Energy pyramids
Passing it up the linePassing it up the line
Imagine passing 100 dollars down a line of friends, but each friend keeps 90 dollars and passes on only 10. The second friend gets 10 dollars, the third gets 1 dollar, and the fourth gets just 10 cents. Energy moves up a food chain the same way.
Worked example: grass captures 10,000 units of energy from sunlight. How much reaches a snake in the food chain grass, grasshopper, frog, snake?
Step 1: Grass is a producer, the first trophic level. It makes food from sunlight, so it holds the most energy.
Step 2: Only about 10 percent of the energy passes to the next level. The rest is used for living, like moving, growing and staying warm, and most of it leaves as heat.
Step 3: The grasshoppers get about 1,000 units, the frogs about 100 and the snakes only about 10.
Step 4: That is why an energy pyramid is wide at the bottom and narrow at the top, and why food chains rarely have more than four or five levels.
Each level up the pyramid gets about one tenth of the energy of the level below.
Try it: build an energy pyramid
Change how much energy the grass captures and how many levels the food chain has.
Only 10 of 10,000 units reach the snakes. That is why top predators are rare.
Key words
- Trophic level: a feeding step in a food chain, like producer or first consumer.
- Producer: a living thing that makes its own food, usually from sunlight.
- Consumer: a living thing that gets energy by eating other organisms.
- Energy pyramid: a model showing how much energy each trophic level holds.
Big mistake to avoid
Thinking energy gets recycled like matter. Energy flows one way through an ecosystem and leaves as heat, so it must keep arriving as new sunlight.
Cool science secret
The 10 percent rule is why a forest can feed thousands of deer but only a few wolves. Top predators need huge areas to find enough food.
Try it yourself
Producers in a pond hold 50,000 units of energy. About how much reaches the third trophic level?
Answer: about 500 units: 50,000 to 5,000 to 500.