Energy is not created or destroyed along a food chain. Only a fraction of the energy at one trophic level reaches the next, and the rest leaves the chain, mostly as heat. You will meet this in ecology and energy flow as a calculation and as a “give two reasons” explanation.
Why is energy lost between trophic levels?
Start with light. Only a small part of the light energy reaching a plant is captured, and some of what the plant makes is used by the plant itself.
At each feeding step, energy is lost in these ways:
- Respiration. The organism releases energy to power movement, growth and keeping warm. Much of it ends up as heat in the surroundings.
- Parts not eaten. The consumer does not eat roots, bones, shells, hair or fur.
- Waste. Some food is not digested and leaves as faeces. Some energy leaves in urine.
The lost energy is not destroyed. It goes to the environment or to decomposers, which is why ecosystems need a constant supply of light energy.
How do you calculate energy transfer efficiency?
Use the percentage formula:
percentage passed on = energy in the next level ÷ energy in the level before × 100
Always divide by the lower level, the one the energy came from.
Worked example
The table shows energy in a stream food chain for one year per square metre (invented data for practice).
| Trophic level | Organism | Energy / kJ |
|---|---|---|
| Producer | algae | 12 000 |
| Primary consumer | mayfly nymph | 1 500 |
| Secondary consumer | small fish | 180 |
Calculate the percentage of energy passed from algae to mayfly nymphs, and from mayfly nymphs to small fish. Then say how much energy was not passed on from algae.
Step 1, algae to mayfly: 1 500 ÷ 12 000 × 100 = 12.5%.
Step 2, mayfly to fish: 180 ÷ 1 500 × 100 = 12%.
Step 3, energy not passed on from algae: 12 000 − 1 500 = 10 500 kJ.
Step 4, give reasons for the loss: some algae were not eaten, some energy was released by respiration in the algae, and some algae eaten were not digested by the nymphs.
Check step 1 again by multiplying back: 12.5% of 12 000 = 0.125 × 12 000 = 1 500. Step 2: 12% of 1 500 = 0.12 × 1 500 = 180. Both match the table.
The mistake to watch for
A frequent slip is to divide by the wrong number, or to write a vague reason.
Mistaken answer: “180 ÷ 12 000 × 100 = 1.5%. Energy is used up.”
The student divided by the producer energy instead of the mayfly energy, and wrote “used up”.
The correction has two parts.
For the calculation, divide by the level the energy came from: 180 ÷ 1 500. For the explanation, name the route: energy is released as heat by respiration, or remains in parts not eaten or in waste. Energy is transferred or lost to the surroundings, not used up.
Check yourself
1. A grassland food chain has 8 000 kJ in grass and 640 kJ in grasshoppers. Calculate the percentage of energy passed from grass to grasshoppers.
Show answer
640 ÷ 8 000 × 100 = 8%. Check: 8% of 8 000 = 0.08 × 8 000 = 640.
2. The grasshoppers in question 1 pass 48 kJ to frogs. What percentage is that, and how does it compare with the first step?
Show answer
48 ÷ 640 × 100 = 7.5%. Check: 0.075 × 640 = 48. It is slightly lower than 8%, so a little less was passed on at this step.
3. Give two reasons why not all the energy in a herbivore is passed on to the carnivore that eats it.
Show answer
Any two of: energy is released as heat by respiration in the herbivore; some parts such as bones or fur are not eaten by the carnivore; some energy is lost in faeces or urine of the herbivore.
Where this leads next
Next, read how to interpret a food web change, which uses energy flow as one piece of reasoning. For the respiration side of the losses, see gas exchange and respiration, and test the arithmetic on the practice set.
If you can calculate efficiency but the written reasons still feel thin, the pattern is worth a conversation in online one-to-one Biology tuition.