Calculating Efficiency of Biomass Transfer

Ecological efficiency can be calculated for any transfer, as long as both values are in the same units (biomass or energy, per unit area per year).ecological efficiency (%) = (biomass or energy transferred to the higher trophic level ÷ biomass or energy in the lower trophic level) × 100Worked example 1: grass has 8000 kJ m⁻² yr⁻¹ of NPP. The rabbits eating it gain 800 kJ m⁻² yr⁻¹.
Efficiency = (800 ÷ 8000) × 100 = 10%.Worked example 2: a pond food chain.
Producers: 87 000 kJ m⁻² yr⁻¹
Primary consumers: 14 000 kJ m⁻² yr⁻¹
Secondary consumers: 1600 kJ m⁻² yr⁻¹
Producers to primary consumers: (14 000 ÷ 87 000) × 100 = 16.1%.
Primary to secondary consumers: (1600 ÷ 14 000) × 100 = 11.4%.Working backwards: if an efficiency is known, you can find a missing value. With 10% efficiency and 5000 kJ m⁻² yr⁻¹ in the primary consumers, the secondary consumers receive 5000 × 0.10 = 500 kJ m⁻² yr⁻¹.Tips
• Always divide the higher trophic level by the lower one; the answer must be below 100%.
• Check the units match before dividing.
• Round sensibly, usually to 3 significant figures or 1 decimal place.
• Efficiencies vary a lot between ecosystems, so a figure far from 10% is not necessarily wrong.