Pyramids of Energy: Extended (CIE IGCSE Biology)

• In order for the energy to be passed on, it has to be consumed (eaten)
• However not all of the energy grass plants receive goes into making new cells that can be eaten
• The same goes for the energy the vole gets from the grass, and the energy the barn owl gets from the vole
• Only the energy that is made into new cells remains with the organism to be passed on
• Even then, some of this energy does not get consumed - for example, few organisms eat an entire organism, including roots of plants or bones of animals - but energy is still stored in these parts and so it does not get passed on
• The majority of the energy an organism receives gets ‘lost’ (or ‘used’) through:
  • making waste products eg (urine) that get removed from the organism
  • as movement
  • as heat (in mammals and birds that maintain a constant body temperature)
  • as undigested waste (faeces) that is removed from the body and provides food for decomposers

• This inefficient loss of energy at each trophic level explains why food chains are rarely more than 5 organisms long
• In the example above, something that preyed regularly on the barn owl would only get 0.1J of energy from each barn owl it ate
• In order to survive, it would have to:
  • eat a huge number of them every day to get the amount of energy it needed to survive (are there that many barn owls close together?)
  • not expend much energy itself hunting them (is this likely?)

 

Energy is lost at each trophic level for several reasons

A typical pyramid of energy would look like this:

Energy Transfer in a Human Food Chain

• Humans are omnivores, obtaining energy from both plants and animals, and this gives us a choice of what we eat
• These choices, however, have an impact on what we grow and how we use ecosystems
• Think of the following food chains both involving humans:

wheat  →  cow   →   human

wheat  →   human

• Given what we know about energy transfer in food chains, it is clear that if humans eat the wheat there is much more energy available to them than if they eat the cows that eat the wheat
• This is because energy is lost from the cows, so there is less available to pass on to humans
• Therefore, it is more energy efficient within a crop food chain for humans to be the herbivores rather than the carnivores
• In reality, we often feed animals on plants that we cannot eat (eg grass) or that are too widely distributed for us to collect (eg algae in the ocean which form the food of fish we eat)