Food Chains & Food Webs (Cambridge (CIE) O Level Biology): Revision Note
Food Chains & Food Webs
Food Chains Definitions Table
Term | Definition |
Producer | An organism that can produce its own food, using sunlight or other energy sources |
Herbivore | An animal that eats only plants |
Carnivore | An animal that eats other animals |
Primary consumer | An herbivore that eats producers |
Secondary consumer | A carnivore that eats primary consumers |
Tertiary consumer | A carnivore that eats secondary consumers |
Decomposer | An organism that feeds on dead or decaying matter by breaking it down |
Food Chain Diagram
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:
Position in the food chain | Organism | Explanation |
Producer | Grass | Makes its own food using energy from 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
Food Web Diagram
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 were to decrease:
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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