IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2~9 min read
Trophic Levels
A trophic level is just a position in a food chain, counted from the producer upwards. It sounds like bookkeeping until you realise the number tells you how many organisms the energy has already passed through – and how little of it is left.
📚 What you need to know
A trophic level is the position of an organism in a food chain, and indicates how many organisms the energy has passed through.
Levels are numbered from producers at level 1, then primary, secondary, tertiary and quaternary consumers.
Energy from sunlight enters the chain at the first trophic level, converted to chemical energy by photosynthesis.
Apex predators sit at the top of a chain and have no predators feeding on them.
Energy stored in an apex predator still passes on to decomposers when it dies.
An organism with a varied diet can occupy more than one trophic level in the same ecosystem.
Counting the levels
Definition
A trophic level is the position of an organism in a food chain. It tells you how many organisms the energy has been transferred through to reach it.
Level
Name
Description
Woodland example
1
Producers
Organisms that produce their own carbon compounds, e.g. using light energy
Oak leaves, grass
2
Primary consumers
Herbivores that feed on plant tissue
Caterpillar, aphid, rabbit
3
Secondary consumers
Carnivores that are predators of primary consumers
Ladybird, blue tit
4
Tertiary consumers
Carnivores that are predators of secondary consumers
Sparrowhawk
5
Quaternary consumers
Carnivores that are predators of tertiary consumers
A larger raptor
Sunlight enters at the bottom row only. Everything above it is living on what the row below could not use.
How the energy moves up
Energy from sunlight enters the food chain at the first trophic level, when producers convert light energy into chemical energy during photosynthesis.
That chemical energy is transferred to primary consumers as they consume producers.
It then transfers from one consumer to the next: primary to secondary, secondary to tertiary, tertiary to quaternary.
Apex predators. These sit at the very top of a food chain and have no predators that prey on them. But their energy is not stuck there – when an apex predator dies and is decomposed, the chemical energy stored in its tissues passes on to decomposers.
The same animal, two different levels
Trophic levels can also be identified in a food web, which gives a clearer picture of the complex feeding relationships in a real ecosystem.
Here is where it gets interesting. Many organisms have a varied diet, which means a species can be present at more than one trophic level in the same ecosystem, depending on which chain you follow.
Nothing about the blue tit changed. Adding one extra step below it pushed it up a level.
If a question asks “state the trophic level of species X”, check whether it says in this food chain or in this food web. In a chain there is one answer. In a web there may be two or three, and listing them all is usually what earns the mark.
Worked examples
WE 1
Identify trophic levels in a chain
In the food chain grass → rabbit → fox, state the trophic level of each organism and name each one. (3 marks)
Grass
Trophic level 1 – producer, it makes its own carbon compounds.
Rabbit
Trophic level 2 – primary consumer, a herbivore feeding on plant tissue.
Fox
Trophic level 3 – secondary consumer, a carnivore preying on a primary consumer.
1 producer, 2 primary consumer, 3 secondary consumergive the number and the name – questions usually want both
WE 2
Explain a species at two levels
Explain how a blue tit can occupy two different trophic levels in the same woodland. (3 marks)
Point 1: the cause
The blue tit has a varied diet, so it appears in more than one food chain within the food web.
Point 2: chain one
Oak leaves → caterpillar → blue tit places it at trophic level 3.
Point 3: chain two
Oak leaves → aphid → ladybird → blue tit places it at trophic level 4, because the energy has passed through one extra organism.
The diet decides the level, and the diet variesalways write out both chains – a bare statement that “it eats different things” is one mark at most
WE 3
Apex predators and decomposers
An apex predator has no predators. Outline what happens to the chemical energy stored in its tissues. (2 marks)
Point 1: while it is alive
Most is released by respiration and lost to the environment as heat.
Point 2: after it dies
The chemical energy still stored in its carbon compounds is passed on to decomposers, which decompose the body.
Nothing is a dead end – decomposers take what is left“no predators” does not mean “no transfer” – decomposers are the answer they are fishing for
💡 Exam tips
Count from the producer as level 1, every time.
Learn the names in order: producer, primary, secondary, tertiary, quaternary.
In a web, check all the routes to a species before you answer with one level.
Say energy enters the chain at the first trophic level as sunlight.
Mention decomposers when asked about the fate of an apex predator’s energy.
Use “trophic level” rather than “stage” or “step” – the mark scheme uses the proper term.
⚠ Common mistakes
Counting the Sun as a trophic level. Level 1 is the producer.
Giving one trophic level for a species in a food web. Check every chain it appears in.
Calling every carnivore a secondary consumer. It depends on what it eats, not on what it is.
Putting decomposers on a trophic level in a chain. They are not considered part of the food chain itself, though they receive energy from every level.
Saying an apex predator’s energy is “wasted”. It goes to decomposers, and some was lost as heat long before that.
Mixing trophic level with population size. A higher level does not mean more organisms – usually the opposite.
Up next: Pyramids of Energy – the skills page. How to draw one properly, what the units are, and why the top bars end up looking like a pin.
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