KS3 · Biology

Pyramids of biomass

Oak tree → caterpillar → blackbird → buzzard. Count them, and one tree holds up ten thousand caterpillars. Weigh them, and the whole picture flips.

Same food chain, two pyramids

One oak tree, drawn two ways
Buzzardtertiary consumerBlackbirdssecondary consumerCaterpillarsprimary consumerOak treeproducerNumber of organisms12010,0001

View: Count them. Showing 2 layers: Food chain, Pyramid of numbers

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Count them, then weigh them. Watch the bottom bar.

Pyramid of numbers. Counting ignores size, so one enormous oak counts the same as one tiny caterpillar, and the pyramid comes out upside down at the bottom.

Illustrative numbers for one imagined oak tree. The bars show which level is bigger; they are not drawn to scale.

Individuals or populations?

Doesn't the big animal at the top have the most biomass?

A buzzard is far heavier than a caterpillar. So why is the buzzard's bar the smallest in the pyramid of biomass?

Which is closest to what you think right now?
How sure are you?

Measuring biomass

?

Reason it through

Why is weighing an organism only a rough estimate of its biomass, and why is an accurate measurement so hard?

Link 1 of 4

First link · your turn

Biomass is the mass of living material. What does that leave out?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Estimating biomass

Where did the caterpillars' 30 kg come from?

A student weighs a sample of 5 caterpillars: 2.8 g, 3.1 g, 3.0 g, 2.9 g and 3.2 g. There are 10,000 caterpillars on the oak tree. Estimate the biomass of the caterpillar population in kilograms, so it can go on the same pyramid as the oak tree's 1,500 kg.

  1. Total of the measured masses = 2.8 + 3.1 + 3.0 + 2.9 + 3.2 = 15.0 gStart with the sample: add up every mass you measured.
  2. missing step
Which line is step 2?

Predict, then check

Think about what each one is doing through the winter.

An oak tree is weighed in summer and again in midwinter. A hedgehog is weighed before it hibernates and again near the end of winter. What happens to their biomass?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Count the organisms in a food chain, then weigh them, and watch the same chain tell two different stories.

What you need to know

  • Food chains and food webs show feeding relationships between populations, but not how many organisms are at each stage, called a trophic level.
  • A pyramid of numbers shows the number of organisms at each trophic level as bars, with the producer at the bottom, then primary, secondary and tertiary consumers above.
  • Biomass is the mass of living material in an organism. It does not include water.
  • A pyramid of biomass shows the biomass at each trophic level as bars drawn either side of a centre line, with the producer at the bottom.
  • Estimated biomass of a population = number of individuals × mean mass of one individual.

The big picture

A food chain shows who eats whom, but not how many organisms there are at each trophic level. A pyramid of numbers counts them, and because it ignores size it isn't always pyramid-shaped: one oak tree can feed 10,000 caterpillars. A pyramid of biomass shows the mass of living material at each level instead. Biomass decreases at each level up a food chain, so the oak chain's pyramid of biomass is a true pyramid. Biomass is hard to measure exactly, so we estimate it from a sample, in matching units, and the pyramid only shows one moment in time.

Key points

1Pyramids of numbers ignore the size of organisms, so they are not always pyramid-shaped: one oak tree can support thousands of caterpillars.
2Biomass decreases at each trophic level of a food chain, so the oak chain's pyramid of biomass is a true pyramid even though its pyramid of numbers is not.
3Think populations, not individuals: one buzzard is heavy, but a pyramid adds up every organism at each level.
4Weighing a living organism only estimates its biomass, because the reading includes water. An accurate measurement means drying it out, which kills it, and catching every organism at that level.
5Mean mass = total of measured masses ÷ number of measurements. Convert grams to kilograms (÷ 1,000) so every level is in the same units before you compare or plot.
6A pyramid of biomass shows one moment in time. Biomass can change during a year, like an oak losing its leaves in winter.

Worked example

Problem

A student samples 4 garden snails and weighs them: 8 g, 11 g, 9 g and 12 g. There are 2,500 snails in the garden. Estimate the biomass of the snail population in kilograms.

⚠ Watch out

Saying biomass increases up a food chain because the animals at the top are bigger. One buzzard does outweigh one caterpillar, but a pyramid of biomass adds up the whole population at each level, and in the oak chain the caterpillars together outweigh the buzzard.

🧠

Memory hook

Count the crowd, then weigh the crowd. A pyramid is about the whole level, never just one organism.

✓

Check yourself

In the oak tree → caterpillar → blackbird → buzzard food chain, why is the pyramid of numbers upside down at the bottom when the pyramid of biomass is not?

Flashcards

(13)
What does a food chain NOT show you?
How many organisms there are at each stage (trophic level).
What does a pyramid of numbers show?
The number of organisms in the population at each trophic level, drawn as bars with the producer at the bottom.
Why isn't a pyramid of numbers always pyramid-shaped?
It doesn't take the size of organisms into account, so one big organism, like an oak tree, counts the same as one tiny one.
What is biomass?
The mass of living material in one or more organisms. It does not include water.
What does a pyramid of biomass show?
The biomass at each trophic level of a food chain, as bars drawn either side of a centre line, producer at the bottom.
What happens to biomass as you go up the trophic levels of a food chain?
It decreases at each level.
Why do people wrongly think biomass increases up a food chain?
They think about one individual (a big buzzard) instead of the whole population at each level.
Why does weighing a living organism give only a rough estimate of its biomass?
The balance reading includes the water in the organism, and biomass doesn't include water.
Give two reasons measuring biomass accurately is very difficult.
The organism must be dried out, which kills it, and every organism in the trophic level would have to be captured.
How do you find the mean mass from a sample?
Mean mass = total of the measured masses ÷ number of measurements.
How do you estimate the biomass of a population?
Estimated biomass = number of individuals × average (mean) mass of one individual.
One biomass is in grams and another in kilograms. What must you do before comparing or plotting them?
Put them in the same units. To change grams to kilograms, divide by 1,000 (1,000 g = 1 kg).
Why is a pyramid of biomass only a snapshot?
It shows biomass at one point in time, and biomass changes during a year: an oak loses its leaves in winter, and a hibernating hedgehog uses up its food reserves.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.