AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to calculate the efficiency of biomass transfers…
Full specification wording
Transfer of biomass: Students should be able to calculate the efficiency of biomass transfers between trophic levels by percentages or fractions of mass.
Biomass transfer efficiency compares the biomass gained by one trophic level with the biomass available from the previous level.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 6 of 7
Study notes
From the checked teaching notes for this specification point.
The idea
Biomass transfer efficiency compares the biomass gained by one trophic level with the biomass available from the previous level. To calculate it, divide the biomass of the higher trophic level by the biomass of the lower trophic level, then multiply by 100 for a percentage. For example, if grass has 20 000 g of biomass and the rabbits eating it gain 2 000 g, efficiency = (2 000 ÷ 20 000) × 100 = 10%. As a fraction this is 2 000 ÷ 20 000 = 0.1 or 1/10. Efficiency is never 100% because energy is lost in respiration, movement, excretion and as heat, and not all biomass is eaten or digested. The same method applies to energy values if the question gives energy rather than mass.
What earns the marks
- Identify the biomass of the lower trophic level as the starting amount and the biomass of the higher trophic level as the transferred amount.
- Use the relationship: efficiency = (biomass transferred ÷ biomass available) × 100 for a percentage.
- Use the relationship: efficiency = biomass transferred ÷ biomass available for a fraction or decimal.
- Substitute values correctly and carry out the division before multiplying by 100.
- Interpret the result, for example 10% means one tenth of the biomass is passed on.
- Explain that efficiency is less than 100% because energy is lost in respiration, movement, excretion and heat, and some biomass is not eaten or digested.
Common mistakes and how to put them right
Dividing the lower trophic level biomass by the higher trophic level biomass.
Put it right by dividing the biomass transferred by the biomass available.
Forgetting to multiply by 100 when a percentage is required.
Put it right by converting the decimal to a percentage.
Mixing units, such as dividing grams by kilograms.
Put it right by converting all masses to the same unit before calculating.
Exam tips
- Write the equation before substituting numbers so the examiner can see your method.
- Check whether the question asks for a percentage or a fraction and give the answer in that form.
- Show the division and any conversion clearly, and include the correct unit or symbol, such as %.
Try the lesson
The start of the Transfer of biomass: Students should be able to calculate the efficiency of biomass transfers between trophic levels by percentages or fractions of mass. lesson, made from checked teaching notes for this specification point.
Have a go first
A caterpillar eats leaves containing 80 g of biomass and uses 12 g to build its own body mass. What is the efficiency of this transfer, written as a fraction in its simplest form?
Listen
Biomass transfer efficiency compares the biomass gained by one trophic level with the biomass available from the previous level. To calculate it, divide the biomass of the higher trophic level by the biomass of the lower trophic level, then multiply by 100 for a percentage. For example, if grass has 20 000 g of biomass and the rabbits eating it gain 2 000 g, efficiency = (2 000 ÷ 20 000) × 100 = 10%. As a fraction this is 2 000 ÷ 20 000 = 0.1 or 1/10. Efficiency is never 100% because energy is lost in respiration, movement, excretion and as heat, and not all biomass is eaten or digested. The same method applies to energy values if the question gives energy rather than mass. A strong answer earns marks for these points. Identify the biomass of the lower trophic level as the starting amount and the biomass of the higher trophic level as the transferred amount. Use the relationship: efficiency = (biomass transferred ÷ biomass available) × 100 for a percentage. Use the relationship: efficiency = biomass transferred ÷ biomass available for a fraction or decimal. Substitute values correctly and carry out the division before multiplying by 100. A common slip: Dividing the lower trophic level biomass by the higher trophic level biomass. Instead, dividing the biomass transferred by the biomass available. Exam tip: Write the equation before substituting numbers so the examiner can see your method.
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