Food chains and food webs

In a nutshell

Living things in a habitat are connected by their food. Food chains show the direction of energy transfer from organism to organism when they are eaten.

Food chains

Definition

A food chain shows the relationship between living things, starting with a producer followed by consumers who eat the organism to their left. All living things need food for energy. Food chains show how energy is transferred through living things when they are eaten.

In a food chain, organisms can be classified as producers or consumers. Consumers can be further identified as either predator, which eats other animals, or prey which are eaten by predators.

Producers

Producers are living things which produce their own food.

Example

Plants are producers that make their food through photosynthesis. Algae and phytoplankton also photosynthesise to make energy.

Consumers

Consumers are living things that can’t produce their own food so they eat producers or other consumers. Consumers can be described as either carnivores, herbivores or omnivores.

Examples

Lions are carnivores as they only eat meat.
Cows are herbivores as they only eat plants.
Foxes are omnivores as they eat meat and plants.

Consumers can be described as either primary, secondary, or tertiary based on their position in the food chain.

Name	Position	Diet
Primary consumer	First after producers.	Herbivores.
Secondary consumer	Second after producers.	Omnivores or carnivores
Tertiary consumer	Third after producers.	Usually carnivores but can be omnivores

In a food chain, relationships are shown using arrows. The blunt side of the arrow points towards the organism that is eaten and the sharp end points toward the organism that is eating.

Example

Science; Animals, including humans; KS2 Year 4; Food chains and food webs

1.	*Grass*	Producer
2.	*Grasshopper*	Primary consumer
3.	*Frog*	Secondary consumer
4.	*Snake*	Tertiary consumer

Food webs

Most predators have more than one type of prey and most prey have more than one type of predator. This means that food chains can be linked by shared predator and prey to form a food web.

Example

Science; Animals, including humans; KS2 Year 4; Food chains and food webs

There are two producers, the grass and the dandelion. The mouse is a primary consumer that eats the dandelion and the grass. The bird is a primary consumer as it eats the grass and a secondary consumer because it eats the mouse. The cat is a tertiary predator because it eats the mouse and bird.

Disruptions to a food web

There are a number of factors that can remove organisms from the food chain and cause disruption.

Example

Science; Animals, including humans; KS2 Year 4; Food chains and food webs

DISRUPTION	Effect
*Drought causes grass (producer) to die.*	There would be fewer grasshoppers as there is less grass available as food. This would cause a decrease in the number of frogs and birds as they rely on the grasshopper for food. There would also be fewer snakes due to the reduced number of frogs and birds.
*Pesticide usage causes grasshoppers (primary consumers) to die.*	There would be more grass because there are fewer grasshoppers to eat it. However, as there are less grasshoppers to eat, the number of frogs and birds will decrease. There would also be fewer snakes due to the reduced number of frogs and birds.
*Hunting causes birds (secondary consumers) to die.*	There would be fewer snakes because there are fewer birds for them to eat. There would also be fewer frogs because snakes eat them as an alternative food. However, there would be more grasshoppers because there are fewer birds as predators. There would also be less grass because there are more grasshoppers to eat it.