Flow of energy and the ten percent law
Energy flows in one direction only, from the sun to producers to consumers. Only about 10% of the energy passes on to each next trophic level.
How energy moves
The sun is the source of energy for almost every ecosystem. Green plants on land capture only about 1% of the sunlight that falls on their leaves, and store it as food energy. A herbivore that eats a plant uses much of this energy in its own life processes, such as moving and growing. Much is also lost as heat. Some of the food is not digested at all. On average, only about 10% of the energy at one level is passed on to the next. This is the ten percent law.
Draw it: energy along a food chain
The numbers are an example. Divide by 10 for each step up the chain.
Why energy flow is one-way and food chains are short
- Energy trapped by plants does not go back to the sun, and energy passed to herbivores does not return to plants. So the flow is unidirectional.
- At each step about 90% of the energy is used up or lost as heat, so very little is left after 3 or 4 levels.
- That is why food chains usually have only 3 or 4 trophic levels.
- There are many more producers than herbivores, and far fewer top carnivores. The number of organisms usually falls at each higher level.
Worked example: using the ten percent law
(a) In a grassland, the grass stores 20,000 J of energy. How much reaches the grasshopper, the frog and the snake in the chain grass → grasshopper → frog → snake? (b) An eagle at the fourth trophic level gets 6 J of energy. How much energy was stored by the producers?
- (a) Grasshopper (T2) = 10% of 20,000 J = 2,000 J.
- Frog (T3) = 10% of 2,000 J = 200 J.
- Snake (T4) = 10% of 200 J = 20 J.
- (b) Going down one level multiplies the energy by 10. The eagle is three steps above the producers.
- Producers = 6 J × 10 × 10 × 10 = 6,000 J.
(a) 2,000 J, 200 J and 20 J; (b) 6,000 J
A plate of rice and dal takes its energy straight from producers, just one step from the sun. Meat, eggs and milk come one level higher, so about 90% of the energy in the animals' feed has already been used up on the way.
3 marks: “State the ten percent law. Why is the flow of energy in an ecosystem said to be unidirectional?” Only about 10% of the energy at one trophic level is passed to the next. Energy taken in by herbivores does not go back to plants, and plants do not return it to the sun, so it moves one way.
1 marks: “If 50,000 J of energy is available at the producer level, how much will reach the secondary consumers?” 500 J: 10% of 50,000 J is 5,000 J for herbivores, and 10% of that is 500 J.
2 marks: “Why do food chains usually have only three or four trophic levels?” About 90% of the energy is lost at each level, so after three or four levels too little energy is left to support another level.
Ten percent travels, ninety percent is used or lost. Divide by 10 going up the chain; multiply by 10 going down.