3.9.5Additional lessons
Pyramids of Energy: Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 23 of 101
Part 1 of 5What a Pyramid of Energy Shows
Definition
- A pyramid of energy is a diagram of stacked horizontal bars, one for each trophic level, with the producers at the base.
Bar width
- The width of each bar is drawn to scale to show the energy available at that level.
Units
- Energy is measured per unit area per unit time, for example kJ m⁻² yr⁻¹, so that ecosystems of different size can be compared.
Shape
- Energy pyramids are always pyramid-shaped, because energy falls at every transfer and never increases from one level to the next.
Comparison
- They give a better picture of an ecosystem than a count of numbers, since a single large tree is not equal to a single insect.
Part 2 of 5Drawing the Pyramid
1
Collect data for the energy entering each trophic level over one year, per square metre.
2
Choose a scale so that the widest bar, for the producers, fits the page.
3
Draw each bar centred on the same vertical line, with its width proportional to the energy value.
4
Label each bar with the trophic level and the value with units, with producers at the bottom.
5
Calculate the percentage transfer between levels to compare the efficiency of different transfers.
Part 3 of 5Calculating Transfer Efficiency
Calculating Transfer Efficiency
Formula
- Percentage transfer = energy at the higher level ÷ energy at the lower level × 100.
Typical values
- Transfers between levels are often around ten percent, but real values vary widely between ecosystems and levels.
Higher values
- Transfers from herbivores to carnivores are often higher than from plants to herbivores, because animal food is easier to digest.
Use of results
- The figures allow us to predict how many top consumers an area can support.
| Trophic level | Energy (kJ m⁻² yr⁻¹) | Transfer to this level |
|---|---|---|
| Producers | 20 000 | - |
| Primary consumers | 2 200 | 11.0% |
| Secondary consumers | 260 | 11.8% |
| Tertiary consumers | 24 | 9.2% |
Part 4 of 5Worked Example
Question
- In an ecosystem, producers have 20 000 kJ m⁻² yr⁻¹ and primary consumers have 2 200 kJ m⁻² yr⁻¹.
- Calculate the percentage of energy transferred to the primary consumers.
- Usepercentage transfer = (energy at higher level ÷ energy at lower level) × 100.
- Substitute2 200 ÷ 20 000 = 0.11.
- Multiply by 100 to give a percentage.
Answer 11% of the energy was transferred to the primary consumers.
Part 5 of 5Exam Checklist & Tips
3.9.5 – Key Points for the Exam
- Pyramid of energy A diagram showing the energy at each trophic level.
- Transfer efficiency Percentage of energy passed from one trophic level to the next.
- kJ m⁻² yr⁻¹ Units of energy per area per year.
- Producer The organism at the base of the pyramid.
- Trophic level The feeding position in a food chain.
Common Mistakes
- Drawing a pyramid with a wider bar above a narrower one, which is not possible for energy.
- Leaving out the time unit, so the data cannot be compared.
- Calculating the percentage using the higher level as the denominator.
- Using the number of organisms instead of energy to draw the bars.
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