Food Chains and Food Webs: Common Mistakes in Class 10 Answers

Food chains look like the easiest question in Class 10 Biology: a few organisms, a few arrows, done. Yet examiners see the same slips every year, such as arrows pointing the wrong way, decomposers placed in the chain, and energy figures that do not add up. Each one costs a mark or two, and each one is avoidable.

This guide builds food chains and webs from the ground up, explains the 10 percent rule and biological magnification, and then lists the five mistakes that cost the most marks, with the fix for each.

Start With the Arrow

In a food chain, the arrow does not mean “eats”. It means “is eaten by”, and more precisely, it shows the direction in which energy flows.

Grass → grasshopper → frog → snake → eagle

Read it as: the grass is eaten by the grasshopper, which is eaten by the frog, and so on. The arrow always points from the food to the eater. If you get this wrong, the rest of the chain is wrong too.

Building Chains, From a Pond to a Forest

Every food chain starts with a producer, an organism that makes its own food by photosynthesis, and continues through consumers, organisms that eat others. Each step in the chain is a trophic level.

A separate group, the decomposers, such as bacteria and fungi, break down dead plants and animals and return nutrients to the soil. They work at every level, but they are not drawn as a link in the chain.

Trophic Levels and the 10 Percent Rule

Energy enters the chain as sunlight, captured by the producers. At each step, only about 10 percent of the energy is passed on to the next trophic level. The rest is used by the organism for its own life processes, such as respiration and movement, or is lost as heat.

This is why food chains are short, usually three or four steps, rarely more than five. By the fifth level, there is hardly any energy left to support another one.

From Chain to Web

In nature, almost nothing eats just one thing, and almost nothing is eaten by just one other thing. A frog eats grasshoppers, but also flies. A snake may eat frogs, rats and birds. When many food chains in an ecosystem connect, they form a food web.

Grass → Grasshopper → Frog → Snake → Eagle

Grass → Rat → Snake

Grass → Rat → Eagle

A web is more stable than a single chain. If one food source declines, an animal that eats several kinds of food can switch to another and survive.

Biological Magnification

Some chemicals, such as certain pesticides, do not break down in the body or in the environment. They enter the food chain at the producer level, perhaps by sprinkling on crops, and are passed from one trophic level to the next. Because each consumer eats many members of the level below it, the chemical builds up, so its concentration is highest in the top consumers. This increase along the food chain is called biological magnification. It is the reason the NCERT chapter points out that humans, who eat at the top of many chains, can carry the highest concentrations of such chemicals.

Five Mistakes That Cost Marks

Two smaller slips are worth watching. First, an omnivore can occupy more than one trophic level, because it eats both plants and animals. Second, a food chain is not the same thing as a food web. A chain is one pathway, and a web is many pathways joined together.

An Energy Calculation, Step by Step

Energy questions are easy once you treat the 10 percent rule as a multiplication by 0.1 at each step.

Question. The producers in a grassland chain, grass → grasshopper → frog → snake → eagle, capture 50,000 J of energy. How much reaches the snake?

• Producers (grass): 50,000 J
• Primary consumers (grasshopper): 50,000 × 0.1 = 5,000 J
• Secondary consumers (frog): 5,000 × 0.1 = 500 J
• Tertiary consumers (snake): 500 × 0.1 = 50 J

Working backwards. If an eagle at the fifth level needs 10 J, how much energy must the grass capture? Multiply by 10 at each of the four steps back: 10 → 100 → 1,000 → 10,000 → 100,000 J. That large figure shows why there are far fewer top predators than grazing animals.

Why a Shorter Chain Feeds More People

The 10 percent rule has a practical meaning for how people eat. Eating plants directly takes energy from the first trophic level. Eating an animal that ate plants takes energy from the second level or higher, after most of the original energy has been lost at each step. Much more land is needed to produce the same amount of food energy through animal products than through plants. This is one of the arguments made for the efficiency of plant-based foods, though people also choose food for taste, culture and nutrition, which this calculation does not capture.

Pyramids: Seeing the Same Idea as a Shape

The numbers above can be drawn as a pyramid of energy, with producers as the broad base and each higher level narrower than the one below.

• A pyramid of energy is always upright, because energy always shrinks at each step.
• Pyramids of numbers or biomass can sometimes be inverted. A single large tree can support thousands of insects, for example, so the number of consumers can exceed the number of producers.

If a question asks why an energy pyramid cannot be inverted, the answer is the 10 percent rule: energy is lost at every step, so a higher level can never have more than the level below it.

Biological Magnification With Numbers

Here is an illustrative example, with made-up figures, to show how the build-up works. Suppose a pesticide in a pond is at 0.02 parts per million in the water.

The chemical is not broken down, so each organism that eats many of the one below stores more than any of them did. These numbers are not real measurements, but the shape of the pattern, rising at every level, is what the textbook describes.

Decomposers Close the Loop

A food chain describes how energy flows one way. Matter, on the other hand, goes round in a cycle. Decomposers such as bacteria and fungi break down dead bodies and wastes at every level and return nutrients to the soil, where producers take them up again. Without decomposers, dead matter would pile up, and nutrients would stay locked in it. That is why they are sometimes called the cleaners of an ecosystem, and why, in a chain diagram, they are noted separately.

Food Chains in Your Textbooks

This topic is part of Our Environment, which is Chapter 13 in the CBSE Class 10 Science textbook for 2026-27 and Chapter 15 in the Karnataka SSLC textbook, according to published listings. Revise with these notes on the overview of the food chain and the ecosystem. Two related topics are the difference between ozone layer depletion and global warming and biodegradable vs non-biodegradable waste. In Class 12 Biology, the same ideas return in the chapter on ecosystems, which is part of the NEET Ecology and Environment unit, as in this complete guide to ecology and environment for NEET. Confirm the current portions with your own board.

Frequently Asked Questions

Q1. What is a food chain?

A food chain is a series of organisms in which each one is eaten by the next, showing how energy flows from producers to consumers.

Q2. What is the difference between a food chain and a food web?

A food chain is a single pathway of energy flow. A food web is a network of many interconnected food chains in an ecosystem.

Q3. What is the 10 percent rule?

Only about 10 percent of the energy at one trophic level passes to the next. The rest is used by the organisms for their life processes or lost as heat.

Q4. Why are food chains short?

Because so much energy is lost at each step, by the fourth or fifth level there is too little energy left to support another level.

Q5. What is biological magnification?

It is the increase in the concentration of a non-degradable chemical, such as a pesticide, at each successive trophic level of a food chain, so it is highest in the top consumers.

Follow the Energy

Everything about food chains makes sense once you follow the energy: it enters through producers, shrinks by about 90 percent at every step, and ends as heat. The arrows, the short chains, the webs and the magnification of chemicals are all consequences of that one fact.