ScienceCBSE Class 10 · Our Environment

Food Chains, Food Webs and Energy Flow

Updated 14 October 2026 · Fact-checked

A food chain is a series of organisms in which each feeds on the one before it; each step is a trophic level, starting with producers. Energy flows one way, and only about 10% passes from one level to the next, so chains have three or four steps. Interlinked chains form a food web.

Notice that energy drops to about one tenth at each step up, so food chains have only three or four levels.

Illustration drawn for QuestionBuddy.

Understand Food Chains and Food Webs

Food chain and trophic levels. A series of organisms feeding on one another is a food chain, for example grass → deer → lion (forest), grass → grasshopper → frog → snake → eagle (grassland), or phytoplankton → zooplankton → fish → bigger fish (pond). Each step is a trophic level:
1. producers (autotrophs), which fix solar energy;
2. herbivores or primary consumers;
3. small carnivores or secondary consumers;
4. larger carnivores or tertiary consumers.

Energy flow. Autotrophs capture energy from sunlight and turn it into chemical energy, which supports the whole living world; it then passes to heterotrophs and decomposers. Every time energy changes form, some is lost to the surroundings in forms that cannot be used again. Studies of energy flow show that:

on land, plants trap only about 1% of the solar energy reaching their leaves and convert it into food energy;
when a consumer eats, much energy is lost as heat, some is used in digestion and work, and the rest goes to growth and reproduction; on average 10% of the food eaten becomes the consumer's own body and is available to the next level (the 10 per cent law);
so little energy reaches higher levels that food chains generally have only three or four steps;
there are generally more individuals at lower levels, with the largest number of producers.

The flow of energy is unidirectional: energy captured by autotrophs does not go back to the Sun, and energy passed to herbivores does not return to plants. The energy available also decreases at each level.

Food web. Each organism is usually eaten by two or more kinds of organisms, which are eaten by several others. So instead of separate straight chains, the feeding relationships form a network of branching lines, a food web. Webs make ecosystems more stable, because a consumer can switch to another food if one becomes scarce.

Biological magnification. Pesticides and other chemicals used on crops wash into soil and water. Plants absorb them with water and minerals; aquatic plants and animals take them up from water bodies. These chemicals are not degradable, so they accumulate progressively at each trophic level. Human beings, at the top of many food chains, get the highest concentration. This is biological magnification. It is why food grains such as wheat and rice, vegetables, fruits and even meat may contain pesticide residues that cannot always be removed by washing.

Removing a trophic level. If all organisms of one level were killed, the levels above would lose their food, and the level below would multiply unchecked for a while; the balance of the ecosystem would be upset. Removing producers would be the most damaging, since every other level depends on them.

Food Chains and Food Webs: formulas and key facts

10 per cent law: $E_{next level} \approx 0.10 \times E_{this level}$; Average transfer between consumers' trophic levels.
Energy after n transfers: $E_{n} = E_{1} \times (0.1)^{n - 1}$; From the producer level (level 1) to level n.
Sunlight captured by plants: $\approx 1% of sunlight falling on leaves$; Terrestrial ecosystems.
Trophic levels: $producer \to herbivore \to small carnivore \to large carnivore$; First, second, third, fourth levels.

How to solve Food Chains and Food Webs questions

For food chain questions, place organisms in order and track energy or chemicals.

1Start with a producer (green plant or phytoplankton); add 'who eats whom' with arrows pointing towards the eater.
2Number the trophic levels from the producer (first level).
3For energy: multiply by 0.1 at each step from one level to the next; for sunlight to plants use 1%.
4For biomagnification: concentration of a non-degradable chemical increases at each step; the top level has the most.
5For 'what if a level is removed', trace effects both up (starvation) and down (unchecked growth).
6For food webs, show that one organism can belong to more than one chain and more than one level.

Food Chains and Food Webs 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.

10 per cent law and diet

Suppose a farmer's field produces crops containing 100000 kJ of food energy in a season. He can either feed the grain directly to people or feed it to goats and give people goat meat.

Using the 10 per cent law, compare the energy available to people in the two cases, and explain why food chains are short.

Try it first, then show the solutionSolution

Grain eaten directly: people are at the second trophic level and get about 10% of 100000 kJ = 10000 kJ.
Grain → goats: goats (second level) get about 10000 kJ; people eating goats (third level) get about 10% of that = 1000 kJ.
So eating the grain directly makes about 10 times more energy available to people.
Each transfer loses about 90% of the energy (as heat, digestion, work), so after three or four levels very little usable energy is left; that is why food chains generally have only three or four steps.

Answer: About 10000 kJ directly versus 1000 kJ through goats; energy drops about tenfold at each step, which keeps food chains short.

Memory hook: Each step up the chain keeps only one-tenth: eat lower, get more.

Tricks to remember Food Chains and Food Webs

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

Divide by ten
Move one level up: divide the energy by 10.
Use it when: Energy numericals.
Poison piles at the top
Non-degradable chemicals multiply up the chain; humans at the top get the most.
Use it when: Biological magnification.
Energy is a one-way street
Sun → producers → herbivores → carnivores, never backwards.
Use it when: Unidirectional energy flow.

Food Chains and Food Webs: solved examples

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

Example 1: Trophic levels · 3 marks · Short answerBoard level

What are trophic levels? Give an example of a food chain with four trophic levels and name each level.

Try it first, then show the solutionSolution

Each step or level of a food chain is a trophic level (1 mark).
Example: grass → grasshopper → frog → snake (1 mark).
Grass: producer (first level); grasshopper: primary consumer (second); frog: secondary consumer (third); snake: tertiary consumer (fourth) (1 mark).

Answer: Steps of a food chain; e.g. grass (producer) → grasshopper (primary) → frog (secondary) → snake (tertiary).

Example 2: Energy numerical · 3 marks · Short answerHarder board level

In a food chain grass → deer → lion, the grass has 50000 J of energy available. How much energy is expected to reach the deer and the lion? If sunlight falling on the grass leaves was 5000000 J, what percentage did the grass capture?

Try it first, then show the solutionSolution

Deer: $0.10 \times 50000 = 5000$ J (1 mark).
Lion: $0.10 \times 5000 = 500$ J (1 mark).
Grass captured $\frac{50000}{5000000} \times 100 = 1%$ , matching the NCERT figure for terrestrial plants (1 mark).

Answer: Deer 5000 J; lion 500 J; grass captured 1% of sunlight.

Example 3: Biological magnification · 3 marks · Short answerHarder board level

What is biological magnification? Will its level be different at different trophic levels? Explain with a food chain.

Try it first, then show the solutionSolution

The progressive accumulation of non-degradable chemicals, such as pesticides, at each trophic level of a food chain (1 mark).
Yes; the concentration increases at each higher level, because organisms eat many individuals from the level below and the chemicals are not broken down (1 mark).
Example: water → phytoplankton → small fish → large fish → humans; humans at the top have the highest concentration (1 mark).

Answer: Accumulation of non-degradable chemicals up the chain; highest at the top (humans).

Example 4: Removing a trophic level · 5 marks · Long answerHOTS / competency

What will happen if we kill all the organisms in one trophic level? Will the impact be different for different levels? Can any level be removed without causing damage?

Try it first, then show the solutionSolution

Organisms at the next higher level lose their food and their numbers fall; organisms at the lower level, freed from being eaten, may increase sharply at first (1½ marks).
Removing producers is most damaging: no level can get energy from sunlight, so the whole food chain collapses (1 mark).
Removing top carnivores lets herbivores or smaller carnivores increase, which can then overgraze producers (1 mark).
Removing decomposers would stop nutrient recycling and let wastes accumulate (½ mark).
No level can be removed without damage, because all levels are interdependent (1 mark).

Answer: The balance is upset; effects differ by level (producers most severe); no level can be removed without harm.

Example 5: Food web stability · 3 marks · Short answerHOTS / competency

In a grassland, grasshoppers are eaten by both frogs and birds, and frogs are eaten by snakes and by birds. Suppose a disease kills most frogs. Using the idea of a food web, predict what happens to the snakes and birds, and explain why a food web is more stable than a single food chain.

Try it first, then show the solutionSolution

Snakes lose a main food source and may decline, unless they can eat other prey (1 mark).
Birds still eat grasshoppers, so they are less affected; grasshoppers may also increase for a time with fewer frogs eating them, giving the birds more food (1 mark).
In a web, organisms have several food sources, so the loss of one population is buffered by others; in a single chain, losing one link breaks the chain (1 mark).

Answer: Snakes decline, birds cope by eating grasshoppers; webs offer alternative foods, so they are more stable.

Where students lose marks in Food Chains and Food Webs

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

Mistake: Arrows pointing the wrong way
Students point from eater to eaten.
Wrong working
lion → deer → grass
Right working
grass → deer → lion (arrow shows direction of energy flow, towards the eater).
Do this: Arrows follow energy: food to feeder.
Usually costs: 1 mark.
Mistake: Applying 10% to sunlight
The two figures are mixed up.
Wrong working
Plants capture 10% of sunlight.
Right working
Plants capture about 1% of sunlight; about 10% passes between trophic levels.
Do this: 1% sun to plant; 10% level to level.
Usually costs: 1 mark.
Mistake: Saying biomagnification is highest in producers
Producers absorb the chemical first.
Wrong working
Producers have the highest pesticide concentration.
Right working
Concentration is highest at the top trophic level.
Do this: It magnifies upward.
Usually costs: 1 mark.
Mistake: Starting a chain without a producer
Students begin with an animal.
Wrong working
Goat → cow → elephant.
Right working
Every chain begins with a producer, e.g. grass → goat → human.
Do this: First link = green plant or phytoplankton.
Usually costs: 1 mark.

Exam tips

Label each organism with its trophic level when writing a food chain.
Show 10% calculations step by step; marks are given for each level.
Use the words 'unidirectional' and 'progressively diminished' for energy flow.
Define biological magnification and give a chain ending with humans.
For 'removing a level', discuss effects both above and below.

Practice questions on Food Chains and Food Webs

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 · MCQBoard level
In the grassland chain grass → grasshopper → frog → snake → eagle, which organism is at the third trophic level?
(a) grasshopper (b) frog (c) snake (d) eagle
Hint
Count from the producer, which is the first trophic level.
Answer
(b) frog. Grass is the first level, the grasshopper the second (primary consumer) and the frog the third (secondary consumer).
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Food chains generally consist of only three or four steps.
Reason (R): Only about 10% of energy is transferred from one trophic level to the next.
(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
What remains after four transfers?
Answer
(a).
Q3 · 2 marks · Very short answerBoard level
A student says: 'The heat a lion gives off can be taken up by the grass again, so energy goes round in a cycle just like nutrients.' Evaluate the claim.
Hint
Can plants use heat from animals to make food? Compare with what decomposers return to the soil.
Answer
The claim is wrong. Grass captures energy only from sunlight; heat lost by the lion (and at every level) escapes to the surroundings in a form that cannot be used again, and energy passed to herbivores never returns to plants, so energy flow is one-way (1 mark). Nutrients, by contrast, are recycled: decomposers return them to the soil as simple inorganic substances that plants take up again (1 mark).
Q4 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
Suppose a lake near farmland receives run-off containing a non-degradable pesticide. Tests show the pesticide in the water at a very low level, higher in small fish, and highest in fish-eating birds.
(a) Name the phenomenon. (1 mark)
(b) Write the likely food chain. (1 mark)
(c) Explain why the concentration rises at each step and why people who eat a lot of fish from this lake may be at risk. (2 marks)
Hint
Non-degradable chemicals are not broken down or excreted easily.
Answer
(a) Biological magnification. (b) Water → phytoplankton/aquatic plants → small fish → fish-eating birds (and humans). (c) Each organism eats many organisms from the level below; the pesticide is not degradable, so it accumulates in its body. Concentration therefore multiplies up the chain. People eating many fish from the lake are at a high trophic level and may take in large amounts.
Q5 · 3 marks · Short answerHarder board level
A producer level has 40000 kJ of energy. Calculate the energy available at the fourth trophic level using the 10 per cent law, and explain why a fifth level is rarely found.
Hint
Three transfers to reach the fourth level.
Answer
Second: 4000 kJ; third: 400 kJ; fourth: 40 kJ (40000 × 0.1³ = 40 kJ). A fifth level would get only about 4 kJ, too little usable energy to support a population, so food chains rarely go beyond four steps.
Q6 · 3 marks · Short answerHOTS / competency
Why are there generally more individuals at lower trophic levels than at higher ones? Use the idea of energy available.
Hint
Less energy supports fewer organisms.
Answer
Energy decreases about tenfold at each step. A lower level has much more energy available, so it can support a larger number of organisms; the highest levels have very little energy and can support only a few individuals. Producers, at the base, are the most numerous.

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.First trophic level?Show answer
Producers (autotrophs).
2.Percentage of sunlight captured by terrestrial plants?Show answer
About 1%.
3.Average energy transferred to the next level?Show answer
About 10%.
4.Why are food chains short?Show answer
Large energy loss at each step leaves little after three or four levels.
5.What is a food web?Show answer
A network of interconnected food chains.
6.Who has the highest concentration of pesticides?Show answer
Organisms at the top trophic level, often humans.

Food Chains and Food Webs: frequently asked questions

What is the 10 per cent law?

On average only about 10% of the food energy at one trophic level is turned into body mass and passed on to the next level.

What is the difference between a food chain and a food web?

A food chain is a single straight series of who eats whom. A food web is a network of many interlinked food chains.

Why is energy flow unidirectional?

Energy passes from the Sun to producers to consumers and never returns to an earlier level; some is lost as heat at each step.

What is biological magnification?

The build-up of non-degradable chemicals like pesticides at each successive trophic level, reaching the highest concentration at the top.

Why are there usually only three or four trophic levels?

So much energy is lost at each step that very little usable energy remains after four levels.