Pyramids of Energy (Cambridge (CIE) IGCSE Biology): Revision Note

Pyramids of Energy: Extended

Extended Tier Only

• The transfer of stored energy in food chains is not always efficient, and energy is lost to the environment between each trophic level; reasons for this inefficiency include:

  • not all of the stored energy consumed by an organism goes into making new cells that can be eaten; energy may instead be transferred to:

    • excretion of metabolic waste, e.g. urine that gets removed from the organism

    • movement

    • heat

  • few organisms eat or digest an entire organism, so any energy stored in the uneaten or undigested parts does not get passed on, e.g.

    • energy stored in uneaten parts, such as roots of plants, or bones of animals does not enter the consumer's body at all

    • undigested waste (faeces) is removed from the body and provides food for decomposers

• Only around 10 % of the energy stored at each trophic level is available to the next trophic level

• This inefficient transfer of energy at each trophic level explains why food chains are rarely more than 5 organisms long

  • In order to survive a consumer would have to eat a huge number of prey organisms every day to get the amount of energy needed; this is unlikely to be possible

    

 Energy is lost at each trophic level

• The energy stored at each trophic level can be represented visually in a pyramid of energy

• Pyramids of energy can have advantages over pyramids of biomass because:

  • they highlight the energy lost at each trophic level

  • different organisms may not all store the same amount of energy in their biomass, so pyramids of energy allow for more accurate comparisons

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 for humans to be the herbivores rather than the carnivores

  • In reality, we often feed animals on plants that we cannot eat (e.g. grass) or that are too widely distributed for us to collect (e.g. algae in the ocean which form the food of fish we eat), so the situation is more complex than for the example shown above