Revision Notes

19.1 Food Chains & Webs

Food Chain & Web Definitions

Types of Variation table, IGCSE & GCSE Biology revision notes

Food Chains

Food chain, IGCSE & GCSE Biology revision notesA food chain with three trophic levels


  • A food chain shows the transfer of energy from one organism to the next, starting with a producer
  • The source of all energy in a food chain is light energy from the Sun
  • The arrows in a food chain show the transfer of energy from one trophic level to the next
  • Energy is transferred from one organism to another by ingestion (eating)
  • In the food chain above:

Food Chains table, IGCSE & GCSE Biology revision notes

Food Webs

  • A food web is a network of interconnected food chains
  • Food webs are more realistic ways of showing connections between organisms within an ecosystem as animals rarely exist on just one type of food source


Food web, IGCSE & GCSE Biology revision notesA food web shows the interdependence of organisms


  • Food webs give us a lot more information about the transfer of energy in an ecosystem
  • They also show interdependence – how the change in one population can affect others within the food web
  • For example, in the food web above, if the population of earthworms decreased:
    • The population of grass plants would increase as there are now fewer species feeding off them
    • The populations of frogs and mice would decrease significantly as earthworms are their only food source
    • The population of sparrows would decrease slightly as they eat earthworms but also have another food source to rely on (caterpillars)
  • Most of the changes in populations of animals and plants happen as a result of human impact – either by overharvesting of food species or by the introduction of foreign species to a habitat
  • Due to interdependence, these can have long-lasting knock-on effects to organisms throughout a food chain or web

Exam Tip

Questions about interdependence in food webs are common and easy to gain marks on if you answer them fully and correctly.

Do not say an animal or plant would ‘die out’ as this is unlikely to happen – stick to using the words decrease or increase. If in doubt, always give your reason for the increase or decrease in population.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.

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