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Food chains and food webs

Organisms and their environment · Energy flow, food chains and food webs · note 2 of 3

Spec B15.2.1, B15.2.2, B15.2.3, B15.2.10Download all Energy flow, food chains and food webs notes (PDF)Download all notes (PDF)5 pages

Food chains and food websSpec B15.2.1, B15.2.2, B15.2.3, B15.2.10

In short

A food chain shows the transfer of energy from one organism to the next, beginning with a producer, and a food web is a network of interconnected food chains. Each arrow points from the organism that is eaten to the one that eats it. Overharvesting and introducing foreign species change food webs and affect other populations.

Food chain
Shows the transfer of energy from one organism to the next, beginning with a producer.
Food web
A network of interconnected food chains.

In a food chain the arrows show the transfer of energy. Each arrow points from the organism that is eaten to the organism that eats it. The chain always begins with a producer.

A food chain: grass, then grasshopper, then frog, then snake, with arrows pointing towards the animal that eats each one, labelled producer, primary consumer, secondary consumer and tertiary consumer. (opens full size in a new tab)
A food chain. The arrows show the transfer of energy from each organism to the one that eats it.
Common mistake:

Arrows point from the food to the feeder. Grass → rabbit means the rabbit eats the grass and energy passes to the rabbit.

Food webs

Most organisms eat more than one type of food, and are eaten by more than one type of predator. A food web shows these links as a network of interconnected food chains.

To interpret a food web, trace each food chain from a producer, following the arrows. You can then say what an organism eats, what eats it, and what might happen if one population changes.

A food web: grass is eaten by rabbits and mice, a shrub by mice and grasshoppers; foxes eat rabbits and mice, frogs eat grasshoppers, and owls eat mice and frogs. (opens full size in a new tab)
A food web. Arrows point from the food to the feeder. The owl is a secondary consumer when it eats mice and a tertiary consumer when it eats frogs.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Human impact on food chains and food webs

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

Food chains and food webs can be used to describe the impact humans have on an ecosystem.

Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
  • Overharvesting of a food species, such as overfishing, reduces the population of that species. Predators that feed on it have less food, so their populations may fall. Species that the harvested species eats may increase in number.
  • Introducing foreign species to a habitat adds a new organism to the food web. It may eat native species, so their numbers fall, or compete with them for food, so those species have less food and their populations fall.
Supplement (what this means)Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean
Exam tip:

For 'what happens if' questions, work along the arrows. Say what happens to the organisms that eat it (less food) and to those it eats (fewer eaten, so more of them). Mention that effects can be indirect.

Written and checked against the Cambridge IGCSE Combined Science (0653) specification · Updated October 2026