Ecological efficiency at Higher Level: An HL-only statement: the percentage of energy passed from one trophic level to the next, worked out from real data, and why the famous 10% is only a rough guide.
Practise this as you read
- Calculate ecological efficiency from a food chain, formula first.
- Explain why efficiency differs between species, levels and ecosystems.
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How much energy is passed on: Ecological efficiency tells you how much of a level's energy reaches the level above it.
It is always worked out between two neighbouring levels, from the energy each one receives.
The formula: Ecological efficiency (%) = (energy passed on to the next level ÷ energy received by this level) × 100
The points to remember
- Ecological efficiency = the percentage of the energy received by one trophic level that is passed on to the next.
- Formula: (energy passed to the next level ÷ energy received by this level) × 100.
- It is always low, because energy is lost at every step: heat from respiration, parts not eaten, faeces (2.2.12).
- Energy not passed on is not destroyed: it leaves as heat or goes to the decomposers.
- Low efficiency is why food chains have only 4 or 5 levels (2.2.14).
Remember it as: Up the chain: top over bottom, times 100.
Example: if zooplankton receive 1,000 kJ and pass 60 kJ on to the fish that eat them, the ecological efficiency is (60 ÷ 1,000) × 100 = 6%.
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Questions give a food chain with the energy at each level, often beside a column you do not need. Four steps work every time.
Working it out from data
- Find the two levels the question names, and read their energy values.
- Divide the higher level by the lower level: the one that receives by the one that gives.
- Multiply by 100 and add the % sign.
- Both values must be in the same units; ignore any other column, such as a pollutant.
- The answer is always below 100%; above 100% means the division is the wrong way round.
Worked example: Silver Springs: Producers to herbivores = (14,100 ÷ 87,100) × 100 = 16%.
Herbivores to carnivores = (1,600 ÷ 14,100) × 100 = 11%.
Carnivores to top carnivores = (90 ÷ 1,600) × 100 = 6%.
Three steps in one river give three different percentages, and none of them is exactly 10%.
Common slips: Dividing the lower level by the higher one gives an answer over 100%, which is impossible. Subtracting the two values gives the energy lost, not the efficiency.
The '10% rule' is often taught as a law, but it is not: the real percentage changes from place to place and from step to step.
Why 10% is not a rule
- Efficiency varies between ecosystems, between trophic levels and between species.
- Values of about 5-20% are common.
- 10% is neither a fixed amount nor a true average: it is only a rough rule of thumb.
- Always work out the percentage from the data; never assume 10%.
| It varies between | Higher efficiency | Lower efficiency |
|---|---|---|
| Species | Insects, fish: they do not keep warm, so less is lost as heat | Birds, mammals: they burn most of their food keeping warm |
| Trophic levels | Carnivores: meat is easy to digest | Herbivores: tough plant fibre leaves as faeces |
| Ecosystems | Oceans: phytoplankton are eaten whole | Forests, grasslands: wood and roots are left uneaten |
Real example: at Silver Springs the efficiency falls from 16% to 6% up the chain. The top carnivores are large, active fish that hunt, so they use more of their food in respiration.
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How this comes up: Paper 2, Section A: a food chain with energy values. Calculate the efficiency of one transfer [1], sometimes followed by why it differs from another transfer [2].
Figure 1 shows the energy in each trophic level of a food chain in a Scottish sea loch.
Calculate, as a percentage, the ecological efficiency of the transfer from zooplankton to herring.
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The transfer from herring to harbour seals is only (3 ÷ 81) × 100 = 3.7%. A follow-up part may ask why.
Figure 1 shows the energy in each trophic level of a food chain in a Scottish sea loch.
Explain why the ecological efficiency from herring to harbour seals is lower than from phytoplankton to zooplankton.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.