Producer — plant/algae that makes food by photosynthesis (1st trophic level)
Primary consumer — herbivore that eats the producer (2nd trophic level)
Secondary consumer — eats the primary consumer (3rd trophic level)
Tertiary consumer — top predator (4th trophic level)
Decomposers — break down dead organisms at every trophic level
Biomass transfer — why it decreases
Only about 10% of biomass passes to the next trophic level
Energy is lost through: respiration, excretion (urine/faeces), heat, parts not eaten
This is why food chains rarely have more than 4–5 levels
Pyramid of biomass: widest at producer level, narrowest at top
Efficiency of biomass transfer
Efficiency (%) = (biomass at next level ÷ biomass at previous level) × 100
Eating lower in the food chain is more energy efficient
Feeding cattle on grain then eating beef wastes far more energy than eating grain directly
Exam questions — 3 questions · 8 marks total
⚡ This module includes extended questions (4–6 marks). These are where most marks are lost. Read the hint carefully. Write in linked sentences. The AI will identify exactly which mark scheme points you missed.
1 markBiomass lossAQA Jun 2024 P2 style
Why is biomass lost between trophic levels in a food chain?
A Organisms grow too slowly to transfer all their biomass
B Energy and biomass are lost through respiration, excretion and heat — not all is available for the next level
C Predators only eat part of their prey
D Producers store biomass underground where consumers cannot reach
3 marksExplain pyramid of biomassAQA 8461 style
Explain why a pyramid of biomass is always widest at the producer level and gets narrower at each trophic level. (3 marks)
Hint: Give the reasons WHY biomass decreases — name specific processes that cause the loss.
+30 XP
4 marksCalculate biomass efficiencyAQA 8461 style
A food chain: Grass (10,000 kg) → Rabbits (1,200 kg) → Fox (84 kg).
(a) Calculate the % efficiency of biomass transfer from grass to rabbits. (2 marks)
(b) Suggest why a farmer would make more economic sense keeping rabbits than foxes if they had 10,000 kg of grass. (2 marks)
Hint: (a) Use: efficiency = (biomass at next level ÷ biomass at previous level) × 100. (b) Think about trophic levels and energy loss at each step.