School › Food chains › Ages 10–11
Food chains, webs and energy
From producers to apex predators: name the levels of a food chain, see how chains link into a food web, and find out why only about a tenth of the energy passes up each step.
Read
Every food chain starts with a producer: a plant, algae or phytoplankton that makes its own food, usually by photosynthesis.[1]
Primary consumers (herbivores) eat producers. Secondary consumers eat herbivores, and tertiary consumers eat secondary consumers. An apex predator at the top has no predators. Example: grass → grasshopper → rat → snake → hawk.[1]
Each food chain is just one path that food and energy can take. In an ecosystem many food chains overlap and link up: together they make a food web.[2]
A change in one population can affect many others. In kelp forests, sea otters eat sea urchins, and sea urchins graze on kelp. If the otters decline, the urchins can destroy the kelp.[2]
Energy is passed up a food chain, but on average only about 10% reaches the next level. The rest is used for living, lost as heat, or left in waste and in parts that are not eaten. That is why most ecosystems have at most about five levels.[3]
Detritivores such as vultures and dung beetles eat dead remains and waste. Decomposers such as fungi and bacteria break them down and return nutrients to the soil for producers.[1]
Try it: The food web string game
You need
- Cards with the names or pictures of: grass, tree leaves, caterpillar, grasshopper, rabbit, sparrow, rat, snake, fox, hawk
- A ball of wool or string
- Calculator
Safety
Pull the wool gently and keep it below shoulder height. Clear bags and chairs from the circle so nobody trips.
What to do
- Give each pupil a card and stand in a circle. Pupils with producers stand together.
- The teacher starts with grass. Pass the wool from the grass to an animal that eats grass; keep hold of your part of the wool.
- Keep passing the wool from food to eater until every card is connected (some cards will hold several strands).
- Now the 'rabbit' gently pulls its strand. Who feels it? These are the living things that would be affected if rabbits disappeared.
- Energy maths: if the grass holds 10,000 units of energy, how much reaches the rabbit (10%)? How much reaches the fox?
- Discuss: why can a food web support far fewer foxes than rabbits?
What happens, and why?
The wool makes a food web: many chains linked together. Pulling one strand shows that a change to one population affects others, like the sea otters, urchins and kelp. With the 10% rule, 10,000 units in the grass give about 1,000 to the rabbits and about 100 to the foxes, so there is energy for far fewer animals at the top.[2][3]
Play: Which level?
Is this living thing a producer, a herbivore, a meat-eating predator or a detritivore/decomposer?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16