A food chain looks like the easiest diagram in the course, and it is where a surprising number of marks are dropped — arrows drawn the wrong way, levels miscounted, decomposers stuck on the end. Get the rules straight once and this becomes free marks for the rest of the topic.
📚 What you need to know
A trophic level is the position an organism occupies in a food chain.
Arrows show the direction of energy flow, so they point from the eaten to the eater.
An apex predator sits at the top of the chain and has no predators of its own.
Consumers use different strategies: herbivores, predators, detritivores, saprotrophs, parasites and scavengers.
Decomposers are not usually drawn inside a food chain because they feed on material from every level.
Chains are short because so much energy is lost between levels.
Naming the levels
Trophic level
Name
What these organisms do
1
Producers
Plants and algae making their own biomass using sunlight
2
Primary consumers
Herbivores that feed on producers
3
Secondary consumers
Carnivores that feed on primary consumers
4
Tertiary consumers
Carnivores that feed on secondary consumers
Four organisms, three arrows, four trophic levels. If you count arrows instead of boxes you will always be one out.
The arrow direction is the single most tested detail here. Draw it pointing at the mouth doing the eating. Energy is moving into that organism, so the arrow points towards it.
Consumers do not all feed the same way
Type of consumer
How it feeds
Example
Herbivores
Feed on plants and plant material
Rabbits grazing on grasses and herbs
Predators
Hunt and kill other animals for food
Wolves hunting deer in packs
Detritivores
Eat dead and decaying material and break it down further
Earthworms feeding on decaying leaves
Saprotrophs and decomposers
Break organic matter down externally and absorb the nutrients, recycling them
Fungi and bacteria on fallen wood
Parasites
Live on or in a host, harming it without killing it quickly
Tapeworms inside a mammal’s gut
Scavengers
Feed on animals that are already dead
Vultures feeding on carrion
Where decomposers fit
Decomposers are usually left out of a drawn food chain, and there is a good reason. They do not sit at one trophic level — they feed on dead material from all of them: fallen leaves, droppings, dead caterpillars and dead hawks alike.
Decomposers are the reason nutrient cycles close. They are unglamorous and completely essential.
Worked examples
WORKED EXAMPLE
For the food chain algae → water flea → minnow → pike, state the trophic level of the minnow and the name of that level.
Step 1: count from the producer
Algae are level 1, the water flea is level 2, the minnow is level 3.
Step 2: name the levelTrophic level 3, a secondary consumerStep 3: check the top
The pike is a tertiary consumer and, if nothing eats it, the apex predator.
Always start counting at the producer, even if the question begins mid-chain.
WORKED EXAMPLE
A student draws a food chain with arrows pointing from the fox to the rabbit. Explain the error.
The rule
Arrows in a food chain show the direction of energy flow, not the direction of eating.
What is happening
The rabbit is eaten by the fox, so energy moves from rabbit to fox.
The correctionThe arrow must point rabbit → foxRead every arrow aloud as “is eaten by” and the error becomes obvious.
WORKED EXAMPLE
Explain why food chains rarely have more than five trophic levels.
Step 1: the energy fact
Only around 10% of the energy at one level is stored as biomass in the next, mostly because of losses to heat from respiration.
Step 2: follow it up the chain
After four or five transfers the energy remaining is a tiny fraction of what the producers captured.
Step 3: the consequence
There is not enough energy left to support a breeding population of another predator.
Energy runs out, so the chain stopsLink it back to the second law of thermodynamics for the final mark.
💡 Exam tip
Number the levels on the diagram before you answer. It stops off-by-one errors under pressure.
If a question gives a chain in words, rewrite it with arrows before counting.
Say direction of energy flow when explaining arrows. That exact phrase appears in mark schemes.
Leave decomposers out of a drawn chain unless the question specifically asks where they fit.
An organism can occupy more than one trophic level if it eats at different levels. Say so if the data shows it.
⚠ Common mix-up
Arrows pointing at the food. They point at the feeder, because energy flows that way.
Counting arrows instead of organisms when asked for the number of trophic levels.
Calling every meat-eater a secondary consumer. It depends what it eats, not what it is.
Putting decomposers at the end of a chain as if they were the top predator.
Confusing detritivores with decomposers. Detritivores eat dead material; decomposers break it down externally and absorb it.
Assuming apex predators are the biggest animals. Apex means nothing preys on them, not that they are large.
Up next: Building and Reading Food Webs — what happens when you join the chains together, and how to predict the knock-on effects of losing a species.
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