AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
The net production of consumers ( N), such as animals, can be calculated…
Full specification wording
Energy and ecosystems (A-level only): The net production of consumers ( N), such as animals, can be calculated as: N = I – (F + R) where I represents the chemical energy store in ingested food, F represents the chemical energy lost to the environment in faeces and urine and R represents the respiratory losses to the environment.
The equation N = I - (F + R) calculates net production for consumers.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 10 of 11
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The equation N = I - (F + R) calculates net production for consumers. 'I' is the chemical energy in ingested food. 'F' represents energy lost to the environment in faeces (undigested food) and urine (excretory products like urea or ammonia). 'R' is the energy lost to the environment as heat from respiration, which powers processes like muscle contraction. The remainder, 'N', is stored as new consumer biomass and is available to the next trophic level. Endotherms (mammals and birds) have a large 'R' to maintain body temperature, resulting in a smaller 'N' compared to ectotherms with the same energy intake. In ecological theory, this lower efficiency suggests endothermic predators might support fewer subsequent trophic levels.
What earns the marks
- State that heat lost from respiration (R) reduces the efficiency of energy transfer.
- Identify F as energy lost in faeces (undigested food) and excretion (accepting urine, urea, uric acid, or ammonia).
- Substitute correctly into N = I - (F + R) with working shown to calculate net production.
- Provide a correctly stated unit for net production, such as kJ m⁻² year⁻¹.
- Explain that endotherms have higher respiratory losses (R) than ectotherms to maintain body temperature, reducing their net production (N).
Common mistakes and how to put them right
Subtracting only F or only R.
Put it right: Subtract the sum of (F + R) from I.
Counting food that was never eaten within I.
Put it right: I represents ingested food only, excluding uneaten parts.
Writing 'energy used in respiration'.
Put it right: Specify 'energy lost as heat from respiration' to explain the loss to the environment.
Exam tips
- Write the equation out and label each term with the figure from the question before calculating anything.
- Questions about mammals and birds almost always want R, because they respire heavily to maintain body temperature.
Try the lesson
The start of the Energy and ecosystems (A-level only): The net production of consumers ( N), such as animals, can be calculated as: N = I – (F + R) where I represents the chemical energy store in ingested food, F represents the chemical energy lost to the environment in faeces and urine and R represents the respiratory losses to the environment. lesson, made from checked teaching notes for this specification point.
Have a go first
A consumer ingests food containing 500 kJ of chemical energy. It loses 150 kJ in faeces and urine and 200 kJ in respiration. Using N = I – (F + R), what is the net production of this consumer?
Listen
The equation N = I - (F + R) calculates net production for consumers. I' is the chemical energy in ingested food. F' represents energy lost to the environment in faeces (undigested food) and urine (excretory products like urea or ammonia). R' is the energy lost to the environment as heat from respiration, which powers processes like muscle contraction. The remainder, 'N', is stored as new consumer biomass and is available to the next trophic level. Endotherms (mammals and birds) have a large 'R' to maintain body temperature, resulting in a smaller 'N' compared to ectotherms with the same energy intake. In ecological theory, this lower efficiency suggests endothermic predators might support fewer subsequent trophic levels. A strong answer earns marks for these points. State that heat lost from respiration (R) reduces the efficiency of energy transfer. Identify F as energy lost in faeces (undigested food) and excretion (accepting urine, urea, uric acid, or ammonia). Substitute correctly into N = I - (F + R) with working shown to calculate net production. Provide a correctly stated unit for net production, such as kJ m⁻² year⁻¹. A common slip: Subtracting only F or only R. Instead, subtract the sum of (F + R) from I. Exam tip: Write the equation out and label each term with the figure from the question before calculating anything.
Key words
Say what each word means before you turn the card.
ingested energy I
Try answering aloud or in your head before turning the card.
faeces and urine F
Try answering aloud or in your head before turning the card.
respiratory loss R
Try answering aloud or in your head before turning the card.
See every definition
- ingested energy I
- The chemical energy store in ingested food.
- faeces and urine F
- The chemical energy lost to the environment in faeces and urine.
- respiratory loss R
- The respiratory losses to the environment.
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Test your understanding of Energy and ecosystems (A-level only): The net production of consumers ( N), such as animals, can be calculated as: N = I – (F + R) where I represents the chemical energy store in ingested food, F represents the chemical energy lost to the environment in faeces and urine and R represents the respiratory losses to the environment.
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