Food Chains and Food WebsCambridge IGCSE Biology: Revision notes
Section 1
What is a food chain?
A food chain shows the transfer of energy from one organism to the next, always beginning with a producer. Each arrow represents the direction energy flows — from what is eaten to what eats it.
Example: grass -> caterpillar -> blue tit -> sparrowhawk
To construct a food chain: identify the producer first, then arrange consumers in the order they feed on one another. To interpret one: read each arrow as 'energy flows to', not as 'is eaten by' in reverse.
Always start a food chain with a producer — the arrows point in the direction energy flows, not the direction of eating.
Section 2
From food chains to food webs
A food web is a network of interconnected food chains, showing how organisms in a community are linked through multiple feeding relationships — most organisms eat, and are eaten by, more than one species.
To interpret a food web:
- follow arrows to trace energy flow between any two organisms
- find all the predators of a species by looking at every arrow leading away from it
- find all the prey of a species by looking at every arrow leading into it
In a woodland food web, a blue tit might eat caterpillars, aphids and seeds, and itself be eaten by sparrowhawks — appearing in several different food chains at once.
Section 3
Roles within a food chain
- A producer makes its own organic nutrients, usually by photosynthesis, and always starts a food chain.
- Consumers feed on other organisms and are classified by their position:
| Consumer | Feeds on |
|---|---|
| Primary | Producers |
| Secondary | Primary consumers |
| Tertiary | Secondary consumers |
| Quaternary | Tertiary consumers |
- A herbivore eats only plants; a carnivore eats only animals.
- A decomposer gets its energy from dead or waste organic material — not usually shown as an end-point of a simple food chain, but essential for recycling nutrients back to producers.
Section 4
How do humans disrupt food chains and webs?
Food chains and food webs can be used to describe how human activity disrupts natural feeding relationships:
- Overharvesting a food species (e.g. overfishing) removes organisms faster than they can reproduce, reducing numbers at that trophic level and affecting every species connected to it in the web.
- Introducing foreign (non-native) species to a habitat can disrupt existing food webs — the new species may have no natural predators, may outcompete native species for the same food, or may prey on species with no natural defences against it.
Introducing cane toads to Australia disrupted native food webs because native predators had no defence against their toxins.
Must Know
- A food chain shows the transfer of energy from a producer through a series of consumers.
- A food web is a network of interconnected food chains.
- Consumers are ranked primary, secondary, tertiary, quaternary by what they feed on.
- Herbivores eat only plants; carnivores eat only animals; decomposers feed on dead/waste material.
- Overharvesting and introducing foreign species are two ways humans disrupt food chains and webs.
- Arrows in a food chain/web always point in the direction energy flows.
That's the notes covered.
Carry on to the next subtopic.