IB Biology SLTopic 3 — Energy & MatterSkillsData & drawing~9 min read
Pyramids of Energy
A pyramid of energy is a bar chart turned on its side and stacked up. It looks simple, but examiners are fussy about it: to scale, correct units, correct labels, stepped sides. Get those four things right and this is free marks.
📘 What you need to know
A pyramid of energy shows the energy contained in the biomass at each trophic level.
The length of each bar is proportional to the energy at that level.
Pyramids of energy must be drawn to scale, with stepped sides rather than smooth sloping ones.
The units are energy per unit area per unit time, usually kJ m−2 yr−1.
They are always widest at the base and can never be inverted.
Roughly 10 % of the energy passes from one level to the next.
What the bars are actually measuring
Each bar represents the energy stored in the carbon compounds of all the organisms at that trophic level, in a set area, over a set period of time.
Those last two conditions matter more than students expect. Because the measurement is per year, a pyramid of energy is a rate, not a snapshot. That is what stops it ever being inverted, as you will see below.
Units to learn
kJ m−2 yr−1 energy, per square metre, per year
If you forget the units, reason them out. Energy is measured in kilojoules. It has to be per unit area or you cannot compare a field with a forest. It has to be per year or a bigger number just means you waited longer.
Drawing one to scale
Suppose the four levels of a grassland hold 10 000, 1 000, 100 and 10 kJ m−2 yr−1. Here is what that looks like when each bar is drawn honestly to scale.
Ten-fold drops at every step. This is why energy pyramids look so lopsided.
That picture is uncomfortable to draw, so in practice a pyramid of energy is often sketched approximately, with each bar simply narrower than the one below it. Exam papers accept an approximate sketch unless the question says “draw to scale”. Read the command carefully.
Why they are never inverted
A pyramid of numbers can easily be upside down. One oak tree can support thousands of insects, so the bottom bar is tiny and the one above it is huge. A pyramid of biomass can be inverted too, in oceans, where a small standing mass of fast-reproducing algae feeds a larger mass of zooplankton at any given moment.
A pyramid of energy cannot do this. Because it measures energy passing through per year, and because energy is always lost between levels, each bar must be smaller than the one below it.
The pyramid of numbers is misleading because one oak tree is enormous. Energy fixes that.
📝 How to draw a pyramid of energy
Choose a scale that lets the largest bar fit the space you have, e.g. 1 cm represents 2 000 kJ m−2 yr−1.
Draw the producer bar first, at the bottom, and make it the widest.
Centre each bar above the one below so the pyramid is symmetrical.
Keep the sides stepped, not sloping. Each bar is a separate rectangle.
Label every level with its name: producer, primary consumer, secondary consumer, and so on.
Write the value and the units on or beside each bar.
State your scale somewhere on the diagram.
Calculating energy transfer efficiency
Pyramid data is usually paired with a percentage calculation.
Efficiency of energy transfer
efficiency (%) = energy at the higher level ÷ energy at the lower level × 100
WORKED EXAMPLE
A pyramid of energy for a meadow shows 18 200 kJ m−2 yr−1 in the producers, 1 560 in the primary consumers and 148 in the secondary consumers. Calculate the percentage efficiency of energy transfer between each pair of levels. [3]
Step 1: producers to primary consumers1 560 ÷ 18 200 × 100 = 8.57 %Step 2: primary to secondary consumers148 ÷ 1 560 × 100 = 9.49 %Step 3: commentboth transfers are close to the usual figure of about 10 %give the answer to 3 significant figures and always include the % sign
Sense check every answer. An efficiency above about 20 % should make you suspicious that you divided the wrong way round. Divide the smaller number by the larger one.
Where the missing 90 % goes
You will meet this in detail on the next page, but in short, energy is lost between levels because:
Not every part of the organism is eaten — roots, bark, bones and fur are left behind.
Not everything eaten can be digested, so some leaves the body as faeces.
Respiration releases heat to the environment.
Metabolic waste such as urea is excreted.
Because biomass follows energy, the biomass at each level also falls as you go up.
💡 Exam tip
The command draw to scale means measure with a ruler. An approximate sketch will not score.
Draw stepped sides. A smooth triangle loses the mark.
Never leave off the units. kJ m−2 yr−1 is worth memorising as one block.
Label the levels by name, not just by number.
If asked why a pyramid of energy cannot be inverted, mention that energy is lost at every transfer.
Show your substitution in efficiency calculations, so you can pick up method marks.
⚠ Common mix-up
Pyramid of energy is not the same as pyramid of biomass. One measures energy per year, the other measures mass at one moment.
10 % is a rough guide, not a law. Real values range widely, so calculate rather than assume.
Bar length, not bar area, is proportional to the energy.
Do not include the sun as a bar at the bottom.
Energy is not “used up”. It is transferred out of the food chain, mostly as heat.
Percentage efficiency is between two levels, so make sure you know which two the question wants.
Up next: Energy Losses — you have seen that only about a tenth gets through. Now we account for the other nine tenths, one route at a time.
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