Levels of organisation

Producers, primary/secondary/tertiary consumers, predator-prey cycles — the formal vocabulary of food chains. Lesson 7.05.

SPEC 4.7.2.1 • PAPER 2

Every Meal Starts With Sunlight

Whatever you ate today, trace it back: meat came from an animal that ate plants (or another animal that ate plants); cereals came from grain; fish ate smaller fish that ate algae. Every food chain begins with a producer — a plant or alga capturing sunlight.

Why this lesson matters

Mark schemes require spec-precise trophic vocabulary. "Producer" is not "plant." "Primary consumer" is not "the second one." Get the role names right or lose marks before you reach the explanation. AQA also tests graph interpretation on predator-prey cycles — the spec specifically requires this skill.

In the next 10 minutes you'll have the four named roles, the food chain arrow rule, and the predator-prey graph interpretation method.

What You'll Be Able To Do

Objective 1

Define producers and the three named consumer roles, and classify any organism in a food chain by its role.

Objective 2

Read a food chain correctly — including which direction the arrows go and what they represent.

Objective 3

Interpret graphs showing predator-prey cycles — identify the lag, describe trends, and explain the pattern.

Producers — The Foundation

Spec wording: "photosynthetic organisms are the producers of biomass for life on Earth."

Producer

An organism that makes its own food by photosynthesis. Producers use light energy to convert CO₂ and water into glucose — the starting biomass for all food chains.

Examples (write 'producer' — not 'plant')

Land plants (grass, oak trees, wheat, lettuce)

Algae (seaweed, pond algae)

Phytoplankton (microscopic algae in oceans and lakes)

Why "producer" not "plant"?

Marine food chains start with algae or phytoplankton, not plants. The spec term producer covers both. Write "plant" in a marine context and you've narrowed the answer incorrectly. Module 4 link: photosynthesis is the producer's chemistry.

The Four Named Roles

The spec names exactly four roles in a food chain. Learn each definition.

Producer

Makes its own food

A photosynthetic organism — plant, alga, or phytoplankton. Always at the start of a food chain.

Primary consumer

Eats producers

A herbivore. Examples: rabbit, cow, grasshopper, zooplankton.

Secondary consumer

Eats primary consumers

A carnivore that eats herbivores. Examples: fox, frog, small fish.

Tertiary consumer

Eats secondary consumers

A carnivore that eats other carnivores. Examples: eagle, large fish, shark.

The same organism can be in multiple roles

A fox eating a rabbit is a secondary consumer. A fox eating a frog (which ate a fly that ate a leaf) is a tertiary consumer. The role depends on which chain you're looking at, not the organism itself.

How To Read A Food Chain

A food chain is a sequence of organisms linked by feeding relationships, written with arrows.

The arrow rule

Arrows in a food chain show the direction of energy / biomass flow. The organism on the left of the arrow is eaten by the organism on the right.

grass → rabbit → fox

Grass is eaten by rabbit. Rabbit is eaten by fox. Energy flows up the chain.

Common error

Students sometimes draw arrows the wrong way (pointing from predator to prey, like "predator points to what it eats"). This loses marks. Arrows always point from prey to predator — in the direction of energy flow.

A Four-Level Food Chain

What this diagram shows

The chain starts with a producer (grass).

Each subsequent organism is one role up.

Arrows show energy / biomass flow — from prey to predator.

Reading "left to right" tells you "eaten by" — grass is eaten by rabbit, rabbit by fox, etc.

Predator and Prey — Spec Definitions

The spec defines these two terms exactly. Use the wording.

Predator

"Consumers that kill and eat other animals." (Verbatim spec.)

Prey

"Those eaten by predators." (Verbatim spec.)

Key point

"Predator" applies to any consumer that kills and eats other animals — so secondary and tertiary consumers can both be predators. A rabbit is prey for a fox; a fox is prey for an eagle. The label depends on the feeding relationship, not the position in the chain.

Don't confuse with similar terms

Predator ≠ carnivore. A carnivore is any meat-eater. A predator specifically kills its food (so scavengers eating already-dead animals are carnivores but not predators).

Prey ≠ primary consumer. Prey is anything eaten by a predator — including secondary consumers (e.g., foxes eaten by eagles).

Predator-Prey Cycles

One of the most important concepts in this spec. Memorise the spec wording.

The cycle (spec wording)

"In a stable community, the numbers of predators and prey rise and fall in cycles." (Verbatim spec.)

The four-stage cycle (mechanism)

Prey numbers rise → more food → predator numbers rise (with a delay).

Predators reach a peak → over-hunt the prey → prey numbers fall.

Prey low → predators run short of food → predator numbers fall.

Predators low → less hunting → prey recover → cycle restarts.

Spec condition: stable community

The cycle only occurs in a stable community — one where conditions are not being disrupted. New predators arriving, habitat loss, or pollution can break the cycle entirely. The spec wording "in a stable community" is what mark schemes credit.

Reading A Predator-Prey Graph

The spec specifically requires you to "interpret graphs used to model these cycles." This is a mark-paying skill.

Two observations that always score

The predator peak follows the prey peak (the lag). Quote the timing: "prey peaks at year 1, predator peaks at year 1.5 — half a year later."

Predator amplitude is smaller than prey amplitude. The prey wave is taller; the predator wave is shorter.

Why the lag?

Predators need time to reproduce in response to extra food. By the time predator numbers rise, prey have already been over-hunted — so prey are already falling when predators reach their peak. The full mechanism was covered in lesson 7.02.

Worked Example — Apply The Roles

A typical 4-mark trophic role question.

Question:

algae → small fish → larger fish → heron

(a)

1 mark

(b)

3 marks

(a)

Producer: the algae. ✓ (1 mark — algae are photosynthetic and at the start of the chain)

(b)

The heron is a tertiary consumer.

State the role: heron = tertiary consumer ✓ (1 mark)

Identify the previous level: the heron eats larger fish; larger fish eat small fish (primary consumers), so larger fish are secondary consumers ✓ (1 mark)

Apply the definition: an organism that eats a secondary consumer is, by definition, a tertiary consumer ✓ (1 mark)

Mark scheme map

Four creditable points: (1) name the producer; (2) name the role of the target organism; (3) identify the role of what it eats; (4) link these via the definition. Don't just name — justify with the chain.

Examiner Alerts

Alert 1

Arrows show energy flow, not "who eats whom". The arrow points from the organism being eaten TO the organism doing the eating. "Predator points to prey" is the wrong way round. This is one of the most common arrow-direction errors examiners flag.

Alert 2

Use "producer" — not "plant". In marine food chains, the producer is usually algae or phytoplankton. Writing "plant" is technically wrong and can lose a marking point.

Alert 3

Predator-prey cycles need a stable community. The spec's phrase "in a stable community" is what mark schemes credit. Cycles break down when new species arrive, habitats change, or pollution disrupts the population. Don't claim cycles happen regardless of conditions.

Common Errors — Wrong vs Right

Wrong

"Plants are primary consumers because they're at the start of the chain."

Right

Plants are producers. Primary consumers are the herbivores that eat the producers.

Wrong

"Arrows point from predator to prey."

Right

Arrows point from prey to predator — in the direction of energy / biomass flow.

Wrong

"Predator population peaks at the same time as prey."

Right

Predator peak lags behind prey peak — predators need time to reproduce in response to extra food.

Wrong

"A tertiary consumer eats producers."

Right

A tertiary consumer eats secondary consumers (carnivore-eats-carnivore).

Your Turn

Question:

year 1

500 rabbits

year 2

80 foxes

Describe and explain what happens to the fox population between year 0 and year 2. 4 marks

Hint: describe the trend, quote specific data, then explain why.

1

Describe: The fox population increases between year 0 and year 2, reaching a peak of around 80 foxes in year 2 ✓ (1 mark — direction and quoted value).

2

Quote the prey data: The rabbit population peaks at ~500 rabbits in year 1 — one year before the fox peak ✓ (1 mark — the lag, quoted).

3

Explain (link): The increased rabbit population provides more food for foxes ✓ (1 mark).

4

Explain (mechanism): More food → foxes have more energy to reproduce → fox population rises (with a lag, because reproduction takes time) ✓ (1 mark).

✓

Total: 4/4 marks. Notice: two marks come from quoting specific numbers from the graph — never describe a graph without quoting values.

Recap — Three Takeaways

Four named roles.

Producer (photosynthesises) → primary consumer (herbivore) → secondary consumer (carnivore eats herbivore) → tertiary consumer (carnivore eats carnivore).

Arrows = energy flow.

Arrows point from prey to predator. "Left is eaten by right." Wrong direction = lost mark.

Predator-prey cycles.

In a stable community, numbers rise and fall in cycles (verbatim spec). Predator lags prey; predator amplitude is smaller. Quote data when interpreting graphs.

Next lesson

7.06 — Cycling of Materials

How carbon and water cycle through ecosystems. Photosynthesis, respiration, decomposition, and the water cycle. Spec 4.7.2.2, Paper 2.

Lesson Summary Card

Key Definitions

Producer: a photosynthetic organism that makes its own food. Always at the start of a food chain.

Primary consumer: an organism that eats producers (a herbivore).

Secondary consumer: a carnivore that eats primary consumers.

Tertiary consumer: a carnivore that eats secondary consumers.

Predator: a consumer that kills and eats other animals.

Prey: an organism eaten by a predator.

Food Chain Rules

Always begins with a producer .

Arrows show direction of energy / biomass flow — from prey to predator.

Reading left to right = "eaten by".

Use "producer" not "plant" (algae and phytoplankton are producers too).

Predator-Prey Cycles (Spec-Verbatim)

"In a stable community , the numbers of predators and prey rise and fall in cycles ."

Predator peak lags prey peak.

Predator amplitude is smaller than prey amplitude.

When interpreting graphs: quote specific data values (not "more in the middle").

Top exam tip

The most common mistake is calling plants "primary consumers" or drawing arrows the wrong way. Both lose easy marks. Master producer, arrow direction, and stable community — these are the three highest-yield items.