Biology · Introductory biology · Worked example
Calculate trophic transfer efficiency
Net production in a grassland is 25,000 kJ/m² a year for producers, 2,100 for primary consumers, 180 for secondary consumers and 12 for tertiary consumers. Find the transfer efficiency between each pair of levels, and from producers to tertiary consumers.
Divide each level by the one below
Efficiency is the energy at the upper level as a percentage of the energy at the level below.
Find the overall efficiency
From producers to tertiary consumers the energy passes up three steps.
Check against the rough rule
Each step is within the usual 5–20% range and a little under 10%. The overall figure is the product of the three step efficiencies: 0.084 × 0.0857 × 0.0667 ≈ 0.00048.
Account for the rest
More than 90% of each level’s energy fails to reach the next: most leaves as heat from respiration, and the rest goes to decomposers as uneaten or undigested material.
Result
The efficiencies are 8.4%, 8.6% and 6.7%; only 0.048% of the producers’ energy reaches the tertiary consumers.
Your turn
Producers in a lake fix 40,000 kJ/m² a year. With 10% efficiency at each step, how much energy reaches the secondary consumers?
Show the answer and explanation
400 kJ/m² a year.
Secondary consumers are two steps above the producers: 40,000 × 0.1 × 0.1 = 400.
Keep exploring
The trophic pyramid tool opens with these four levels and lists each transfer percentage.
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