Food chains

A food chain shows a sequence of energy transfers between organisms as they feed. It begins with a producer, such as a green plant, which uses energy from sunlight to make its own food. The organisms that feed after it are consumers: they obtain energy by eating other organisms.

Each arrow points from the organism being eaten to the organism that eats it. The arrow therefore shows the direction in which energy is transferred, not the direction in which the consumer is looking.

Constructing and interpreting a food chain

To construct a food chain, place the producer first. Then place each consumer after the organism it eats, with an arrow pointing towards the consumer. For example:

The grass is the producer. The grasshopper eats the grass, so the first arrow points from the grass to the grasshopper. The frog eats the grasshopper, so the second arrow points from the grasshopper to the frog. Following the arrows shows the direction of energy transfer through the chain.

Predator–prey cycles

A predator is a consumer that kills and eats another animal. The animal eaten is its prey. In a stable community, predator and prey populations can rise and fall in linked cycles: changes in one population affect the other.

When prey become more abundant, more food is available for predators. This can allow more predators to survive and reproduce, so the predator population increases. As predator numbers rise, more prey may be eaten, contributing to a decline in the prey population.

With fewer prey available, predators have less food, so the predator population may then fall. Reduced predator numbers mean less predation, allowing the prey population to recover. As prey become more abundant again, the cycle can repeat.

Reading predator–prey population graphs

A population graph shows how population sizes change over time. To compare the two populations, read their values at the same points in time and identify where each population rises or falls. A peak is a high point before a population falls; a trough is a low point before it rises.

Predator population peaks typically occur after prey population peaks. The predator population takes time to respond to increased prey availability: predators need time to survive and reproduce before their numbers increase. Predator numbers can therefore continue rising even after prey numbers have begun to fall.

Photosynthetic production of biomass

Photosynthetic organisms use light energy to make glucose from carbon dioxide and water. Oxygen is released during this process, called photosynthesis.

Glucose provides material for making other organic molecules. For example, plants use glucose to make cellulose for cell walls and starch for storage. These substances contribute to the organism’s biomass: the mass of its living material.

Green plants and algae are producers because they make their own organic material rather than obtaining it by eating other organisms. When a consumer eats a producer, it obtains organic material that can be used to build its own biomass. Through feeding relationships, producers therefore support the biomass of other organisms.

Food chains and food webs

Organisms are connected by feeding relationships: when one organism eats another, food and energy are transferred between them.

Producers, usually green plants or algae, use sunlight to make organic material by photosynthesis. Consumers obtain food by eating other organisms. A primary consumer eats a producer; a secondary consumer eats a primary consumer. Higher-level consumers eat other consumers.

Food chains: one feeding pathway

A food chain shows one pathway along which food and energy are transferred. Each arrow points from the organism being eaten towards the organism that eats it.

In a small UK freshwater pond, algae can be eaten by water fleas, which can be eaten by small fish:

The algae are the producer, the water fleas are primary consumers, and the small fish are secondary consumers in this chain.

Food webs: connected feeding pathways

A food web links several food chains to show the feeding relationships within a community. It gives a fuller picture than a single chain because an organism may eat several species and may itself be eaten by more than one species.

In the pond, consumers other than water fleas can feed on algae, and small fish can eat prey other than water fleas. These connections mean that a change in one population can affect both the organisms it eats and the predators that eat it.

For example, if the water flea population falls, less algae may be eaten and small fish may have less food. The effects can extend beyond the algae–water flea–fish chain because other consumers and prey are connected to it in the food web.