A food chain is just a map of who eats whom, drawn so that the arrows follow the energy. Get the arrow direction right and the trophic numbering right, and this becomes some of the easiest marks in the paper.
📘 What you need to know
A trophic level is the position an organism occupies in a food chain.
Level 1 is always the producers; level 2 is primary consumers, and so on.
Arrows point towards the eater, because they show the direction energy and biomass move.
Apex predators sit at the top with no predators of their own.
Consumers feed in many different ways — herbivores, predators, parasites, detritivores, scavengers, decomposers.
Decomposers are usually left off food chains because they feed on every level at once.
Reading a food chain
Take a simple grassland chain: grass, then grasshopper, then frog, then snake, then eagle. Each step up is a trophic level, and each arrow means “is eaten by”.
Counting levels is easy if you always start at the producer and call it 1. A five-step chain like this is about as long as real ecosystems manage.
The four named levels
Level
Name
What lives there
1
Producers
Plants and algae — they make their own biomass using sunlight
2
Primary consumers
Herbivores — they feed on producers
3
Secondary consumers
Predators — they feed on primary consumers
4
Tertiary consumers
Predators — they feed on secondary consumers
If several organisms all eat producers, they all sit in level 2 together. Trophic level is about position in the chain, not about how big or clever the animal is.
Not all consumers eat the same way
“Consumer” covers a lot of very different lifestyles, and ESS asks you to recognise them.
Type
How they feed
Example
Herbivores
Eat plants and plant material
Deer grazing on grasses and shrubs
Predators
Hunt and kill other animals
Wolves hunting deer in packs
Parasites
Live on or in a host, harming it without killing it quickly
Tapeworms in the gut of a mammal
Detritivores
Eat dead and decaying material and break it up further
Earthworms feeding on rotting leaves
Scavengers
Feed on animals that are already dead
Vultures at a carcass
Decomposers
Break organic matter down chemically and release nutrients
Bacteria and fungi in the soil
Detritivore or decomposer? A detritivore eats dead material and digests it inside its body. A decomposer breaks it down outside its body and absorbs what is released. Earthworm versus fungus is the cleanest pair of examples.
Apex predators and decomposers
An apex predator is a carnivore or omnivore with nothing above it — eagles, lions, killer whales. Nothing hunts them, but the energy in their bodies is not wasted. When they die, decomposers get it.
That is exactly why decomposers are usually drawn off to the side rather than in the chain. They take carbon compounds from every trophic level, so putting them at one position would be misleading.
🧩 Working out a trophic level in three moves
Find the producer. Whatever makes its own food is level 1.
Follow the arrows. Each arrow you cross moves you up one level.
Check for shortcuts. If the organism also eats something lower down, it occupies more than one level — say so.
WORKED EXAMPLE
For the food chain oak tree → caterpillar → blue tit → sparrowhawk, name the trophic level of each organism.
Step 1: Start at the produceroak tree = level 1, producerStep 2: Move up one level per arrowcaterpillar = level 2, primary consumerblue tit = level 3, secondary consumersparrowhawk = level 4, tertiary consumerLevels 1, 2, 3 and 4The sparrowhawk is also the apex predator here
WORKED EXAMPLE
A squirrel eats acorns from an oak tree and also eats caterpillars that feed on oak leaves. State the trophic level or levels of the squirrel and explain your answer.
Step 1: Take each food separatelyAcorns come from a producer; caterpillars are primary consumersStep 2: Number each routeeating acorns → level 2eating caterpillars → level 3The squirrel occupies levels 2 and 3Omnivores very often sit in two levels at once – always check both foods
💡 Exam tip
Draw arrows towards the eater. This is probably the most common single mark lost in this topic.
Always name the level and the number if the question gives you room: “level 3, secondary consumer”.
If an organism eats from two levels, say so explicitly. Examiners want both.
Say arrows show the transfer of energy and biomass, not “what eats what”, if the question asks what an arrow represents.
Use real named organisms in your examples. Vague answers like “a big animal” score nothing.
⚠ Common mix-up
Arrows drawn backwards. The arrow does not point at the food; it points at whoever eats it.
Starting the count at the herbivore. Level 1 is always the producer, even if it is not drawn.
Calling everything that eats plants a “primary producer”. Producer and consumer are opposites, so read your own answer back.
Putting decomposers at the end of the chain. They feed on every level, so they normally sit outside it.
Assuming apex predators are always the largest animal. Apex means nothing eats it — size is irrelevant.
Up next: Building and Reading Food Webs — what happens when you join all the chains together and one species disappears.
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