AQA GCSE Biology Foundation (8461), Foundation tier · Ecology › Trophic levels in an ecosystem
Practise Transfer of biomass. 7 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Only a small fraction of the biomass at one trophic level passes to the next. You need to explain where the rest is lost, calculate the efficiency of biomass transfers, and explain how this limits the number of organisms at higher trophic levels. This subtopic is in GCSE Biology only, not Combined Science.
Key facts
Producers transfer about 1% of incident light energy for photosynthesis
About 10% of biomass passes to the next trophic level
Efficiency = biomass at higher level ÷ biomass at lower level × 100
Losses: parts not eaten; faeces (egestion); CO2 and water (respiration); urea and water (urine)
Much of the absorbed glucose is used in respiration
Less biomass higher up → fewer organisms and short food chains
Notes
How much is transferred
Of the light energy falling on producers (mostly plants and algae), only about 1% is transferred for photosynthesis.
On average, only about 10% of the biomass in one trophic level ends up in the level above.
Where the biomass goes
diagram
Not all of an organism is eaten, e.g. roots, bones or fur may be left.
Some of the food eaten is never absorbed and leaves the body as faeces (egestion).
Some of what is absorbed is lost as waste: carbon dioxide and water from respiration, and urea and water in urine (excretion).
Much of the absorbed glucose is respired to release energy, e.g. for movement and, in mammals and birds, to keep warm, so it never becomes new biomass.
Most biomass is lost (red) before it reaches the next trophic level.
Calculating efficiency
Efficiency of transfer (%) = (biomass transferred to the higher level ÷ biomass at the lower level) × 100.
Less biomass is available at each higher trophic level, so fewer organisms can be supported there.
This is why pyramids of biomass get narrower going up, and why food chains rarely have more than four or five trophic levels.
Eating plants rather than animals means fewer transfers, so less biomass is lost along the way.
How to answer each type of question
Explain how biomass is lost between trophic levels
3 to 4 marksGrade 5
Give separate ways: not all eaten; egested as faeces; lost in respiration (carbon dioxide and water); excreted (urea and water in urine).
Say that respiration uses much of the absorbed glucose, releasing energy that is not stored as biomass.
Example. Only a small percentage of the biomass of the grass a cow eats becomes biomass in the cow. Explain why. (3 marks)
Show the model answerHide the model answer
Some of the grass is not absorbed and is egested in faeces (1). Much of the absorbed glucose is used in respiration, and the carbon dioxide and water produced are lost (1). Some is lost as urea (and water) in urine (1). Also credit: energy released in respiration is used for movement or to keep warm.
Don’t lose marks
Dividing the lower level by the higher level, giving an answer over 100%.
Saying biomass is 'used up as energy'. Say glucose is used in respiration and the carbon dioxide and water produced are lost.
Confusing egestion (faeces: material that was never absorbed) with excretion (urine: waste made by the body, such as urea).
Using 1% for transfers between animals. The 1% is the light energy used by producers; about 10% applies to biomass between levels.
More tips
Memory tricks
'1 and 10': about 1% of the light energy is used by producers; about 10% of biomass is passed up each level.
Efficiency sense check: the answer must be less than 100%, and is usually around 10%.
Losses, FRUN: Faeces, Respiration, Urine, Not eaten.
Exam technique
Show the division and the × 100 in your working; each step can earn a mark.
Give each way biomass is lost as a separate point, with its process (egestion, respiration, excretion).
Round efficiencies sensibly, e.g. to 2 or 3 significant figures, unless the question tells you otherwise.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
State the typical transfer percentagesProducers transfer about 1% of the incident light energy for photosynthesis; about 10% of biomass passes to the next level.
Grade 5
Describe how biomass is lost between levelsParts not eaten, egestion in faeces, carbon dioxide and water from respiration, and urea and water in urine.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Animals also lose water and one other waste substance in urine. Name this waste substance.
Urea
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Only some of the biomass at one trophic level is transferred to the next trophic level.
(a) Approximately what percentage of the biomass at one trophic level is transferred to the level above? Tick (✓) one box.[1]
1%
10%
50%
90%
(b) Which two of the following are reasons why biomass is lost between trophic levels? Tick (✓) two boxes.[2]
Some ingested material is egested as faeces
Some material is lost as carbon dioxide and water in respiration
Organisms at higher trophic levels are larger
Producers absorb all the light that reaches them
(c) Animals also lose water and one other waste substance in urine. Name this waste substance.[1]
Show the answer and mark scheme
(a)Answer: 10%
(b)Answer: Some ingested material is egested as faeces; Some material is lost as carbon dioxide and water in respiration
(c)Answer: Urea
urea
Question 2Medium7 marks
Food chains rarely have more than four or five trophic levels.
(a) Explain why.[3]
(b) A lake contains 50 000 kg of algae (trophic level 1). Assume that 10% of the biomass is transferred between each trophic level. Calculate the biomass at trophic level 4.[2]
(c) Explain how the loss of biomass between trophic levels affects the number of organisms at each level.[2]
Show the answer and mark scheme
(a)
only about 10% of the biomass is transferred from one trophic level to the next
so the biomass decreases rapidly along the food chain
there is not enough biomass at the top to support another trophic level
(b)Answer: 50 kg
50 000 × 0.1 × 0.1 × 0.1
50 (kg)
(c)
there is less biomass at each higher trophic level
so fewer organisms can be supported at higher trophic levels (especially as they are often larger)