Food Webs

Food Chains

• Energy is stored in organisms as chemical energy.
• When one organism eats another, this energy is passed on.
• A food chain is a diagram which shows the flow of energy from one organism to another as they eat one another.
• A trophic level is another name for 'energy level'
• Not all the energy is passed on in a food chain. Some is lost in other ways.
• Some is released in respiration to allow movement and keep the organism warm (if endothermic)
• Lots is egested (undigested food in faeces)
• There are fewer organisms as you go along a food chain because energy is lost at each stage.

Pyramids

• Pyramids of numbers show stages in a food chain.
• Each level on a pyramid of numbers is called a trophic level.
• Pyramids of biomass show the total dry mass of the organisms at each trophic level.
• Pyramids of numbers are not always pyramid shape whereas pyramids of biomass are.
• Data for pyramids of biomass must be obtained from dry mass, so the organisms have to be killed. Therefore only a small sample can be used which may not be representative.
• Pyramids of energy are produced by sampling and burning the organisms from each trophic level in a calorimeter, to determine how much heat energy is produced per gram.
• This is very time-consuming and very destructive to the environment.
• They only take a snapshot of an ecosystem at one moment in time.
• Pyramids of energy are always pyramid shaped because energy is lost at each level, so the size of each bar decreases at each successive trophic level.

Conclusion

• The amounts of biomass and energy contained in living things always gets less at each stage of a food chain, from producers onwards. Biomass is lost as waste products and used to release energy in respiration. This is used for movement and to control body temperature. Only a small amount is used for growth.