Biology

3.14 - Food Chains & Food Webs

7 min

Food chains

Energy up the chain

Predator-prey cycles

note 1 of 2

Food chains

A food chain shows the feeding relationships within a community - what eats what. It's a simplified single line of who-eats-whom.
Every food chain begins with a producer: a photosynthetic organism such as a green plant or an alga.
A producer uses light energy to make glucose by photosynthesis and builds this into its tissues - the material called biomass.

The order of a food chain

The producer is eaten by a primary consumer.
The primary consumer may be eaten by a secondary consumer.
The secondary consumer may be eaten by a tertiary consumer.
Some chains have more steps and some fewer.

What the arrows mean

The arrows always point from the organism that is eaten to the organism that eats it.
They represent the transfer of energy (and the biomass that carries it) up the chain, so an arrow points in the direction the energy flows, not the direction of feeding.
Each organism passes on only a fraction of what it received, so the amount of energy and biomass available falls at each step.

A food chain: grass, a grasshopper, a bird and a fox above named boxes joined by arrows, labelled producer, primary consumer, secondary consumer and tertiary consumer, with one arrow marked transfer of energy.

note 2 of 2

Food webs

In a real ecosystem, an organism rarely eats - or is eaten by - only one other kind of organism, so individual food chains overlap and interconnect.
A food web is this network of many interconnected food chains, showing lots of feeding relationships in a community at once.
Read a web the same way as a single chain: each arrow points from an organism to the one that eats it - but now with many branching links rather than one line.

A food web with green boxes for carrots, grass and wheat at the bottom, blue boxes for rabbits, mice and grasshoppers, and pink boxes for foxes, owls and birds, arrows marked transfer of energy.

Organisms in a web are interdependent

Because the organisms are linked, a change to one population has knock-on effects on the others.
If one organism is removed - for example a predator dies out or a prey species is lost - the populations connected to it rise or fall in response.
Those changes then pass along the web to organisms further away.
This is why the members of a community are described as interdependent: what happens to one can affect many.

key terms

TermDefinition

ProducerAn organism, usually a green plant or alga, that makes its own food by photosynthesis and begins a food chain.

BiomassThe material that living organisms are made of, or the mass of this material in an organism or at a trophic level (a feeding level in a food chain).

Primary consumerAn animal in a food chain that eats producers.

Secondary consumerAn animal in a food chain that eats primary consumers.

Food webA network of interconnected food chains showing multiple feeding relationships in a community.

end of notes

Key points to remember

Food chains

A food chain shows the feeding relationships within a community - what eats what. It's a simplified single line of who-eats-whom.

Every food chain begins with a producer: a photosynthetic organism such as a green plant or an alga.

A producer uses light energy to make glucose by photosynthesis and builds this into its tissues - the material called biomass.

The order of a food chain

The producer is eaten by a primary consumer.

The primary consumer may be eaten by a secondary consumer.

The secondary consumer may be eaten by a tertiary consumer.

Some chains have more steps and some fewer.

What the arrows mean

The arrows always point from the organism that is eaten to the organism that eats it.

They represent the transfer of energy (and the biomass that carries it) up the chain, so an arrow points in the direction the energy flows, not the direction of feeding.

Each organism passes on only a fraction of what it received, so the amount of energy and biomass available falls at each step.

Food webs

In a real ecosystem, an organism rarely eats - or is eaten by - only one other kind of organism, so individual food chains overlap and interconnect.

A food web is this network of many interconnected food chains, showing lots of feeding relationships in a community at once.

Read a web the same way as a single chain: each arrow points from an organism to the one that eats it - but now with many branching links rather than one line.