The big idea: Energy enters almost every ecosystem as sunlight. Producers capture it by photosynthesis, and it then passes from organism to organism when one is eaten by another.
A food chain shows that one-way flow of energy, drawn as a line of organisms joined by arrows.
Each step in the chain is a trophic level — a feeding position. The position tells you what an organism eats and what eats it.
- Food chain
- A diagram showing a single path of energy through an ecosystem, drawn as a row of organisms joined by arrows.
- Food web
- Several food chains linked together, showing the many feeding relationships in an ecosystem more realistically.
- Trophic level
- An organism's feeding position in a food chain — for example producer, primary consumer or secondary consumer.
- Producer
- An organism that makes its own food by photosynthesis (an autotroph). It is always the first trophic level.
- Consumer
- An organism that gets its energy by eating other organisms (a heterotroph).
The arrow has a meaning: In a food chain or web, the arrow always points from the organism being eaten TO the organism that eats it.
It shows the direction energy flows — from prey to predator — NOT 'who eats whom' the other way round.
Drawing the arrow backwards is the single most common exam mistake on this topic.
Read the arrow as 'is eaten by': Say the arrow out loud as 'is eaten by': grass is eaten by rabbit is eaten by fox.
That keeps the arrows pointing the right way every time — towards the eater, the way the energy travels.
Every organism in a food chain sits at a numbered trophic level, and each level has a name. Learning the names — and the order — lets you label any chain or web in the exam.
Start at the producer and count upwards: each time you move one arrow along the chain, you move up one trophic level.
The trophic levels in order
- 1st level — producer: makes its own food by photosynthesis (e.g. grass)
- 2nd level — primary consumer: a herbivore that eats producers (e.g. rabbit)
- 3rd level — secondary consumer: a carnivore that eats primary consumers (e.g. fox)
- 4th level — tertiary consumer: a carnivore that eats secondary consumers (e.g. hawk)
| Trophic level | Name | What it does | Example |
|---|---|---|---|
| 1st | Producer | Makes its own food by photosynthesis (an autotroph) | Grass, algae, a tree |
| 2nd | Primary consumer | A herbivore — eats producers | Rabbit, grasshopper, locust |
| 3rd | Secondary consumer | A carnivore — eats primary consumers | Fox, frog, small fish |
| 4th | Tertiary consumer | A carnivore — eats secondary consumers | Hawk, large fish, eagle |
How to read an organism's level: To find any organism's trophic level, count the arrows from the producer up to it and add one.
Grass = level 1. Eaten by a rabbit (one arrow) = level 2, a primary consumer. The rabbit eaten by a fox (two arrows) = level 3, a secondary consumer.
So the same organism's name depends on what it eats — not on its size or how fierce it looks.
One organism, more than one level: In a real food web, an organism can sit at more than one trophic level at the same time.
If a fox eats both a rabbit (a primary consumer) and a frog (a secondary consumer), then the fox is acting as a secondary consumer in one chain and a tertiary consumer in another.
This is normal in webs and is a favourite exam point.
Food chain vs food web: A food chain shows just one path of energy. A food web links many chains together, so it shows the ecosystem more realistically — most organisms eat, and are eaten by, several others.
To pull a single food chain out of a web, just follow one continuous set of arrows from a producer up to a top consumer.
| Feature | Food chain | Food web |
|---|---|---|
| Shows | One single path of energy | Many food chains linked together |
| Number of organisms per level | Usually one | Often several |
| How realistic | A simplified slice of an ecosystem | Closer to the real ecosystem |
| Arrows | Point from prey to the eater | Point from prey to the eater |
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How this is tested: On Paper 2 a 2-mark Draw / Construct question gives you a list of organisms and asks you to build a food chain — you score for the correct order (producer first) and for arrows pointing the right way (towards the eater).
On Paper 3 and Paper 1A, questions usually give you a food web and ask you to identify an organism's trophic level (producer / primary / secondary / tertiary consumer), or to trace its energy source back along the arrows.
Watch for the classic twist: an organism that occupies two trophic levels in the same web.
IB-style question — construct a food chain
A meadow contains clover, field mice, grass snakes and barn owls. Construct a food chain using all four organisms, beginning with the producer. [2]
How to score both marks
- Put the producer first. Clover is the only organism that makes its own food by photosynthesis, so it starts the chain as the producer (trophic level 1).
- Order the consumers by what eats what. Field mice eat clover, grass snakes eat mice, and barn owls eat snakes — so the order is clover → field mouse → grass snake → barn owl.
- Add arrows pointing to the eater. Every arrow points FROM the organism being eaten TO the one that eats it (the way energy flows). (Mark 1: correct order, producer first. Mark 2: all arrows pointing the correct way.)
Final answer
clover → field mouse → grass snake → barn owl (each arrow pointing towards the organism that does the eating).
✓ Why this scores full marks: The chain starts with the producer (clover), the consumers are in the right feeding order, and every arrow points towards the eater.
The two easy marks to lose are starting with a consumer instead of the producer, or drawing the arrows backwards.
IB-style question — identify a trophic level
In the food chain clover → field mouse → grass snake → barn owl, identify the trophic level of the grass snake. [1]
How to score the mark
- Count the arrows from the producer. Clover is level 1; field mouse is level 2; the grass snake is two arrows from the producer, so it is at trophic level 3.
- Name the level. Level 3 is the secondary consumer — it eats the primary consumer (the field mouse). (Mark 1: secondary consumer / third trophic level.)
Final answer
The grass snake is a secondary consumer (the third trophic level) — it eats the primary consumer, the field mouse.