Food Chains & Food Webs (Cambridge (CIE) IGCSE Biology)
Revision Note
Written by: Lára Marie McIvor
Reviewed by: Lucy Kirkham
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Food Chains & Food Webs
A 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 chain example table
Position in food chain | Organism | Explanation |
|---|---|---|
Producer | Grass | Makes its own food using energy from sunlight in photosynthesis |
Primary consumer | Mouse | Eats the producer |
Secondary consumer | Barn owl | Eats the primary consumer |
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
A 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
Examiner Tips and Tricks
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.
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