Food chains and food webs

Ecology and the environment · Feeding relationships · note 2 of 4

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Food chains and food websSpec 4.7

Feeding relationships in a community can be shown by food chains. All food chains begin with a producer, usually a green plant or alga that makes glucose by photosynthesis. The arrows show the direction in which food, and so energy and substances, pass.

Feeding levels in a food chain
Name	What it does	Example
Producer	Makes glucose by photosynthesis	Grass
Primary consumer	Eats the producer	Grasshopper
Secondary consumer	Eats the primary consumer	Frog
Tertiary consumer	Eats the secondary consumer	Snake

Consumers that kill and eat other animals are predators, and those eaten are prey.

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. — The arrows show the direction the food goes: from the food to the animal that eats it.

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 how all the food chains in a habitat are linked together. In a food web an organism can be at more than one trophic level. For example, an owl that eats mice and also eats shrews that have eaten beetles is both a secondary and a tertiary consumer.

Because the food chains are linked, a change in the population of one organism can affect many others. If one prey species dies out, its predators may eat more of another prey species, whose numbers then fall.

Meadow food web: grass and clover (producers) are eaten by the rabbit and field mouse, and grass by the grasshopper; the frog eats grasshoppers, the fox eats rabbits and mice, and the owl eats mice (secondary consumer) and frogs (tertiary consumer); dashed arrows show dead organisms and waste from every trophic level going to decomposers (bacteria and fungi). — The owl feeds at two trophic levels: a secondary consumer when it eats mice and a tertiary consumer when it eats frogs.

Common mistake:

Arrows in a food chain or web point from the food to the animal that eats it. Grass → rabbit means the rabbit eats the grass.