard level

Mushrooms and bacteria that feed on dead plants are called
(a) producers
(b) herbivores
(c) decomposers
(d) carnivores

Try it first, then show the solutionSolution

They break down dead matter and return nutrients: decomposers (saprotrophs).

Answer: (c)

Example 3: Food web analysis · 5 marks · Long answerHarder board level

In a food web: grasses are eaten by the hare, mouse and grasshopper; the mouse is eaten by the snake, owl and fox; the grasshopper by the frog; the frog by the snake; the snake by the hawk; the hare by the fox.
(a) Write three food chains. (1.5)
(b) Name the organisms at the second trophic level. (0.5)
(c) Which organism is eaten by the most kinds of predators? (1)
(d) What happens to the hare population if the fox disappears? (1)
(e) Why is it a web and not a chain? (1)

Try it first, then show the solutionSolution

(a) Grasses > mouse > owl; grasses > grasshopper > frog > snake > hawk; grasses > hare > fox. (1.5)
(b) Hare, mouse and grasshopper. (0.5)
(c) The mouse, eaten by the snake, owl and fox. (1)
(d) The hare population would tend to increase and then be limited by its food (grasses). (1)
(e) The food chains share organisms, so they are linked into a network. (1)

Answer: (b) Hare, mouse, grasshopper; (c) mouse; (d) hare increases, then limited by food; (e) linked chains form a web.

Example 4: Domino effect in a pond · 4 marks · Case-basedHarder board level

Read the passage and answer.

Pollution kills many pond plants. Oxygen in the water drops, fish die, insects increase, spread to farms and farmers use more pesticides.
(a) Why does oxygen fall? (1)
(b) Why do insects increase? (1)
(c) What is the effect of the pesticides on the environment? (1)
(d) Give one action to prevent this chain of events. (1)

Try it first, then show the solutionSolution

(a) Plants release oxygen in photosynthesis; with fewer plants less oxygen is produced. (1)
(b) Fewer fish means fewer consumers of insects. (1)
(c) Synthetic pesticides can harm the environment, soil, water and health. (1)
(d) Reduce pollution of the pond, or use environment-friendly pest control. (1)

Answer: (a) Less photosynthesis; (b) fewer predators; (c) pesticides harm the environment; (d) prevent pollution.

Example 5: Sundarbans under threat · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

The Sundarbans, the world's largest mangrove forests, slow down storm winds and waves. A group says cutting mangroves for fuelwood has no effect on villages, and another says the forests are only useful to wildlife.
(a) Evaluate the first statement. (1)
(b) Evaluate the second. (1)
(c) Name two other threats. (1)
(d) Name two kinds of protected areas that can help. (1)

Try it first, then show the solutionSolution

(a) Wrong: mangroves protect people by slowing down winds and waves, so cutting them reduces that protection. (1)
(b) Wrong: they also absorb carbon dioxide and release oxygen and support people's well-being. (1)
(c) Any two: illegal hunting, overuse of resources, industrial waste, untreated sewage. (1)
(d) Any two: national park, wildlife sanctuary, biosphere reserve, community conserved area. (1)

Answer: (a) Wrong; (b) wrong; (c) hunting and pollution; (d) e.g. national park and sanctuary.

Where students lose marks in Food Chains, Food Webs, Decomposers and Ecological Balance

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

Mistake: Drawing arrows from the eater to the food.
Direction is confused with 'eats'.
Wrong working
Hare → grass
Right working
Grass → hare
Do this: Arrow shows the flow of food.
Usually costs: 1 mark.
Mistake: Not starting a food chain with a producer.
Animals are in focus.
Wrong working
Frog → snake → eagle
Right working
Grass → grasshopper → frog → snake → eagle
Do this: Always start with the plant.
Usually costs: 1 mark.
Mistake: Classing decomposers as consumers.
They also feed.
Wrong working
Fungi are herbivores.
Right working
Fungi on dead matter are decomposers (saprotrophs).
Do this: Dead matter = decomposers.
Usually costs: 1 mark.
Mistake: Saying a balanced ecosystem never changes.
Balance is mistaken for being fixed.
Wrong working
In balance, populations are constant.
Right working
The balance is dynamic and can be disrupted.
Do this: Use the word dynamic.
Usually costs: 1 mark.

Exam tips

Start every food chain with a producer and draw arrows towards the eater.
Give the trophic level number with each organism.
For cascade questions, write each link in order using 'so' or 'which leads to'.
Quote chapter examples: frog legs, the pond pollution chain, Sundarbans.
Mention a conservation measure to finish a long answer.

Practice questions on Food Chains, Food Webs, Decomposers and Ecological Balance

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

Q1 · 2 marks · Very short answerBoard level
What is a food web? How does it differ from a food chain?
Hint
Interlinked chains.
Answer
A food chain is a simple sequence showing who eats whom (1); a food web is a network of interlinked food chains in an ecosystem (1).
Q2 · 1 mark · MCQBoard level
Which of the following is an omnivore?
(a) Deer
(b) Leopard
(c) Mouse
(d) Hare
Hint
Eats both plants and animals, per the chapter.
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Frogs help farmers by reducing insect pests.
Reason (R): Frogs feed on insects, so a fall in frog numbers can lead to a rise in pests.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Think of the 1980s frog legs case.
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
In a grassland there are 6000 grass plants, 480 grasshoppers, 60 frogs, 6 snakes and 1 eagle (invented numbers). (a) Write the food chain. (b) How many times more grasshoppers than frogs are there? (c) Find the ratio of grass plants to eagles. (d) State the shape produced by arranging these numbers and explain it.
Hint
Divide the numbers.
Answer
(a) Grass > grasshopper > frog > snake > eagle. (b) 480 ÷ 60 = 8 times. (c) 6000 : 1. (d) A pyramid with the largest number at the base, because each level supports fewer organisms above it (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A village stops hunting snakes, so snake numbers rise. In the local food web, snakes eat mice and frogs, owls eat mice and snakes, and mice eat grain.
(a) Predict what happens to the mouse population. (b) What happens to grain losses? (c) Explain why the rise in snakes cannot continue for ever. (d) Name the relationship between a snake and a mouse.
Hint
The mouse is eaten by snakes in the food web.
Answer
(a) Mice decrease, since more snakes eat them. (b) Grain losses to mice fall. (c) As mice and frogs become scarce, snakes compete for food and their numbers are limited; more snakes also give owls more food. (d) A predator and its prey (a feeding relationship) (1 mark each).

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.What are producers?Show answer
Green plants that make their own food by photosynthesis.
2.Name a herbivore, a carnivore and an omnivore from the chapter.Show answer
Deer; leopard; crow, fox or mouse.
3.What are decomposers also called?Show answer
Saprotrophs.
4.Which cause of a pest outbreak did the frog legs case show?Show answer
Fewer frogs meant fewer predators of insects.
5.When did UNESCO declare the Sundarbans a World Heritage Site?Show answer
1987.

Food Chains, Food Webs, Decomposers and Ecological Balance: frequently asked questions

Why is a pyramid of numbers broad at the base?

Producers are the most numerous, and fewer organisms are found at each higher level in the chapter's examples.

Does nature waste anything?

Decomposers recycle dead matter into nutrients, so in nature nothing is wasted.

What was the Green Revolution?

The mid-20th-century rise in food production through tractors, machines, synthetic fertilisers and pesticides, after India's food crisis between 1950 and 1965.

Why are corridors needed?

They connect forest habitats so that wildlife, like elephants, can move safely.