Food Chains, Food Webs, Pyramid of Numbers, Symbiosis, Parasitism and Predation (ICSE Class 8)
Updated 17 October 2026 · Fact-checked
A food chain shows who eats whom, with arrows pointing from the eaten to the eater, and a food web is many linked chains. A pyramid of numbers shows how many organisms live at each trophic level. Organisms depend on each other through symbiosis (both benefit), parasitism (parasite gains, host harmed) and predation (predator kills prey).
Arrows point from the eaten to the eater; going up, numbers and energy shrink, so there are many grass plants but only a few eagles.
Illustration drawn for QuestionBuddy.
Understand Food Chains, Food Webs, Pyramid of Numbers and Interdependence
Food chain. A sequence of organisms in which each is eaten by the next, showing the flow of food and energy. The arrow points from the eaten to the eater.
- Grass → deer → tiger.
- Leaf → caterpillar → small bird → snake → eagle.
Every chain starts with a producer. Each step is a trophic level: producer (1), herbivore (2), carnivores (3 and above).
Energy and chain length. Energy is lost at each step as movement and body heat, so chains rarely have more than four or five links. As a rough rule only about one-tenth passes to the next level.
Food web. A network of interlinked chains. A snake eats frogs, rats and birds; a hawk eats snakes and rats. A web makes the ecosystem stable: if one food is scarce, animals can switch to another.
Pyramid of numbers. Bars showing the number of organisms at each trophic level, with producers at the base.
- Upright (grassland): many grass plants → fewer grasshoppers → fewer frogs → fewer snakes → one or two eagles. It narrows because energy decreases at each step.
- Forest (spindle-shaped): a few big trees support a very large number of insects, which support fewer birds; the base is narrow.
- Inverted (parasite chain): one tree → many birds → very many lice. Numbers increase upward.
Interdependence of organisms.
- Symbiosis: close association in which both benefit. Lichen (alga makes food, fungus gives shelter and water); Rhizobium bacteria in root nodules of pea and gram (bacteria fix nitrogen for the plant and get food and shelter); bee and flower.
- Parasitism: the parasite lives on or in a host and takes food from it; parasite benefits, host is harmed. Cuscuta (amarbel) on a host plant; ticks and leeches; tapeworm in an animal.
- Predation: a predator kills and eats its prey (tiger and deer, owl and mouse). Predators keep prey numbers in check; if tigers vanish, deer multiply and overgraze the forest.
| Symbiosis | Parasitism | Predation | |
|---|---|---|---|
| First partner | benefits | parasite benefits | predator benefits |
| Second partner | benefits | host harmed | prey killed |
Flow chart: energy through a chain. Sun → producer (100%) → herbivore (about 10%) → carnivore (about 1%) → top carnivore (about 0.1%).
Food Chains, Food Webs, Pyramid of Numbers and Interdependence: key facts
- Food chain
- Starts with producer
- Arrow from eaten to eater
- Trophic levels
- About one-tenth energy passed on
- Chains seldom exceed five links.
- Pyramid of numbers
Upright grassland Spindle forest Inverted parasite chain Base is producers - Shape depends on the size of producers.
- Interdependence
Symbiosis both gain Parasitism parasite gains, host harmed Predation predator kills prey - Ask who gains and who loses.
How to answer Food Chains, Food Webs, Pyramid of Numbers and Interdependence questions
Typical tasks: build a chain from given organisms, use a web to predict the effect of removing one organism, draw or choose a pyramid, or name the relationship in a description.
- 1Find the producer first and put it at the start; then order by what eats what.
- 2Draw arrows from the eaten to the eater and write the trophic level of each.
- 3For removal questions, follow the arrows: the eater of the removed organism loses food, its food organisms increase.
- 4For a pyramid, count individuals at each level; a few big producers can give a spindle shape.
- 5For relationships, ask whether each partner gains, is harmed or is killed.
Food Chains, Food Webs, Pyramid of Numbers and Interdependence in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Fewer snakes, more rats
Suppose villagers kill most of the snakes in a field next to a forest and, a year later, find that rats have multiplied and eaten much of the stored grain.
Explain using a food chain.
Try it first, then show the solutionSolution
- In the chain grain → rat → snake, the snake is the predator of the rat.
- With fewer snakes, fewer rats are eaten, so rat numbers rise.
- More rats eat more grain.
Answer: Removing the predator allowed the prey (rats) to multiply and damage the grain.
Memory hook: Remove the predator, the prey explodes.
Energy at each step
Suppose the plants in a small meadow capture 8000 units of energy as food in a year, and about one-tenth passes to each next level.
How much energy reaches the fox (the carnivore, at the third trophic level) in the chain plant → rabbit → fox?
Try it first, then show the solutionSolution
- Plants hold 8000 units.
- Rabbits receive one-tenth: 800 units.
- Foxes receive one-tenth of that: 80 units.
Answer: 80 units.
Memory hook: Divide by ten at every step.
Tricks to remember Food Chains, Food Webs, Pyramid of Numbers and Interdependence
Short memory aids. Say each one aloud once, then use it on a question the same day.
Arrow means eaten by
Grass → deer reads as grass is eaten by deer.
Use it when: Direction of arrows.
Both, one, none
Symbiosis: both gain; parasite: one gains and the other loses; predator: one gains and the other dies.
Use it when: Three relationships.
Tree is not a tiny base
Few big trees, many insects: the forest pyramid bulges in the middle.
Use it when: Pyramid shapes.
Food Chains, Food Webs, Pyramid of Numbers and Interdependence: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: First link · 1 mark · MCQBoard level
The first member of every food chain is always a:
(a) herbivore (b) carnivore (c) producer (d) decomposer
Try it first, then show the solutionSolution
- A chain begins with the organism that makes its own food.
Answer: (c).
Example 2: Two partnerships · 2 marks · Short answerBoard level
A bee takes nectar from a flower and carries its pollen; a leech sucks blood from a buffalo. Name the relationship in each and say who gains.
Try it first, then show the solutionSolution
- Bee and flower: symbiosis; both gain (1 mark).
- Leech and buffalo: parasitism; the leech gains and the buffalo is harmed (1 mark).
Answer: Symbiosis (both gain); parasitism (leech gains).
Example 3: Energy in a chain · 3 marks · Short answerHarder board level
In a forest chain, producers hold 5000 kJ of energy. Assuming one-tenth passes to each level, find the energy at (a) the herbivore, (b) the secondary consumer, (c) the third consumer in the chain leaf → caterpillar → bird → snake.
Try it first, then show the solutionSolution
- (a) 5000 ÷ 10 = 500 kJ (1 mark).
- (b) 500 ÷ 10 = 50 kJ (1 mark).
- (c) 50 ÷ 10 = 5 kJ at the snake (1 mark).
Answer: (a) 500 kJ. (b) 50 kJ. (c) 5 kJ.
Example 4: Shape of pyramids · 4 marks · Long answerHarder board level
Draw the shape of the pyramid of numbers and explain it for (a) grass → grasshopper → frog → snake and (b) one tree → 50 birds → 5000 lice.
Try it first, then show the solutionSolution
- (a) Upright pyramid: many grass plants, fewer grasshoppers, fewer frogs, very few snakes (1 mark).
- It is upright because energy decreases at each level, so fewer individuals can be supported (1 mark).
- (b) Inverted pyramid: one tree at the base but numbers increase upward (1 mark).
- It is a parasite chain, in which many small parasites feed on a few hosts (1 mark).
Answer: (a) Upright; energy falls at each level. (b) Inverted; many parasites on few hosts.
Example 5: A web in trouble · 4 marks · Case-basedHOTS / competency
In a village-edge web, hawks eat both snakes and rats, snakes eat rats and frogs, frogs eat insects and rats eat grain. Poison kills all the snakes. Predict what happens to (a) rats, (b) frogs, (c) hawks, giving reasons, and say what the web teaches.
Try it first, then show the solutionSolution
- (a) Rats increase: they lose a predator, though hawks still eat some (1 mark).
- (b) Frogs increase as no snake eats them, so insects then fall (1 mark).
- (c) Hawks survive by eating rats, which are now plentiful (1 mark).
- With several food paths, the loss of one link does not collapse the ecosystem, though numbers change (1 mark).
Answer: Rats and frogs rise; hawks survive on rats; the web gives stability.
Where students lose marks in Food Chains, Food Webs, Pyramid of Numbers and Interdependence
The slips examiners see most often, and what to write instead.
Mistake: Arrows drawn from the eater to the eaten
Students read the arrow as eats instead of is eaten by.
Do this: The arrow shows the direction of energy: grass → deer.
Usually costs: 1 mark.
Mistake: Starting a chain with an animal
The producer is left out.
Do this: Begin with a green plant.
Usually costs: 1 mark.
Mistake: Calling every association symbiosis
Close living together is taken to mean mutual benefit.
Do this: Check whether the second partner gains or is harmed.
Usually costs: 1 mark.
Mistake: Saying forest pyramids are always upright
Large trees are counted as many.
Do this: A forest pyramid is spindle-shaped.
Usually costs: 1 mark.
Exam tips
- Producer first; arrows from eaten to eater.
- A food web is many chains and gives stability.
- Lichen and Rhizobium are symbiosis; Cuscuta is parasitism.
- Quote the one-tenth rule for chain length.
Practice questions on Food Chains, Food Webs, Pyramid of Numbers and Interdependence
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · Very short answerBoard level
Define a food web.
Hint
Many chains.
Answer
A network of interconnected food chains in an ecosystem.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): If all the green plants of an area die, every consumer there will eventually suffer.
Reason (R): Producers are the only way the Sun's energy enters the food chain.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).Hint
Where does the energy enter?
Answer
(c). With no producers no energy enters, so herbivores and then carnivores starve.
Q3 · 3 marks · Short answerHarder board level
Rhizobium bacteria live in the root nodules of a gram plant. Name the relationship, say what each partner gives and receives, and explain why it is not parasitism.
Hint
Who gains?
Answer
The relationship is symbiosis. The bacteria fix nitrogen for the plant and receive food and shelter from it. It is not parasitism because the plant is not harmed: both partners benefit.
Q4 · 3 marks · Long answerHarder board level
Why do food chains rarely have more than four or five links? Use the idea of energy.
Hint
Think about 10%.
Answer
Only about one-tenth of the energy at one level reaches the next, because the rest is used up in movement, body heat and life processes. After four or five steps too little energy remains to support another level.
Q5 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a forest has these organisms: grass, deer, tiger, bacteria, Cuscuta growing on a shrub.
(a) Write a food chain from the list. (1)
(b) Name the relationship of Cuscuta with the shrub. (1)
(c) What would happen to deer if tigers were removed, and what harm follows? (1)Hint
Arrows from eaten to eater.
Answer
(a) Grass → deer → tiger. (b) Parasitism. (c) Deer would increase, overgraze the grass and damage the forest.
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 is each step in a food chain called?Show answer
A trophic level.
2.Name a parasitic plant.Show answer
Cuscuta (amarbel).
3.Which partners make a lichen?Show answer
An alga and a fungus.
4.In which direction does the arrow point in a food chain?Show answer
From the eaten to the eater.
5.What does a predator do to its prey?Show answer
Kills and eats it.
Food Chains, Food Webs, Pyramid of Numbers and Interdependence: frequently asked questions
Is the one-tenth rule exact?
No, it is a rough rule used to show that energy falls steeply at each step.
Do I need to draw pyramids?
Be ready to draw and label the three shapes with an example.
Is a predator always a carnivore?
In this syllabus a predator is an animal that kills and eats another animal.
Are producers and decomposers in a chain?
The chain starts with a producer; decomposers act on dead remains at every level.