18.1.3Organisms and Their Environment

Trophic Levels — Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 6

📚 Cambridge IGCSE Co-ordinated Sciences 0654 - Biology Hub / Topical Revision

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Part 1 of 6Feeding relationships and trophic levels

Every ecosystem is held together by feeding relationships.

Food chain

A food chain shows who eats whom, with each arrow pointing from the organism that is eaten to the organism that eats it, in the direction energy moves.

Trophic level

A trophic level is a feeding position in a food chain, counted from the producer upwards.

Food web

Several interlinked food chains make a food web, which gives a more realistic picture because most consumers eat more than one type of food.

Example chain

grass, then rabbit, then fox.
Grass is at trophic level 1, rabbit at 2 and fox at 3.

Food webs can be very complex.

Feeding relationships and trophic levels

In a woodland, a caterpillar may be eaten by a blue tit, a beetle or a spider, and each of those has other predators,

so a single species can sit at more than one trophic level depending on what it has eaten.

An omnivore such as a badger is a primary consumer when it eats roots and a secondary consumer when it eats worms.

Studying these links helps ecologists predict the effect of removing or adding a species.

Part 2 of 6Trophic level 1: producers

Producers make their own organic molecules by photosynthesis, using light energy, carbon dioxide and water.

Trophic level 1: producers

Green plants and algae are producers.

They are always at the start of a food chain because nearly all the energy and biomass in an ecosystem comes originally from the light energy they capture.

Producers are said to be the base of the chain because every other organism depends on them directly or indirectly.

Some organisms in deep oceans produce food by chemosynthesis using chemical energy, but at GCSE the focus is on photosynthesising producers.

Part 3 of 6Higher trophic levels: consumers

Consumers cannot make their own food and must eat other organisms.

Primary consumers

(trophic level 2) eat producers. They are herbivores such as rabbits, caterpillars and zooplankton.

Secondary consumers

(level 3) eat primary consumers. They are carnivores or omnivores, such as foxes or small fish.

Tertiary consumers

(level 4) eat secondary consumers, for example a hawk eating a snake that has eaten a mouse.

Apex predator

An apex predator is at the top of the chain and has no natural predators in that ecosystem.

Consumers are also described by diet

herbivores eat plants, carnivores eat animals and omnivores eat both.
A predator hunts and kills other animals, which are its prey.

Names help in examsa "producer" is not a plant by definition but any organism that photosynthesises, so seaweed and phytoplankton count.

Higher trophic levels: consumers

Scavengers such as vultures feed on dead animals but are still consumers, unlike decomposers, which digest material outside the body.

Parasites are consumers too, but they live on or in their host and feed from it without killing it quickly.

Part 4 of 6Decomposers

Decomposers are bacteria and fungi that break down dead plants and animals and waste such as faeces.

They secrete enzymes onto the material and absorb the soluble products.

They release mineral ions such as nitrates and phosphates back into the soil,

where plants can take them up again,
and carbon dioxide into the air through respiration.

This recycling means that materials are not used up.

Decomposers are not shown at a particular trophic level in a standard chain, but they are essential in every ecosystem.

Group	Gets food by	Example
Producer	Photosynthesis	Oak tree, algae
Primary consumer	Eating producers	Caterpillar
Secondary consumer	Eating primary consumers	Blue tit
Tertiary consumer	Eating secondary consumers	Sparrowhawk
Decomposer	Breaking down dead matter	Fungi, bacteria

Part 5 of 6Food webs and changes in populations

In a food web the population of one species affects others.

Food webs and changes in populations

If the number of prey falls, the predators have less food and their numbers decline; if predators are removed, the prey may increase and over-graze the producers.

A change in one population can cascade through the web.

Predator and prey populations often rise and fall in repeating cycles, with the predator curve lagging a little behind the prey curve because predators need time to breed when food is plentiful.

Human activity such as the use of pesticides can break links in a food web.

Food webs and changes in populations

Removing a producer removes the energy source for everything above it,

whereas removing a top predator may allow populations lower down to rise sharply.

Relationships also include competition: two species that eat the same food compete,

and the one that is better adapted may increase while the other declines.

Food chains are written from the producer upwards, and in an exam you should label the arrows as showing the flow of energy, not simply "eats".

Part 6 of 6Exam-style questions

Use the food chain: algae, then shrimp, then small fish, then seal. Name the secondary consumer and state its trophic level. [2]2 marks
In a pond, the number of tadpoles rapidly decreases after a heron arrives. Explain how this could change the amount of pondweed. [3]3 marks
Explain why decomposers are important in an ecosystem. [3]3 marks

Key terms

Term	Meaning
Trophic level	A position in a food chain, counted from the producer.
Producer	An organism that makes its own food by photosynthesis.
Primary consumer	An organism that eats producers.
Secondary consumer	An organism that eats primary consumers.
Apex predator	A predator at the top of a food chain with no natural predators.
Decomposer	A microorganism that breaks down dead matter and recycles nutrients.
Food web	A network of interlinked food chains.

Avoid these mistakes

Drawing arrows from predator to prey. The arrow shows the direction of energy transfer, so it points to the eater.

Saying that plants are not part of the food chain, or that all plants are consumers. Plants are the producers.

Writing that decomposers are always at the top of the food chain. They feed on dead material from every level.

Thinking that a herbivore is a secondary consumer. Herbivores are primary consumers.

Last updated: 20 October 2026