Food Chains, Food Webs and Energy Transfer — AQA GCSE Biology

Food chains show how energy and biomass are transferred between organisms in an ecosystem.

Food chains

A food chain shows the feeding relationships in a community, always starting with a producer:

producer → primary consumer → secondary consumer → tertiary consumer

• Producers are usually green plants or algae that make their own food (biomass) by photosynthesis. They are the source of all biomass in the food chain.
• Primary consumers (herbivores) eat producers; secondary and tertiary consumers are carnivores.
• A predator is an animal that hunts and eats other animals (prey).

Food webs

A food web shows how many food chains in an ecosystem are linked, giving a more realistic picture of feeding relationships and interdependence.

Predator–prey cycles

In a stable community, the populations of predators and prey rise and fall in cycles:

• more prey → predators have more food → predator numbers rise,
• more predators → more prey eaten → prey numbers fall,
• fewer prey → predators have less food → predator numbers fall, and so on.

The predator peaks usually lag behind the prey peaks.

Exam tips

• Food chains start with a producer (makes biomass by photosynthesis).
• Use the correct terms: producer, primary/secondary/tertiary consumer, predator, prey.
• Explain predator–prey cycles in terms of food availability.
• Predator population changes lag behind prey changes.