The energy pyramid: why food chains are short
Follow the energy in a food chain from grass to grasshoppers, frogs, snakes and eagles. Change how much energy the plants store and how much is passed on, and see why there are so few eagles.
Core concept
Energy flows one way along a food chain; at each step only about 10% is passed on and the rest is used for living and lost as heat, so each level has less energy and food chains rarely have more than four or five links
Topic: Ecological balance
Plants are producers: they store the Sun's energy: What is needed to make things move, heat up, light up or grow. All key words in food by photosynthesis: How green plants make their own food from light, water and carbon dioxide. All key words. Animals are consumers. In a food chain: Who eats whom: grass is eaten by a goat, and the goat by a tiger. All key words in a Nepali field, grasshoppers eat the grass, frogs eat the grasshoppers, snakes eat the frogs and eagles eat the snakes. But a grasshopper does not turn all the grass it eats into its own body: it uses most of the energy to move, keep alive and stay warm, and that energy leaves as heat. Only about 10% is stored in its body for the next animal. This is the ten percent law. Drawn as a pyramid, each level is much narrower than the one below. In this lab you change how much energy the plants store and what share is passed on at each step.
What you will learn
- Describe the flow of energy along a food chain.
- Apply the ten percent law to work out the energy at each level.
- Explain why food chains are short and top predators are few.
- Explain how losing the plants or the animals at one level upsets the balance.
Before you start
- Know what producers and consumers are.
Materials
- Pictures of grass, a grasshopper, a frog, a snake and an eagle
Equipment
- A calculator
Procedure
- Record the energy at each level when the plants store 10000 kJ and 10% is passed on.
- Record with 5% and with 20% passed on.
- Record with more plants (50000 kJ and 100000 kJ).
How this simulation is simplified
- Each animal eats only the level below it, and the share passed on is the same at every step.
- The energy is for one year, in a small field.