What Is Ecological Efficiency and How Do You Calculate It?

Ecological efficiency is the percentage of energy received by one trophic level that is passed on to the next trophic level. Calculate it by dividing the energy transferred to the next level by the energy received by the previous level, then multiplying by $100$.

The Calculation

This concept belongs to Subtopic 2.2: Energy and biomass in ecosystems and is HL only in the 2026 ESS syllabus.

Use:

$$\text{Ecological efficiency} = \frac{\text{energy passed to the next trophic level}}{\text{energy received by the previous trophic level}} \times 100$$

For example, suppose producers receive and store $50{,}000\ \text{kJ m}^{-2}\text{ yr}^{-1}$, while primary consumers obtain $6{,}000\ \text{kJ m}^{-2}\text{ yr}^{-1}$:

$$\text{Ecological efficiency} = \frac{6{,}000}{50{,}000} \times 100 = 12\%$$

Therefore, $12\%$ of the energy received by the producer trophic level is passed to primary consumers. The units cancel because the numerator and denominator use the same units.

Most energy is not transferred because organisms use energy in cellular respiration, transforming some into heat. Energy and organic matter are also lost through movement, maintenance, uneaten material, excretion and egestion. These losses explain why ecological pyramids narrow and why food chains usually contain relatively few trophic levels.

A common misconception is that ecological efficiency is always $10\%$. It varies among ecosystems, trophic levels and species; $10\%$ is neither a fixed rule nor a true average. For example, energy transfer within the Serengeti grassland food web varies according to organisms’ diets, metabolism and the digestibility of consumed biomass.

Ecological efficiency also helps explain why diets lower in trophic levels are generally more sustainable: fewer energy-transfer stages are required to provide food for humans.

Exam Technique

In a calculation, write the formula, substitute values, show working and include the percentage sign. Check that both energy values describe adjacent trophic levels and have matching units. ESS questions provide any required equation, but you must apply and interpret it correctly.