Energy falls at every step up a food chain, because each organism uses most of its energy and loses some as heat. The percentage transferred tells you how much reaches the next level.
This lesson is part of ecosystems. The idea that energy flows one way and does not cycle is covered in energy transfer without claiming that energy cycles.
How do you read an energy pyramid?
Each bar is one trophic level, with producers at the base. The bar length shows the energy stored at that level, so each bar is shorter than the one below.
Read the unit carefully. A value such as 8 000 kJ m⁻² year⁻¹ means energy per square metre each year.
Worked example: a four-level lake chain
The figures are invented for practice.
| Trophic level | Organism | Energy (kJ m⁻² year⁻¹) |
|---|---|---|
| 1 | Phytoplankton | 8 000 |
| 2 | Zooplankton | 900 |
| 3 | Small fish | 90 |
| 4 | Large fish | 8 |
Transfer 1 to 2: 900 ÷ 8 000 × 100 = 11.25%. Transfer 2 to 3: 90 ÷ 900 × 100 = 10%. Transfer 3 to 4: 8 ÷ 90 × 100 ≈ 8.9%.
Each step passes on roughly a tenth. After three steps, only 8 of the 8 000 units remain, which is 0.1% of the producer energy.
Why are food chains short?
- Energy is lost as heat at each level through respiration.
- Some of the body is not eaten or digested, so that energy stays out of the chain.
- By the fourth or fifth level, too little energy remains to support another population.
A full-mark answer uses all three points and ends with the conclusion about the number of levels.
The mistake that costs marks
The usual error is to divide the wrong way, for example 8 000 ÷ 900. The transfer percentage always has the higher level on top and the lower level beneath it.
A second error is to say the energy is “used up”. Write instead “lost as heat through respiration”.
Check yourself
A pyramid shows grass 12 000 kJ, grasshoppers 1 500 kJ and frogs 120 kJ. Find the percentage transferred from grass to grasshoppers and from grasshoppers to frogs.
Answer
Grass to grasshoppers: 1 500 ÷ 12 000 × 100 = 12.5%.
Grasshoppers to frogs: 120 ÷ 1 500 × 100 = 8%.
Each transfer passes on less than a fifth, so the energy falls quickly along the chain.
What to study next
Move on to interpreting population sampling data. The biological data interpretation trainer has more tables, and the mistake log is a place to record calculation slips.
If you want a teacher to go through your energy answers, see online one-to-one Biology tuition.