CIE IGCSE Biology: 复习笔记:19.1.2 Food Chains & Food Webs

CIE IGCSE Biology: 复习笔记:19.1.2 Food Chains & Food Webs

Food Chains & Food Webs



Food-chainA 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 Webs

  • 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-webA 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.