17.1 Feeding Relationships

A free GCSE Biology lesson for AQA (8461) from OBA School: food chains, producers and consumers, and the cycles of predator and prey populations. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe a food chain and explain why it always begins with a producer
• Name the feeding levels in a food chain, including primary, secondary and tertiary consumers
• Explain what the arrows in a food chain show
• Interpret graphs of predator and prey populations and explain why they cycle

Key words

• Producer: An organism that makes its own food by photosynthesis, such as a green plant or alga. It makes glucose, which it uses to build biomass.
• Consumer: An organism that gets its energy by eating other organisms.
• Trophic level: A feeding level in a food chain. Producers are level 1, primary consumers level 2, and so on.
• Predator: A consumer that kills and eats other animals.
• Prey: An animal that is killed and eaten by a predator.
• Apex predator: A carnivore at the top of a food chain, with no predators of its own.
• Biomass: The mass of living material in an organism or a group of organisms.

Food chains

Feeding relationships in a community can be shown as food chains. All food chains begin with a producer that makes glucose by photosynthesis, using energy from the Sun. The glucose is used to make other molecules, building the producer’s biomass. Animals cannot make their own food, so all biomass in a food chain comes from producers.

Exam tip: The arrow means “is eaten by”. It points from the food to the feeder, in the direction energy and biomass move.

• 1; Name Producer; What it eats Makes its own food; Example above Oak leaves
• 2; Name Primary consumer (herbivore); What it eats Producers; Example above Caterpillar
• 3; Name Secondary consumer (carnivore); What it eats Primary consumers; Example above Blue tit
• 4; Name Tertiary consumer (carnivore); What it eats Secondary consumers; Example above Sparrowhawk

Producers on land and in water

On land, most producers are green plants. In seas, lakes and ponds the main producers are tiny algae (phytoplankton) and seaweeds.

Worked example: naming the levels

Pond food chain: algae → water flea → stickleback → heron.
Algae: producer. Water flea: primary consumer. Stickleback: secondary consumer. Heron: tertiary consumer (apex predator).

Answer: the stickleback is both a predator (of water fleas) and prey (of herons)

Predators and prey

Consumers that kill and eat other animals are predators; those eaten are prey. In a stable community, the numbers of predators and prey rise and fall in cycles.

• 1. When there is plenty of food for the prey, prey numbers rise.
• 2. Predators now have more food, so more survive and breed. Predator numbers rise.
• 3. More prey are eaten, so prey numbers fall.
• 4. Predators now have less food, so some starve or breed less. Predator numbers fall, and the cycle starts again.

Worked example: reading the graph

Voles peak at 2, 6 and 10 years. Stoats peak at 3, 7 and 11 years.
Time lag between prey peak and predator peak = 3 − 2 = 1 year.
Time for one complete cycle = 6 − 2 = 4 years.

Answer: predator peaks come about 1 year after prey peaks; each cycle lasts 4 years

Key fact: Prey peaks always come before predator peaks. There are also usually fewer predators than prey.

Common mistakes

• Drawing arrows the wrong way. The arrow points towards the organism doing the eating.
• Calling the Sun a producer. The Sun is the energy source; the producer is the plant or alga.
• Saying predators “control” prey numbers alone. Prey numbers also depend on their own food supply, disease and other factors.
• Saying the predator peak comes first. Predator numbers follow prey numbers, so they peak later.

Revision questions

Try these, then check your answers below each one.

Question 1. What is a producer? (Choose from: An organism that makes its own food by photosynthesis, An animal that eats plants, An organism that breaks down dead material, An animal at the top of a food chain.)

Answer: An organism that makes its own food by photosynthesis

Working: Producers (plants and algae) make glucose by photosynthesis.

Question 2. True or false? In a food chain, each arrow points from the organism that is eaten to the organism that eats it.

Answer: True

Working: True. The arrow means “is eaten by”, showing energy and biomass moving up the chain.

Question 3. Why does the number of predators fall soon after the number of prey falls? (Choose from: There is less food for the predators, Predators have run out of space, The prey start eating the predators, Predators always die after breeding.)

Answer: There is less food for the predators

Working: With fewer prey, there is less food, so fewer predators survive and breed.

Related lessons

• Previous lesson: 16.9 Required Practicals: Adaptations, Interdependence, and Competition
• Next lesson: 17.2 Materials Cycling
• In this chapter: 17.3 The Carbon Cycle
• In this chapter: 17.4 Rates of Decomposition
• In this chapter: 17.5 Required Practicals: Organising an Ecosystem

Continue in the full course

This lesson is section 17.1 (AGCBH1701) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.