B16.1 Feeding Relationships

A free GCSE Combined Science: Trilogy lesson for AQA (8464) 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
• Estimate the size of a population from quadrat data

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. Producers capture only a small fraction of the light energy that reaches them, and at each step along a chain only part of the biomass is passed on, so chains rarely have more than four or five links.

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 Clover
• 2; Name Primary consumer (herbivore); What it eats Producers; Example above Snail
• 3; Name Secondary consumer (carnivore); What it eats Primary consumers; Example above Song thrush
• 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

Southern Ocean food chain: phytoplankton → krill → penguin → leopard seal.
Phytoplankton: producer. Krill: primary consumer. Penguin: secondary consumer. Leopard seal: tertiary consumer (apex predator).

Answer: the penguin is both a predator (of krill) and prey (of leopard seals)

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, 7 and 12 years. Stoats peak at 3 and 8 years.
Time lag between prey peak and predator peak = 3 − 2 = 1 year.
Time for one complete cycle = 7 − 2 = 5 years.

Answer: predator peaks come about 1 year after prey peaks; each cycle lasts 5 years

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

Counting the organisms in a food chain

To study feeding relationships, ecologists need to know how many of each organism there are. Counting every individual takes too long, so they sample. You carried out this method in the required practical on field investigations (B15).

• Random sampling with quadrats estimates a population or compares two areas. Use random numbers as coordinates on a grid so there is no bias.
• Systematic sampling along a transect shows how the distribution of a species changes across a habitat, e.g. from a hedge into a field. Measure an abiotic factor (such as light) at each point too.
• Quadrats suit plants and slow-moving animals; fast-moving animals leave the quadrat.

Worked example: estimating a population

Fifteen 0.25 m² quadrats were placed at random in a meadow 120 m × 80 m. The mean was 6.4 clover plants per quadrat.
Plants per m² = 6.4 ÷ 0.25 = 25.6. Area = 120 × 80 = 9600 m².
Population ≈ 25.6 × 9600 = 245 760.

Answer: about 250 000 clover plants

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 (green plants and algae) make glucose by photosynthesis and use it to build biomass.

Question 2. The diagram shows a woodland food web. Which organism is a tertiary consumer? (Choose from: Owl, Shrew, Grasshopper, Fox.)

Answer: Owl

Working: The owl eats shrews and blue tits, which are secondary consumers, so the owl is a tertiary consumer (and the apex predator here).

Question 3. 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”, so it points towards the feeder.

Related lessons

• Previous lesson: B15 RP Required Practical: Field Investigations
• Next lesson: B16.2 Materials Cycling
• In this chapter: B16.3 The Carbon Cycle

Continue in the full course

This lesson is section B16.1 (AGCSC1601) of our free GCSE Combined Science 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 Combined Science course.