Efficiency is the percentage transferred
Definition
Energy transfer efficiency
The percentage of energy or biomass at one trophic level that is passed on to the next, found by dividing the amount transferred by the amount available and multiplying by 100.
- Energy transfer efficiency compares the energy passed to the next trophic level with the energy available at the trophic level below.
- Use the same energy unit in the numerator and denominator so the units cancel and the final answer is a percentage.
- The general equation is efficiency=amount transferredamount available×100\text{efficiency} = \dfrac{\text{amount transferred}}{\text{amount available}} \times 100efficiency=amount availableamount transferred×100.
- For energy data, write it as efficiency of energy transfer=energy at the next trophic levelenergy at the previous trophic level×100\text{efficiency of energy transfer} = \dfrac{\text{energy at the next trophic level}}{\text{energy at the previous trophic level}} \times 100efficiency of energy transfer=energy at the previous trophic levelenergy at the next trophic level×100.
Example
- A caterpillar population receives 12 000 kJ12\,000\,\mathrm{kJ}12000kJ from leaves and stores 1 800 kJ1\,800\,\mathrm{kJ}1800kJ as biomass available to birds.
- Substitution gives efficiency=1 80012 000×100=15%\text{efficiency} = \dfrac{1\,800}{12\,000} \times 100 = 15\%efficiency=120001800×100=15%.
- The energy transfer efficiency from leaves to caterpillars is therefore 15%15\%15%.
The denominator is the previous level
- The amount transferred belongs on top of the fraction and the amount available before the transfer belongs underneath.
- Dividing in the opposite order can produce a value above 100%100\%100%, which is not possible for trophic transfer efficiency.
- If the figures use different units, convert them before calculating, such as changing both values to kJ\mathrm{kJ}kJ or both to J\mathrm{J}J.
- A correct final value can receive full calculation marks, but showing the substitution makes an arithmetic slip easier to identify.
Common Mistake
Do not divide by the original producer value unless the question asks for the overall efficiency from the producer to the stated level.
Biomass uses the same calculation
Definition
Biomass
The total mass of living material, usually measured as dry mass, present in an organism or at a trophic level.
- Biomass is usually measured as dry mass because water content changes and does not represent stored organic material fairly.
- For biomass data, use biomass transfer efficiency=biomass at the next trophic levelbiomass at the previous trophic level×100\text{biomass transfer efficiency} = \dfrac{\text{biomass at the next trophic level}}{\text{biomass at the previous trophic level}} \times 100biomass transfer efficiency=biomass at the previous trophic levelbiomass at the next trophic level×100.
- The biomass values may be totals or values per unit area, but both values in one calculation must use the same basis and units.
Example
- A field contains 360 kg360\,\mathrm{kg}360kg of plant biomass and 54 kg54\,\mathrm{kg}54kg of herbivore biomass.
- Substitution gives efficiency=54360×100=15%\text{efficiency} = \dfrac{54}{360} \times 100 = 15\%efficiency=36054×100=15%.
- The biomass transfer efficiency is 15%15\%15%, so 85%85\%85% of the plant biomass was not converted into herbivore biomass.
Percentages can find a missing value
- Convert a percentage to a decimal before multiplying when the transferred amount is unknown.
- For an efficiency of 8%8\%8%, the decimal multiplier is 8100=0.08\dfrac{8}{100} = 0.081008=0.08.
- The transferred amount is found from amount transferred=amount available×efficiency100\text{amount transferred} = \text{amount available} \times \dfrac{\text{efficiency}}{100}amount transferred=amount available×100efficiency.
Example
- If plants store 2 500 kJ2\,500\,\mathrm{kJ}2500kJ and the transfer efficiency is 8%8\%8%, the consumer receives 2 500×8100=200 kJ2\,500 \times \dfrac{8}{100} = 200\,\mathrm{kJ}2500×1008=200kJ.
- The answer keeps the energy unit because the calculation finds an amount of energy rather than a percentage.
Use the data given
- Read the labels carefully to identify the two adjacent trophic levels involved in the transfer.
- Keep full calculator values during working and round only the final answer to the precision requested.
- A transfer percentage is not the same as a percentage decrease, so use the numerator named in the efficiency equation.
Exam technique
- Write the fraction with the next level on top before entering numbers into the calculator.
- Include the multiplication by 100100100 and the percent sign when the question asks for efficiency.
- Check that the result lies between 0%0\%0% and 100%100\%100%.
Check the method and units
- A sensible answer is smaller than 100%100\%100% because some energy and biomass fail to reach the next trophic level.
- An energy answer needs an energy unit such as kJ\mathrm{kJ}kJ, whereas an efficiency answer needs %\%%.
Self review
- Write the equation for energy transfer efficiency.
- Which trophic level supplies the denominator?
- Why is dry mass preferred when biomass is compared?
- How much energy is transferred from 4 000 kJ4\,000\,\mathrm{kJ}4000kJ at an efficiency of 12%12\%12%?
