Each arrow in a food web carries energy. How much? Ecologists measure it in kilocalories (kcal) per square meter per year, and the numbers shrink fast as you climb. You will use percents of a number.
Key idea
About 10% of the energy at one level is stored in the bodies of the next level. The other 90% is used to live (moving, keeping warm, growing) and leaves as heat, or is never eaten or digested. Energy flows one way; it does not cycle.
Why it works
A mouse eats a grasshopper, but most of the grasshopper's energy was already spent: it powered hopping and chewing and escaped as heat. Only the energy stored in the grasshopper's body can pass on. This is why a field supports millions of grass plants, thousands of grasshoppers and only a few hawks.
Worked example: Climb the pyramid
Grass in a field stores 20,000 kcal. With about 10% passed up each step, how much reaches the snakes in grass → grasshopper → mouse → snake?
- Grasshoppers: 10% of 20,000 = 2,000 kcal.
- Mice: 10% of 2,000 = 200 kcal.
- Snakes: 10% of 200 = 20 kcal.
Answer: 20 kcal.
Worked example: Work backward
Hawks at level 4 get 15 kcal. About how much energy did the producers store?
- Each step down multiplies by 10.
- Level 3: 150 kcal. Level 2: 1,500 kcal. Level 1: 15,000 kcal.
Answer: About 15,000 kcal.
Common mistake
Students think energy is recycled like matter, or that each level passes on all its energy. Matter (carbon, nitrogen) cycles. Energy enters as sunlight, flows up, and leaves as heat at every step. Check: the numbers in an energy pyramid must get smaller going up.
The 10% figure is a rule of thumb. Real transfers range from about 5% to 20%, depending on the organisms.