Net production of consumers
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of food chains and respiration. You can test your knowledge on these below.
What is the difference between a producer and a consumer?
Producers make their own food through photosynthesis. Consumers feed on other organisms for energy.
What is the role of respiration in animals?
Respiration releases energy from organic molecules for biological processes like muscle contraction, protein synthesis and thermoregulation.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains net production of consumers or read the full notes below. Once you've gone through the whole note, try out the practice questions!
What is Net Production in Consumers?
- Net production (N) in consumers is the energy available for growth and reproduction and therefore the amount of energy stored in biomass. It can also be thought of as the energy available to the next trophic level
- Consumers eat food (ingested energy), but not all of this energy is transferred usefully.
Equation
N=I-(F+R), where:
- N = net production
- I = chemical energy ingested
- F = energy lost in faeces and urine
- R = energy lost through respiration
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Why is Energy Lost?
Consumers lose energy at each stage due to:
- Not all parts of the organism are consumed (e.g. bones, shells or roots or bark)
- Egestion (indigestible material passes out as faeces)
- Excretion (urine and urea)
- Respiration (used for metabolic processes like muscle contraction or protein synthesis) and heat loss (especially in mammals and birds which are warm blooded)
Efficiency of Energy Transfer
To measure trophic efficiency between trophic levels:
This helps explain why:
Only ~10% of energy is passed on from one level to the next.
Most energy is lost, limiting energy available to support higher trophic levels.
Why Energy Loss Limits Trophic Levels
- Due to large energy losses at each trophic level (~90% loss), only 4–5 trophic levels can be supported.
- Insufficient energy is left to support top-level consumers in large numbers.
- This is why biomass and energy pyramids decrease sharply with each level.
Key Terms
- Net production (N): Energy available to the organism after respiratory and excretory losses.
- Ingested energy (I): Total energy consumed in food.
- Respiration (R): Energy used for metabolic processes.
- Faeces & urine (F): Waste losses that remove undigested/unused energy.
- Efficiency: How much energy is passed from one level to the next.
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Exam Tips
Use the terms egestion (faeces) and excretion (urine or urea) correctly.
Be prepared to explain why the efficiency of transfer of energy is so low.
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Explain why there is a limit to the number of trophic levels in a food chain. (3 marks)
- Energy is lost at each trophic level, mainly through faeces, urine, and respiration.
- Only around 10% of the energy is passed on to the next level.
- Less energy is available to support the biomass of higher trophic levels.
- Eventually, not enough energy remains to support another trophic level.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!