10 min readPathway sheetSection A MCQ (1 mark) and objective (2 marks, true or false or fill in the blanks); Section B very short answer (2 marks) or short answer (3 marks)

Food chains, food webs, trophic levels and the flow of energy

A food chain passes food energy from producers to herbivores to carnivores; each step is a trophic level. Energy flows one way and shrinks at every step.

Food chainA series of organisms through which food energy passes, each eating the one below it and being eaten by the one above

Trophic levelEach step of a food chain (trophos means feeding); producers are always level I

Two kinds of food chainGrazing (starts with green plants) and detritus (starts with dead organic matter)

Energy flowUnidirectional: energy never comes back to an earlier level, and less is passed on at each step

Draw it: a grazing food chain on land

Arrows point from the food to the eater, the way food and energy move. Your NIOS lesson also gives trees and shrubs → giraffe → lion.

Grazing and detritus food chains

Feature	Grazing food chain	Detritus food chain
Starts with	Living green plants (producers)	Dead organic matter (detritus: dead leaves, dead animals)
Next link	Herbivores that eat the plants	Fungi and bacteria that break the dead matter into simple nutrients
Then	Carnivores, then larger carnivores	Small animals that eat these nutrients and decomposers, then larger carnivores
Where found	Land and water; the most common chain on land	Land and water, for example the floor of a forest or the bottom of a pond
Example	Phytoplankton → small fish → big fish	Dead leaves → bacteria and fungi → insect larvae → frog

Food webs and why they matter

Most animals feed on several kinds of food, so each belongs to several food chains. Linked together, these chains make a food web. A snake may eat frogs and rats; a hawk may eat snakes and rats. Food chains and webs keep the ecological balance, show who feeds on whom, and are the paths along which energy flows and nutrients cycle. Because animals in a web have other foods to turn to, a web is more stable than a single chain. Still, removing one species upsets the whole web.

One-way flow of energy

The sun is the source of energy for every ecosystem. Plants trap it and store it as food. When a herbivore eats a plant, only a small part of that energy becomes its new body mass. The rest is lost as heat or used up in life processes such as moving, digesting, breathing and reproducing. So a carnivore that eats the herbivore gets even less, and it must eat many herbivores to grow. Energy passed up the chain never comes back down: the flow is unidirectional. Because so much is lost at each link, food chains usually have only four or five trophic levels.

Activity: build food chains from specimens or pictures (practical)

1
Collect pictures or specimens of plants and animals, or list those you see in a crop field, garden or pond.
2
Sort them into three groups: plants, plant-eaters (herbivores) and flesh-eaters (carnivores).
3
On a large sheet, place one plant and draw an arrow to its right. Place a herbivore that eats it after the arrow.
4
Add a carnivore that eats the herbivore, then a larger one, to reach four trophic levels, for example plant → rat → snake → hawk.
5
Make as many chains as you can and write I, II, III, IV under each organism.
6★
Observation: every chain starts with a green plant, and the top consumers come at the end.

★ key step

How to draw a food chain or a food web

Write the producer first, on the left or at the bottom.
Write each eater after it, in order, with an arrow pointing from the food to the eater.
Under each organism, write its trophic level: I, II, III, IV.
For a food web, draw several chains that share organisms. For example: grass → rat → snake → hawk, and grass → grasshopper → frog → snake.
Never draw an arrow backwards: it shows the direction in which food and energy move.

In real life

Eating rice, dal or vegetables puts you at level II, like a herbivore. Eating a fish that ate smaller fish puts you higher up. A plant-based meal loses less energy along the chain.

Exam angle

1 marks: “Which trophic level do green plants always occupy?” The first trophic level, as producers.

2 marks: “A lion must eat many herbivores to grow, but a deer feeds only on plants. Explain this using the flow of energy.” At each step most energy is used in life processes or lost as heat, so the lion, higher up, gets only a small part of the energy the herbivores took in.

3 marks: “A heap of fallen leaves on a forest floor starts one kind of food chain, and a field of grass starts another. Name the two kinds, trace each through three links, and give one difference between them.” Detritus food chain: dead leaves → bacteria and fungi → insect larvae; it starts from dead organic matter. Grazing food chain: grass → grasshopper → frog; it starts from living green plants.

Remember it

The arrow means 'is eaten by'. Energy goes up and out, never back: Grass, Grasshopper, Frog, Snake, Hawk.