DECOMPOSERS — break down dead organisms at ALL trophic levels, returning nutrients to the soil.
Organisms can occupy more than one trophic level — e.g. an omnivore (like a human or bear) that eats both plants AND animals.
Pyramids of Biomass
A PYRAMID OF BIOMASS shows the TOTAL MASS of organisms at each trophic level (measured as DRY MASS — all water removed).
Pyramids of biomass ALWAYS narrow going upward — because biomass decreases at each trophic level.
Biomass is lost between trophic levels because:
Organisms use energy in RESPIRATION — released as heat and not passed on.
Not all of each organism is EATEN — bones, shells, hair, roots remain.
Some material is excreted as waste (faeces, urine) and not absorbed.
Pyramids of biomass are more accurate than pyramids of number because:
Pyramids of NUMBER can be inverted (e.g. one oak tree supports thousands of insects — the tree would be a tiny bar at the bottom).
Biomass accounts for the actual mass at each level — giving a true picture of energy available.
Efficiency of Biomass Transfer
The efficiency of energy/biomass transfer between trophic levels can be calculated:
EFFICIENCY (%) = (biomass at next trophic level ÷ biomass at previous trophic level) × 100
Typically only about 10% of biomass is transferred from one trophic level to the next — the rest is lost.
Example:
Grass (producers): 10,000 kg dry mass
Rabbits (primary consumers): 1,000 kg dry mass → efficiency = (1000 ÷ 10,000) × 100 = 10%
Foxes (secondary consumers): 100 kg dry mass → efficiency = (100 ÷ 1000) × 100 = 10%
WHY THIS MATTERS for food production:
Eating plants (lower trophic level) is more energy-efficient than eating meat (higher trophic level).
Feeding grain to cattle then eating the cattle wastes ~90% of the energy at each step.
A vegetarian diet can feed more people per unit of agricultural land than a meat-based diet.
⚠️ Common Mistake
Pyramids of BIOMASS always narrow upward — they are NEVER inverted. Pyramids of NUMBER can be inverted (e.g. one tree, thousands of insects). Students often mix these up. Remember: biomass is dry mass — always decreases up the pyramid because energy is lost at each level.
📐 Variables
B_nextBiomass at next trophic level (B_next) is measured in kg (kg)
B_prevBiomass at previous trophic level (B_prev) is measured in kg (kg)
Trophic levels 1–4: producer → primary → secondary → tertiary. Biomass decreases upward — lost as heat, waste, uneaten material. Efficiency = (next level ÷ previous level) × 100%. Typically ~10%. Eating lower trophic levels is more efficient.
🎯 Matching Activity — Match the Trophic Level Concept
Match each term to its correct description. — drag the symbols on the right to match the component names on the left.
Trophic level 1
Drop here
Pyramid of biomass
Drop here
10% efficiency
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Biomass lost as heat
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Pyramid of number
Drop here
Producers — plants and algae — fix energy by photosynthesis
Can be inverted — e.g. one oak tree supporting thousands of insects
Shows dry mass of organisms at each trophic level — always narrows upward
Typical proportion of biomass transferred from one trophic level to the next
Energy used in respiration — not passed to the next trophic level
⚽ FIFA Worked Examples
Biomass Transfer Efficiency
A meadow contains 5000 kg of grass. Rabbits feeding on the grass have a total mass of 400 kg. Calculate the efficiency of energy transfer.
F
Efficiency = (biomass at next level ÷ biomass at previous level) × 100
I
Efficiency = (400 ÷ 5000) × 100
F
Efficiency = 0.08 × 100
A
Efficiency = 8%
🎯 Test Yourself
Question 1 of 2
1. A food chain has 20,000 kJ of energy in producers. Only 200 kJ reaches tertiary consumers. What is the efficiency of each trophic level transfer, assuming equal efficiency at each step?
2. Why is a pyramid of number sometimes inverted but a pyramid of biomass never is?
⭐ How Well Do You Understand This Topic?
Be honest with yourself — this helps you know what to revise!
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