IB Biology SLTopic 3 — Energy & MatterPaper 1 & 2Core idea~8 min read
Trophic Levels
A trophic level is just a position number in a food chain. Simple enough — until you meet an animal that eats both plants and other animals and suddenly sits on three different levels at once. This page shows you how to handle that.
📘 What you need to know
A trophic level is the position an organism occupies in a food chain.
Level 1 is always the producers, then primary, secondary, tertiary and quaternary consumers.
The number tells you how many organisms the energy has passed through to reach that point.
An apex predator sits at the top of a chain and has no predators of its own.
In a food web one species can occupy more than one trophic level, depending on which chain you follow.
Chemical energy in an apex predator eventually passes to decomposers.
The ladder
Counting always starts at the producer. Every step up the chain adds one.
Producers are always level 1, whatever the ecosystem.
Level
Name
What it feeds on
1
Producers
Nothing — they make their own carbon compounds
2
Primary consumers
Producers (herbivores)
3
Secondary consumers
Primary consumers
4
Tertiary consumers
Secondary consumers
5
Quaternary consumers
Tertiary consumers
Students often ask why we bother numbering. It is because the number predicts something real: roughly ninety per cent of the energy is lost at every step, so an organism at level 5 has far less energy available to it than one at level 2.
Apex predators
An apex predator is at the top of a food chain and nothing hunts it. Lions, orcas and golden eagles are the standard examples.
Being at the top does not mean the energy stops. When an apex predator dies, detritivores and saprotrophs break its body down, and the chemical energy in its tissues passes to them.
Careful with the wording: apex predator describes a position, not a level number. A shark can be an apex predator at level 4 in one chain and level 5 in another.
One species, several trophic levels
This is where it gets interesting. In a real ecosystem, most animals do not stick to one food source, so their trophic level depends on which chain you trace.
Trace two different paths to the blue tit and you get two different trophic levels.
Follow the two paths to the blue tit:
oak tree → caterpillar → blue tit puts the blue tit at trophic level 3.
oak tree → aphid → ladybird → blue tit puts it at trophic level 4.
Both are correct. The blue tit occupies levels 3 and 4 in this web. Follow the same logic upwards and the sparrowhawk sits at levels 4 and 5.
WORKED EXAMPLE
Using the food web above, state the trophic levels occupied by the sparrowhawk and explain your answer. [3]
Step 1: trace the shorter chainoak tree (1) → caterpillar (2) → blue tit (3) → sparrowhawk (4)Step 2: trace the longer chainoak tree (1) → aphid (2) → ladybird (3) → blue tit (4) → sparrowhawk (5)Step 3: concludethe sparrowhawk occupies trophic levels 4 and 5always start counting at the producer, and write out the chain to show your reasoning
Where decomposers fit
Detritivores and saprotrophs are usually drawn to one side rather than as another rung on the ladder. That is because they do not feed from a single trophic level — they receive dead material and waste from every level, producers included.
Some questions do ask you to assign them a level. If so, judge it by what they are consuming: a fungus growing on a dead rabbit is feeding on a primary consumer, so in that chain it behaves like a secondary consumer.
📝 How to work out a trophic level
Find a producer that leads to your organism. Label it level 1.
Follow the arrows upwards, one organism at a time.
Add one to the count at each step.
Check for other routes. If a different path reaches the same organism with a different count, it occupies more than one level.
Write the chain out in your answer. Examiners give credit for the working, not just the number.
💡 Exam tip
Producers are always trophic level 1. Never start counting at the first animal.
If asked for “the” trophic level of an omnivore in a web, check whether several answers are valid and say so.
Use the phrase number of organisms the energy has passed through when defining a trophic level.
Food chains rarely go beyond four or five levels — be ready to explain that this is because of energy losses.
An apex predator has no predators, but it is still eventually decomposed.
Write chains left to right with arrows, and label the producer, to show your working.
⚠ Common mix-up
Primary consumer is level 2, not level 1. The word “primary” refers to being the first consumer, not the first level.
Trophic level is not the same as species. One species can sit on several levels.
Carnivore does not automatically mean level 3. A carnivore eating another carnivore is at level 4.
Apex predator is not a trophic level. It is a description of position in a particular chain.
Decomposers are not simply “the top level”. They take material from every level.
Do not count the sun as a trophic level. Energy source, not organism.
Up next: Pyramids of Energy — now that each level has a number, we can measure how much energy sits at each one and draw it to scale. That drawing tells a dramatic story.
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