Efficiency and biomass calculationsSpec 9.8B
You may be asked to work out how efficiently energy or biomass is transferred from one trophic level to the next.
Energy transfer efficiency
The grass in a field contains 40 000 kJ of energy. The cattle that eat the grass gain 4 000 kJ in their biomass. Calculate the efficiency of energy transfer from grass to cattle.
- Efficiency = energy transferred ÷ energy available × 100
- = 4 000 ÷ 40 000 × 100
- = 10 %
Answer: 10 %
Percentage of biomass
A caterpillar eats 40 g of leaf (dry mass) and its own dry mass increases by 6 g. Calculate the percentage of the leaf biomass that became caterpillar biomass.
- Percentage = mass gained ÷ mass eaten × 100
- = 6 ÷ 40 × 100
- = 15 %
Answer: 15 %
The numbers are always less than 100 %, because energy and biomass are lost at every stage. The remainder is the part lost as heat, waste or uneaten material.
Quick check
Give two ways energy is lost between trophic levels.
Show answer
Any two of: respiration (heat), movement, waste (faeces, urine), parts not eaten.
Why are food chains short?
Show answer
Energy is lost at each level, so too little remains to support more trophic levels.
What shape is a pyramid of biomass?
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Wide at the bottom, narrower at each higher trophic level.
State the equation for efficiency of transfer.
Show answer
Energy transferred to next level ÷ energy available at previous level × 100.