GCSE · Biology · AQA · Spec 8461

Trophic levels (biology only)

Is an owl a level 3 animal or a level 4 animal? It’s a trick question: the answer depends on which food chain you are looking at.

Count along the food chain

Stage 1 of 4: Algae. paused

Algae · 1/4scrub along the chain →

Level 1 · producer. Algae make their own food, so the chain starts here. Put your finger on them and say “one”.

The chain: algae → krill → seal → orca. Each arrow means “is eaten by”. Scrub along it, and the number on each organism is its trophic level.

Use the count

Where does the owl go?

Pick an organism, then put it at its trophic level in the chain shown. When you have sorted Chain A, switch to Chain B.

Chain A: grass → vole → owl

Still to sort

Level 1 · producer (0)

Makes its own food.

Level 2 · primary consumer (0)

A herbivore.

Level 3 · secondary consumer (0)

A carnivore that eats a herbivore.

Where the line is: Levels 3 and 4 are both carnivores. The difference is the prey: a herbivore for level 3, another carnivore for level 4.

Level 4 · tertiary consumer (0)

A carnivore that eats another carnivore.

Not in this chain (0)

This organism does not appear in the chain shown.

5 of 5 still to sort.

The top of the chain

Where does the lion stand?

A savanna food chain: grass → zebra → lion. Lions are carnivores with no predators.

Which idea is closest to what you think right now?

The lion can’t be an apex predator. Apex predators are always at level 4, and the lion is only at level 3.The lion is at level 4, because it is at the top of the chain.The lion is a secondary consumer at level 3, and it is also the apex predator.Decomposers come after the lion, so decomposers are the highest trophic level in this chain.

How sure are you?

I'm sureI think soI'm guessing

The lion can’t be an apex predator. Apex predators are always at level 4, and the lion is only at level 3.

This is a common misconception

“Apex predator” is not a level number. It means a carnivore with no predators, and that describes the lion. Here it eats a herbivore, so it sits at level 3, and it is still the apex predator. The orca at the top of the lesson’s first chain was an apex predator at level 4. The lion is one at level 3.

The lion is at level 4, because it is at the top of the chain.

This is a common misconception

Count from the grass: grass is 1, zebra is 2, lion is 3. Being at the top does not make an organism level 4. The level is set by how many steps it is from the producer, and this chain only has three organisms.

The lion is a secondary consumer at level 3, and it is also the apex predator.

This matches the evidence

Yes. It is at level 3 because it eats a herbivore, two steps from the producer. It is the apex predator because it is a carnivore with no predators. The two labels answer different questions: where it feeds in the chain, and whether anything preys on it.

Decomposers come after the lion, so decomposers are the highest trophic level in this chain.

This is a common misconception

Decomposers are not the next step after the top carnivore. They break down dead matter from every level (dead grass, dead zebras, dead lions), so they feed on the whole chain rather than sitting at the end of it.

The level counts steps from the producer. “Apex predator” means no predators. Decomposers feed on dead matter from every level.

How a decomposer feeds

Open each stage. The digesting happens outside the decomposer, and only then is the food absorbed.

Dead matter is broken downOutside the microorganism, the enzymes break the dead matter down into small soluble food molecules.

Exam practice

Describe it in your own words

A river contains this food chain: algae → mayfly larva → trout → heron. No animal preys on the heron. Describe the trophic levels in this food chain, and describe how decomposers obtain food from the organisms in it. [7 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

Algae are at level 1: producers, because they make their own food. (1 mark)
Mayfly larvae are at level 2: primary consumers (herbivores that eat the algae). (1 mark)
Trout are at level 3: secondary consumers (carnivores that eat a herbivore). (1 mark)
Herons are at level 4: tertiary consumers (carnivores that eat another carnivore). (1 mark)
The heron is the apex predator. (1 mark)
Decomposers secrete enzymes onto dead matter from any of these levels, breaking it down into small soluble food molecules. (1 mark)
The small soluble food molecules diffuse into the decomposer. (1 mark)

Model answer

The algae are producers at level 1 because they make their own food. Mayfly larvae eat the algae, so they are herbivores at level 2: primary consumers. Trout eat mayfly larvae, so they are carnivores eating a herbivore at level 3: secondary consumers. Herons eat trout, a carnivore, so they are tertiary consumers at level 4. Because nothing preys on the heron, it is also the apex predator. Decomposers feed on dead algae, larvae, trout and herons by secreting enzymes onto the dead matter. The enzymes break it down into small soluble food molecules, which diffuse into the decomposer.

For each organism, give the level number and its name, and back both up with what it eats.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Start at 1 on the producer and add one for every step along the food chain.

What you need to know

Trophic levels are numbered from level 1, the producers (plants and algae, which make their own food), and go up by one for each step along the food chain.
Level 2 is the primary consumers (herbivores). Level 3 is the secondary consumers (carnivores that eat herbivores). Level 4 is the tertiary consumers (carnivores that eat other carnivores).
An apex predator is a carnivore with no predators.
Decomposers secrete enzymes onto dead plant and animal matter, breaking it into small soluble food molecules that then diffuse into them.

The big picture

Trophic levels number the organisms in a food chain by how far along the chain they feed. Producers (plants and algae, which make their own food) are level 1. Herbivores are level 2, the primary consumers. Carnivores that eat herbivores are level 3, the secondary consumers. Carnivores that eat other carnivores are level 4, the tertiary consumers. An apex predator is a carnivore with no predators. Decomposers break down dead plant and animal matter by secreting enzymes into their surroundings, and the small soluble food molecules then diffuse into them.

Key points

1A trophic level is a position, not a kind of organism. The same owl is at level 3 in one chain and level 4 in another.

2Levels 3 and 4 are both carnivores. What separates them is whether the prey is a herbivore or another carnivore.

3“Apex predator” and “tertiary consumer” answer different questions, so a lion can be the apex predator at level 3.

4Decomposers are not the top of the chain. They feed on dead matter from every level and digest it outside their bodies.

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Worked example

Problem

A woodland food chain is: oak tree → caterpillar → blue tit → sparrowhawk. Give each organism’s trophic level number and name. Then say whether the chain shows that the sparrowhawk is an apex predator.

⚠ Watch out

Calling the top organism of any chain “level 4” or “the apex predator”. Count the steps from the producer to find the level, and only call an organism an apex predator if it is a carnivore with no predators.

🧠

Memory hook

Finger on the producer, say “one”, then add one for every arrow. The number you land on is the level.

✓

Check yourself

In the chain grass → rabbit → fox, what is the fox’s trophic level, and what is that level called? What would you need to know before calling it an apex predator?

Flashcards

(13)

What does a trophic level number tell you?

How far along the food chain an organism feeds, counting the producer as level 1.

Which organisms are at trophic level 1?

Producers: plants and algae, which make their own food.

What name is given to level 2 organisms?

Primary consumers: herbivores that eat plants or algae.

What do secondary consumers eat?

Herbivores. Secondary consumers are carnivores at level 3.

What makes a carnivore a tertiary consumer rather than a secondary consumer?

It eats other carnivores, which puts it at level 4.

Define apex predator.

A carnivore with no predators.

Can an apex predator be at level 3?

Yes. If a carnivore that eats herbivores has no predators, it is an apex predator at level 3.

Why can one animal have different trophic levels in two food chains?

Its level depends on its position in each chain (what it eats there), not on what kind of animal it is.

What do decomposers feed on?

Dead plant and animal matter, from every trophic level.

How does a decomposer start to digest its food?

It secretes enzymes into its surroundings, onto the dead matter.

What do a decomposer’s enzymes turn dead matter into?

Small soluble food molecules.

How do food molecules get into a decomposer?

They diffuse into the microorganism.

What does an arrow in a food chain mean?

“Is eaten by”. It points from the food to the organism that eats it.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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Trophic levels (biology only)

Count along the food chain

Where does the owl go?

Where does the lion stand?

How do the feeding groups differ?

Describe it in your own words

More AQA GCSE Biology topics