7.17 Biomass transfer and efficiency
Why only about 1% of light energy is used by producers and only about 10% of biomass passes to the next trophic level, the losses by egestion, excretion and respiration, efficiency calculations, and why food chains are short.
How much biomass is passed on?
Key terms
- Egestion: getting rid of undigested food as faeces.
- Excretion: getting rid of waste products of metabolism, such as urea and carbon dioxide.
- Efficiency of biomass transfer: the percentage of biomass at one trophic level that becomes biomass at the next.
Producers transfer only about 1% of the light energy that reaches them into biomass. Only about 10% of the biomass at each trophic level is transferred to the level above.
Why biomass is lost
- Not all the ingested material is absorbed: some is egested as faeces. Herbivores lose a lot this way, because they cannot digest cellulose easily.
- Some absorbed material is lost as waste: carbon dioxide and water in respiration, and water and urea in urine (excretion).
- Large amounts of glucose are used in respiration, releasing energy for movement and, in mammals and birds, for keeping warm. This glucose does not become new biomass.
- Some parts of organisms are not eaten at all, such as bones and roots.
Calculating efficiency
Example: grass has 2000 kg of biomass; the rabbits that eat it have 180 kg. Efficiency = 180 ÷ 2000 × 100 = 9%.
Why food chains are short
So much biomass is lost at each level that there is not enough to support many trophic levels. Most food chains have no more than four or five levels, and there are far fewer organisms at the top.
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