Food chains and food webs

Every living thing needs food. A food chain shows what eats what, and so how energy passes from one living thing to the next. It always starts with a plant, which makes its own food using light from the Sun. In real habitats, food chains join up into food webs, and a change to one animal or plant affects many others.

Key facts

  • In a food chain the arrows point from the food to the animal that eats it. They show the direction the energy goes.
  • Producers are green plants and algae: they make their own food using energy from sunlight. Every food chain starts with a producer.
  • Consumers eat other living things. Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
  • Herbivores eat only plants, carnivores eat only animals and omnivores eat both. A predator hunts and kills other animals, its prey.
  • A food web is many food chains joined together. The animal at the top, which nothing else eats, is the top (apex) predator.
  • If one population changes, others change too: if a predator dies out, its prey usually increases, and then the prey’s food decreases.
  • Decomposers, such as bacteria and fungi, break down dead plants and animals and return nutrients to the soil.

Worked examples

Example 1

In the food chain grass → rabbit → fox, name the producer, the primary consumer and the secondary consumer.

  1. Grass makes its own food using sunlight: it is the producer.
  2. The rabbit eats the grass: it is the primary consumer (a herbivore).
  3. The fox eats the rabbit: it is the secondary consumer (a carnivore and a predator).

Answer: producer: grass; primary consumer: rabbit; secondary consumer: fox

Example 2

In this food web, a disease kills most of the rabbits. What will happen to the grass and to the foxes?

  1. Rabbits eat grass. With fewer rabbits, less grass is eaten, so the grass increases.
  2. Foxes eat rabbits. With fewer rabbits, foxes have less food, so the number of foxes may fall.
  3. Foxes also eat mice, so they may eat more mice. Then there may be fewer mice for the owls.

Answer: the grass will increase (fewer rabbits eat it); the foxes will decrease (less food), or eat more mice

Example 3

What do the arrows in a food chain show?

  1. Energy from the Sun is stored in plants when they make food.
  2. When an animal eats, it takes in the energy stored in its food.
  3. So each arrow points from the thing that is eaten to the thing that eats it: the way the energy goes.

Answer: the direction energy moves: from the food to the animal that eats it

Example 4

Why are decomposers important in an ecosystem?

  1. When plants and animals die, or animals produce waste, decomposers such as bacteria and fungi feed on it.
  2. They break it down, releasing nutrients (mineral ions) into the soil and carbon dioxide into the air.
  3. Plants take up the nutrients to grow, so the materials are used again and again.

Answer: they break down dead organisms and waste, returning nutrients to the soil for plants