Three pyramids, three different things being measured, and three different sets of rules about when they can turn upside down. This is one of the few places in ESS where you are marked on the quality of your drawing, so the details matter.
📚 What you need to know
The three types are pyramids of numbers, biomass and energy (also called productivity).
All three are quantitative models measured for a given area and time.
Pyramids of numbers count organisms, so they are easily inverted — one oak tree feeds thousands of insects.
Pyramids of biomass show dry mass at one moment, a standing crop. They can be inverted in aquatic systems.
Pyramids of energy show flow per area per time, and are never inverted.
Units for energy pyramids are per area per time, for example kJ m-2 yr-1.
Drawing rules: bars touching, centred on a midline, drawn to scale, each level labelled, axis labelled with units.
Pyramids of numbers
The width of each bar shows how many organisms are at that level. Numbers usually fall going up, because less energy is available — but not always, and that is what makes these worth a question.
This is the main weakness of pyramids of numbers, and it is exactly why pyramids of biomass were invented.
Limitations of pyramids of numbers
They ignore the size of the organisms, so a single oak counts the same as a single blade of grass.
They become misleading when parasites are involved, because one host supports many parasites.
They give no information about the energy or biomass at each level.
Pyramids of biomass
These use dry mass, so a tree is no longer counted as one item — it is counted as the large amount of organic material it actually is. Biomass normally falls going up, so the shape is usually a proper pyramid.
Notice the scale runs outwards from zero in both directions. That is what lets the bars stay centred while still being drawn accurately.
🧩 Drawing a pyramid that scores full marks
Choose a scale that fits the page and goes up in regular steps.
Draw a centre line and build every bar symmetrically around it.
Keep the bars touching, with no gaps between levels.
Label each trophic level with the organism’s name.
Label the axis with units, for example g m-2 or kJ m-2 yr-1.
Producers go at the bottom, always.
When a pyramid of biomass inverts
In some marine ecosystems the producers are phytoplankton: microscopic, short-lived and eaten almost as fast as they appear. Measure their biomass at any single moment and it can be lower than the zooplankton feeding on them, producing an upside-down pyramid.
The reason is that a pyramid of biomass is a snapshot. It records the standing crop on the day of measurement, not how fast that crop is being replaced. Phytoplankton have a low standing biomass and a very high productivity, so a small mass supports a much larger one above it.
Imagine a small bakery supplying a whole town. At any moment there is only one tray of bread on the counter, far less than the bread sitting in everyone’s kitchens. The counter is the standing crop; the ovens are the productivity.
Pyramids of energy
These show the flow of energy through each trophic level, per unit area per unit time. Because they measure a rate over a whole year rather than a snapshot, and because the second law guarantees losses at every transfer, they always narrow towards the top.
Pyramid
What the bars show
Can it be inverted?
Numbers
How many organisms are at each level
Yes — easily, with large producers or with parasites
Biomass
Dry mass at each level at one moment (standing crop)
Sometimes — in aquatic systems with fast-growing producers
Energy
Energy flow per area per time (a rate)
No — never
Limitations of each type
Numbers: ignore organism size, distorted by parasites, and say nothing about energy.
Biomass: only a snapshot, so they miss the rate of production; and measuring dry mass means killing the organisms.
Energy: the most accurate and never inverted, but the hardest to build — data must be collected over a full year to cover seasonal variation.
Worked examples
WORKED EXAMPLE
Describe how you would draw a pyramid of biomass for these data: algae 850, mayfly larvae 210, minnows 45, kingfisher 6 (all g m²).
Step 1: pick a scale
The largest value is 850, so a scale marked every 100 g m² fits comfortably.
Step 2: draw the base
Algae at the bottom, 850 units wide, centred on the midline.
Step 3: stack the rest, touching
Mayfly larvae 210, then minnows 45, then kingfisher 6, each centred and drawn to the same scale.
Step 4: finish it
Label every level and label the axis with units.
A normal pyramid shape, narrowing sharply towards the topThe top bar will be very thin. Draw it thin — do not exaggerate it to make it visible.
WORKED EXAMPLE
Explain why a pyramid of biomass for a lake can be inverted but a pyramid of energy for the same lake cannot.
Step 1: what biomass measures
Biomass is a standing crop — the mass present at one moment.
Step 2: why that can invert
Phytoplankton reproduce very quickly and are eaten almost immediately, so their standing biomass can be smaller than the zooplankton they support.
Step 3: what energy measures
A pyramid of energy measures flow over a whole year, and energy is always lost as heat between levels.
A rate cannot increase up the chain, so energy pyramids never invertThe contrast between snapshot and rate is the whole answer. Say both words.
WORKED EXAMPLE
A pyramid of energy shows 10 000, 1 200, 150 and 21 kJ m² per year. Calculate the efficiency of the transfer from producers to primary consumers, and comment.
Step 1: substituteefficiency = (1 200 ÷ 10 000) × 100efficiency = 12%Step 2: comment on the pattern
The next transfers are 12.5% and 14%, so all three are close to the usual 10% figure.
Step 3: explain the losses
The missing energy left as heat from respiration, in egested faeces, and as material that was never eaten.
If asked to comment, quote the other transfers too — it shows you read the whole data set.
💡 Exam tip
Producers always go at the bottom, even when the pyramid ends up inverted.
Use a ruler and a real scale. Marks are given for bars drawn proportionally.
Write the units on the axis. An unlabelled axis costs a mark every time.
For “why is this pyramid inverted”, decide first which type it is. The reason is different for numbers and for biomass.
Remember energy pyramids need data collected over a full year because productivity changes with the seasons.
⚠ Common mix-up
Saying pyramids of energy can be inverted. They cannot, and that is a favourite one-mark question.
Explaining an inverted biomass pyramid with “there are fewer producers”. There are not fewer — they simply have a lower standing biomass at that moment.
Drawing bars with gaps between them, or lined up at the left edge instead of centred.
Using wet mass. Biomass pyramids use dry mass.
Leaving the axis unlabelled or omitting the per-year part of an energy unit.
Making the top bar bigger than the data says so it can be seen. Draw it to scale and label it.
Up next: How Humans Disrupt Energy and Matter Flow — bioaccumulation, biomagnification, microplastics, and the activities that reroute the flows in this topic.
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