International A Level Biology / Environment and climate change

Energy transfer between trophic levels revision notes

Edexcel International A Level Biology (YBI11), chapter 16: Environment and climate change, specification point 5.10. Free lesson.

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Energy transfer between trophic levels is inefficient, so only a small amount becomes biomass available to the next level. This limits how many trophic levels a food chain can support.

How energy changes across trophic levels

A trophic level is a feeding level in a food chain. A pyramid of energy shows the energy stored at each level over a set area and time. In a grassland, energy falls sharply from producers to consumers. Producers (grasses): 20 800 kJ m⁻² year⁻¹. Primary consumers (herbivores): 1 920 kJ m⁻² year⁻¹. Secondary consumers (carnivores): 160 kJ m⁻² year⁻¹. The unit kJ m⁻² year⁻¹ means energy per square metre per year. Per square metre allows areas to be compared fairly, and per year shows energy flowing through each level over time.

Why little energy reaches the next level

Only energy stored as new biomass can be passed on when an organism is eaten. Much of the energy at a trophic level is lost in other ways, so often only around a tenth reaches the next level, sometimes much less. Not all is eaten: roots and dead leaves are not eaten by herbivores. In animals, bones, fur and teeth may not be eaten. Not all is digested: cellulose in plant cell walls is difficult to digest, so energy leaves the body in faeces. This is called egestion. Excretion: urea, made when spare amino acids are broken down, carries energy out in urine. Respiration:…

How to calculate transfer efficiency

Efficiency of transfer is the percentage of energy in one level that reaches the next. Put the energy in the level doing the eating on top, then divide by the energy in the level being eaten and multiply by 100. The higher trophic level is the one doing the eating, and has the smaller energy value. Use matching units for both values. An answer over 100% suggests the values have been put the wrong way round. For biomass transfers, use dry mass in g m⁻² because water content varies.

How to write the energy ratio

A ratio compares the energy in several trophic levels. To finish it in its simplest form, divide every value by the smallest value. This makes one value equal to 1. For the grassland values, divide 20 800 : 1 920 : 160 by 160. The ratio is 130 : 12 : 1, so the herbivores hold twelve times as much energy as the carnivores.

Why food chains are short

Because so little energy passes on at each transfer, little remains for higher trophic levels. A further level may not have enough energy to support a population. At an efficiency of 8.3%, a fourth trophic level would receive about 13 kJ m⁻² year⁻¹ from the 160 kJ m⁻² year⁻¹ in the secondary consumers. This is too little energy to support a population of tertiary consumers, so the grassland has no fourth trophic level.