3.7Chapter 3: Microorganisms and Decomposers

Food chains, food webs and decomposers — Alhussan Biology - Stage 7 Revision Notes & Topical Questions

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3.7 Food chains, food webs and decomposers — Alhussan International Academy — Biology (Stages 7–9) notes: Build and interpret food chains and food webs and explain the importance of decomposers in them.Food chain — a simple model that shows the feeding relationships between organ…

Further examples
Primary consumer
Secondary consumer
Tertiary consumer
Length limit
Keystone species

Section 1 of 8Food chains

Food chain

Food chain a simple model that shows the feeding relationships between organisms in a habitat.

Arrows

Arrows they show the transfer of biomass and energy from one organism to the next, from the food to the feeder.
- (biological material)
A rabbit eating grass is drawn as grass → rabbit.

Example

Example acacia tree → giraffe → lion.

Further examples

Further examples maize → chicken → human, and leaf → beetle larva → oriole.

Length

Length food chains may have two, three, four or even five links.

Section 2 of 8Roles in a food chain

Producer

Producer the first organism, often a plant, which traps energy from the Sun and makes food by photosynthesis.

Primary consumer

Primary consumer a herbivore that eats producers, digests plants and uses the energy and materials for all the functions of life.

Secondary consumer

Secondary consumer a carnivore that eats primary consumers.

Tertiary consumer

Tertiary consumer a carnivore that eats secondary consumers; the one at the end is the top predator.

Prey

Prey an animal eaten by other animals.

Predator

Predator an animal that eats other organisms.

Role	Example
Producer	Algae, grass
Primary consumer	Rabbit, snail
Secondary consumer	Fox, thrush
Decomposer	Fungi, bacteria

Section 3 of 8Energy and biomass in a chain

Losses

Losses at each stage some material cannot be digested and is lost as waste.
Energy is transferred from the living organisms to their surroundings or used to move about.
Roughly 10% of the energy at one level reaches the next.

Length limit

Length limit the longer the chain, the less biomass reaches the organism at the top.
Food chains rarely have more than four or five links.

Numbers

Numbers there are fewer animals at the top than producers at the bottom.
A pyramid of numbers shows the number of organisms at each level, although one large tree supporting many insects gives an odd shape.

Section 4 of 8Food webs

Food web

Food web a model that links several food chains together.

Reason

Reason most animals eat more than one kind of organism, and most foods are eaten by more than one kind of animal.
Chickens eat maize but also grass, almost any grain, worms and beetles.
Lions eat giraffes but will eat many other animals if they get the chance.

Realism

Realism food webs are still simple models, but they are more realistic than food chains.

Changes

Changes removing or adding one species affects others.
If frogs vanished, snakes would have less food and fall in number, while grasshoppers would increase and eat more grass.

Keystone species

Keystone species one that many others depend on and so has a large effect if removed.
Sea otters eat sea urchins and so protect kelp forests, which shelter fish.

Section 5 of 8Where decomposers fit

Death

Death every organism in a chain eventually dies, and its waste and remains are used by decomposers.

Position

Position decomposers feed on every level, so arrows can be drawn from every organism to them rather than putting them at one level.

Action

Action bacteria and fungi break down dead producers, consumers and waste.
They release minerals into the soil and carbon dioxide into the air.

Carbon

Carbon the carbon compounds in dead material return to the air as carbon dioxide from respiration.

Section 6 of 8The circle of life

Problem

Problem undigested material is lost along a chain, energy is transferred to the environment and plants keep taking mineral salts from the soil.
If this continued, no minerals would be left, plants could not grow and all food chains and webs would fail.

Solution

Solution decomposers break down the bodies of plants and animals and return nutrients to the soil.
Plants take the nutrients up and use them to build new plants, the start of a new food chain.

Energy and nutrients

Energy and nutrients nutrients are recycled, while energy flows through the web and is eventually lost as heat.

Models

Models all models have limitations, so identify the strengths and weaknesses of any model used.

Section 7 of 8Key terms

Term	Meaning
Food chain	A model showing the feeding relationships and transfer of biomass between organisms.
Food web	A model linking several food chains together.
Producer	An organism that makes its own food by photosynthesis.
Herbivore	An animal that eats plants.
Carnivore	An animal that eats other animals.
Predator	An animal that eats other animals, its prey.
Biomass	Biological material.

Section 8 of 8Exam technique

Drawing arrows from the eater to the food — arrows point from the food to the feeder.

Putting decomposers at the start of the chain as producers — producers make food by photosynthesis.

Saying energy is recycled — energy flows and is lost as heat, but nutrients are recycled.

Forgetting that a food web is more realistic than a single chain — most animals eat several foods.

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Last updated: 13 October 2026