AQA A Level Biology

Revision Notes

5.3.6 Trophic Levels

Trophic Levels

  • Net primary production (NPP) is the chemical energy that is stored in the plant’s biomass (after respiratory losses have been taken into account)
    • This means NPP is the energy that is available to a plant for growth and reproduction
  • NPP also represents the energy that is available to organisms in higher trophic levels in the ecosystem
    • Trophic levels describe the position of an organism in a food chain, web or pyramid
    • Trophic levels can be represented by numbers or by the name of that trophic level e.g. plants and algae are in trophic level 1, otherwise known as producers
    • Further trophic levels are numbered subsequently according to how far the organism is along the food chain
  • Like plants and other producers, these consumers store chemical energy in their biomass

Trophic levels table

Trophic levels, downloadable IGCSE & GCSE Biology revision notes

Food chain showing trophic levels, downloadable IGCSE & GCSE Biology revision notes

Trophic levels for a simple food chain – the blue arrows show how the chemical energy originally produced by the primary producer (grass) is transferred to other organisms in the community

Food web showing trophic levels_2, downloadable AS & A Level Biology revision notes

Trophic levels for a simple food web – note that some organisms can belong to more than one trophic level (such as the squirrel, fox and eagle in this food web)

Decomposers

  • The net primary production (i.e. the chemical energy) of producers is also available to another group of organisms known as decomposers
  • The two main groups of decomposers are bacteria and fungi
  • Decomposers carry out a very important function in ecosystems – they break down dead plant and animal material (in the process gaining the chemical energy still stored in the dead matter)
  • They do this by:
    • Secreting digestive enzymes onto the surface of the dead organism
    • These enzymes break down the dead matter into small soluble food molecules
    • These molecules are then absorbed by the decomposers
  • This process of decomposition also helps to release organic nutrients back into the environment (eg. the soil) which are essential for the growth of plants and other producers

Exam Tip

Don’t forget – animals (known as consumers) can be at different levels within the same food web as they could be omnivores (animals that can eat both plants and animals) or could be predators that eat both primary, secondary and/or tertiary consumers!

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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