IB Biology HLEnergy & Matter in EcosystemsPaper 1 & 2Practical skill~9 min read
Pyramids of Energy
This is a skills page. Drawing a pyramid of energy is worth easy marks, and losing them almost always comes down to the same three things: scale, units and labels. Get those right and the shape draws itself.
📚 What you need to know
A pyramid of energy shows the energy contained within the biomass of organisms at each trophic level.
The length of each bar represents the energy present, and bars should be drawn to scale.
Pyramids of energy are always widest at the base and get smaller going up.
They show a stepped decrease, not smooth sloping sides.
Levels are labelled producer, primary consumer, secondary consumer, and so on.
Units are energy per unit area per year, e.g. kJ m−2 year−1.
Roughly 10 % of the energy is passed on at each trophic level.
What the pyramid is showing
Each bar stands for one trophic level. The length of the bar is the energy contained in the biomass of the organisms at that level. Longer bar, more energy.
The base is wide because there is more energy contained in the biomass of producers than anywhere else in the ecosystem.
As you go up, the energy available decreases, because not all energy is transferred to the biomass of the next level.
Roughly 10 % of the energy is passed on at each step.
Because the energy falls, the biomass at each level falls too.
Where the missing energy goes
Loss
What it means
Incomplete consumption
Not every part of the organism gets eaten
Incomplete digestion
Some of what is eaten cannot be digested, so it leaves as faeces
Heat loss during respiration
Energy released by respiration is lost to the environment as heat
Excretion
Waste products of metabolism such as carbon dioxide, water and urea carry energy away
Drawing one properly
🧩 The method
Pick a scale that makes the largest bar fit the space you have. Write the scale down.
Calculate each bar length as a fraction of the largest one: bar length = (energy at that level ÷ energy at level 1) × length of level 1 bar.
Draw the bars centred on the same vertical line, widest at the bottom, stacked upwards.
Keep the sides stepped, not sloped. Each bar is a rectangle with its own width.
Label each level: producer, primary consumer, secondary consumer, tertiary consumer.
Write the units on the diagram: energy per unit area per year, e.g. kJ m−2 year−1.
The bars really are 380, 38, 3.8 and 0.38 units wide here. Nothing is exaggerated – that is what a tenfold drop four times over looks like.
This is the honest version, and it is worth seeing once. In practice an exam will usually accept an approximate sketch, where each bar is a rough representation of the energy contained. Just never draw the top bar wider than the one below it.
The triangle on the right is the single most common way students lose a drawing mark. Rectangles, always.
Worked examples
WE 1
Calculate bar lengths to scale
A grassland contains 8 000 kJ m−2 yr−1 in producers, 900 in primary consumers and 85 in secondary consumers. The producer bar is drawn 80 mm long. Calculate the length of the other two bars. (3 marks)
Step 1: find the scale
80 mm represents 8 000 kJ m−2 yr−1, so 1 mm = 100 kJ m−2 yr−1.
Step 2: primary consumers
900 ÷ 8 000 × 80 = 9.0 mmStep 3: secondary consumers
85 ÷ 8 000 × 80 = 0.85 mm9.0 mm and 0.85 mmyes, 0.85 mm really is that thin – say so in your answer rather than quietly drawing it bigger
WE 2
Calculate the percentage transferred
Using the same data, calculate the percentage of energy transferred from producers to primary consumers, and from primary to secondary consumers. (2 marks)
Step 1: producers to primary
(900 ÷ 8 000) × 100 = 11.25 %Step 2: primary to secondary
(85 ÷ 900) × 100 = 9.44 %Step 3: comment
Both are close to the rough 10 % figure, so this ecosystem behaves as expected.
11.25 % and 9.44 %always divide by the level below, never by the producers, unless the question asks for the overall transfer
WE 3
Explain the shape
Explain why a pyramid of energy is always widest at the base and shows a stepped decrease. (3 marks)
Point 1: the base
Producers contain the most energy in their biomass, because energy enters the ecosystem at this level.
Point 2: the decrease
Only about 10 % of the energy at one level is transferred to the biomass of the next, because of incomplete consumption, incomplete digestion, heat lost in respiration and excretion.
Point 3: why stepped
Each bar shows a fixed quantity of energy for one whole trophic level, so the change happens in steps between levels, not gradually within them.
Energy only ever falls going up, and it falls in jumpsnaming two or three specific losses is usually worth more than repeating “energy is lost”
💡 Exam tips
Write the units on the diagram every time: kJ m−2 year−1.
Show your scale calculation even if the drawing goes wrong – the working carries marks.
Use a ruler and keep the sides stepped.
Label levels by name, not just by number.
If asked why energy falls, give named losses, not just “energy is lost as heat”.
Round sensibly and keep the units in your final answer.
⚠ Common mistakes
Drawing a smooth triangle. It is a set of rectangles.
Leaving the units off. Energy per unit area per year, not just kJ.
Forgetting “per year”. A pyramid of energy is a rate, measured over time.
Dividing by the wrong level. Percentage transferred compares a level with the one directly below it.
Making the top bars bigger so they can be seen. If you must, say clearly that the sketch is not to scale.
Confusing it with a pyramid of biomass. Energy pyramids measure energy per area per year; biomass pyramids measure mass at one moment.
Up next: Energy Losses – the full account of where that missing 90 % goes, and why food chains run out of room after four or five levels.
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