The idea of the transfer of energy allows us to consider the efficiency with which light energy is transferred to energy in producers, as well as the efficiency with which energy in the producers is then transferred from trophic level to trophic level.

The diagram shows the percentage of energy transferred to each trophic level in the ecosystem. We can look at this another way. For every 10 000 kJ of energy absorbed by the producer, 100 kJ are incorporated into its tissues, 10 kJ will eventually be incorporated into the tissues of primary consumers, and 1 kJ into the tissues of secondary consumers. The rest will be lost as heat. This is the basic pattern of energy transfer, but there are a number of points that are worth making about each stage. These points are often required in order to answer questions which involve the interpretation of information. 

Transfer of sunlight energy to energy in plant tissues
Not all the light energy falling on a plant is used to make new tissues: values have been rounded

• Some is of the wrong wavelength for photosynthesis. 

• Some fails to strike a chlorophyll molecule.

• Some will be reflected from the plant surface. 

• Other factors such as soil nutrients or carbon dioxide concentration may be in short supply. This will limit the rate of formation of new tissue
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  Crop plants often convert a higher percentage of the light energy which falls on them into energy in new tissue than plants growing in the wild do. This is because:  

• Crops are often irrigated and supplied with fertiliser. Shortage of water and mineral ions does not limit growth. 

• Crop plants have been bred for high productivity. They therefore have genes which ensure that they are efficient at converting light energy into energy in plant tissue. 

• Crops are often treated with pesticides. As a result, there is little damage to their leaves and they can photosynthesise more efficiently. 

Transfer of energy from one trophic level to another
In the diagram, the efficiency of energy transfer was given as 10%, but it is very variable. Some of the factors which result in this variation include the following:

• Carnivores are rather more efficient than herbivores at converting energy in food into energy in body tissues. This is partly because plants contain much more indigestible material than animals.

• Poikilothermic animals, such as fish, are much more efficient at transferring energy in food into energy in body tissues than homoiothermic animals, such as pigs and chickens. This is because a homoiothermic animal maintains a high body temperature. It does this by producing heat by metabolic processes. A lot of the food that a homoiotherm eats goes into heat production.

• A small mammal has a larger surface area compared with its volume than a large mammal has. It therefore loses more body heat. Since this heat comes from its metabolic processes, it follows that a lot more of the energy in the food that a small mammal eats is lost as heat and does not go into making new tissues.