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3.4.2Unit 3 — Life on Earth

Ecological Pyramids — SQA National 5 Biology Revision Notes & Topical Questions

5 min read 5 parts Lesson 22 of 31

📚 SQA National 5 Biology Hub / Topical Revision
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Part 1 of 5Pyramids of Numbers

3.4.2 Ecological Pyramids
What it shows
  • A pyramid of numbers has a bar for each stage of a food chain, with length showing the number of individuals.
Labels
  • Each bar needs the name of the organism, and the number of individuals should be labelled at each level.
Shape varies
  • Many small organisms support few large ones, but a single large tree supports huge numbers of insects.
Limitation
  • It ignores the size of organisms, so a bar for one tree and a bar for one aphid look equal.

Part 2 of 5Pyramids of Energy

What it shows
  • A pyramid of energy shows the energy contained in the biomass at each trophic level.
Always a pyramid
  • The shape is always widest at the base and narrower at each higher level.
Reason
  • Producers hold the most energy, and some energy is lost at every transfer, so less is stored higher up.
Units
  • Energy is labelled with units, for example kJ per m² per year, so levels can be compared fairly.
FeaturePyramid of numbersPyramid of energy
Bar length showsNumber of individualsEnergy in biomass
ShapeCan varyAlways a pyramid
Size of organismsNot consideredTaken into account

Part 3 of 5Drawing a Pyramid

1
Write out the food chain in order, starting with the producer.
2
Draw a horizontal bar for each organism, stacked in food chain order.
3
Make each bar's length match the data, using the same scale for every bar.
4
Label each bar with the name of the organism.
5
Add numbers, or energy values with their units, to complete the pyramid.

Part 4 of 5Explaining the Difference

Explaining the Difference
Numbers pyramid inverted
  • One oak tree can feed thousands of caterpillars, so the number of individuals increases at the second level.
Energy pyramid stays upright
  • The oak holds far more energy than all the caterpillars, so the energy bar at the base is still the widest.
Conclusion
  • Pyramids of energy are always pyramid shaped because energy is lost at each transfer.
Use
  • Energy pyramids show why long food chains cannot be supported and why few top predators exist.

Part 5 of 5Exam Checklist & Tips

3.4.2 – Key Points for the Exam

  • Pyramid of numbers A diagram showing the number of organisms at each stage of a food chain.
  • Pyramid of energy A diagram showing the energy stored in the biomass at each trophic level.
  • Trophic level Each feeding stage in a food chain.
  • Producer The organism at the base of a food chain that makes its own food.
  • Scale The fixed relationship between bar length and the quantity shown.

Common Mistakes

  • Saying pyramids of numbers must always be pyramid shaped;
    • they can be an odd shape depending on organism size.
  • Omitting units on a pyramid of energy.
  • Drawing bars of unequal scale so that the quantities cannot be compared.
  • Explaining the energy pyramid shape without mentioning energy loss between levels.

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