Consumers convert only part of the energy and organic matter they acquire into new consumer biomass.

Secondary production is the rate at which heterotrophs produce new biomass through growth and reproduction.

Between two trophic levels, trophic transfer efficiency can be defined as

$$\mathrm{TTE}=\frac{\text{production at the higher trophic level}}{\text{production at the lower trophic level}}\times100%.$$

Worked example

If herbivores produce $180\ \mathrm{kJ,m^{-2},yr^{-1}}$ of new biomass from a producer level with NPP of $1500\ \mathrm{kJ,m^{-2},yr^{-1}}$, then

$$\mathrm{TTE}=\frac{180}{1500}\times100%=12%.$$

The energy that is not transferred into new herbivore biomass was not destroyed. Some producer material was never eaten; some ingested material was not assimilated, meaning absorbed and incorporated into the consumer's metabolism; and much assimilated energy was used in cellular respiration and ultimately dissipated as heat rather than stored as new biomass.

Transfer efficiency therefore depends on physiology, food quality, digestibility and ecological conditions. The often-quoted value of about 10% is only a rough rule of thumb, not a universal constant.

Because less production is normally available after each transfer, long food chains become increasingly difficult to support energetically. Trophic structure is therefore constrained by metabolic energy losses at every level.