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

In short

A food chain shows the feeding relationships in a community. It always begins with a producer, and the arrows show the direction food and energy pass, from the food to the organism that eats it. A food web shows how the food chains in a habitat are linked, so one organism can feed at more than one trophic level.

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.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

Why is energy lost between trophic levels?

Energy is lost between trophic levels because not all of an organism is eaten, and some of what is eaten is not digested and is lost in faeces. Energy is also released by respiration and transferred to the surroundings as heat, used for movement and keeping warm, and lost in waste such as urine.

What is a pyramid of biomass?

A pyramid of biomass is a diagram whose bars show the mass of living material at each trophic level, with producers at the bottom. It almost always narrows at each higher level, because energy and biomass are lost at every step. Unlike a pyramid of number, it accounts for size, so one large oak tree gives a wide base.

Why are food chains short?

Food chains are short, usually four or five trophic levels, because only about 10% of the energy is transferred from one level to the next. The rest is lost as heat from respiration, in faeces and in waste. After a few levels, too little energy is left to support another population of organisms.

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