The 10% rule
Only a tenth passes on.
A big meal
A herbivore eats a lot of grass. It runs, digests its food and grows.
About how much of the food it eats becomes its own body matter, available to the next level?
What this lesson covers
The idea
A consumer loses much of the energy it eats as heat and uses some for digestion and work; on average only 10% of food eaten becomes body matter available to the next level, so energy diminishes at each trophic level.
A big meal
A herbivore eats a lot of grass. It runs, digests its food and grows.
About how much of the food it eats becomes its own body matter, available to the next level?
- About 10%
- About 90%
- About 50%
Pass it on
Pick an example amount. The amounts are examples. The 10% is the book's average.
A tenth each time
A consumer loses a great deal of the energy it eats as heat. Some goes into digestion and doing work. The rest goes to growth and reproduction. On average 10% of the food eaten is turned into its own body, and it becomes available to the next level. So the energy gets smaller at each trophic level.
A consumer loses much of the energy it eats as heat and uses some for digestion and work; on average only 10% of food eaten becomes body matter available to the next level, so energy diminishes at each trophic level.
Notes
On average only 10% of the energy passes to the next level, so energy diminishes at each level.
Check yourself
On average, how much of the food a consumer eats becomes its own body and is available to the next level?
A herbivore eats food containing 2000 J of energy (an example). Using the 10% average, how much becomes its own body matter, in J?
Answer: 200 J
10% of 2000 J is 2000 ÷ 10 = 200 J.
The producers hold 5000 J of energy (an example). Using 10% at each step, how much reaches the third trophic level, in J?
Answer: 50 J
Level 2 has 5000 ÷ 10 = 500 J, and level 3 has 500 ÷ 10 = 50 J.
As we go from one trophic level to the next, what happens to the energy available?
- 10% — correct. Yes!
- 90%. Much of the energy is lost as heat or used for digestion and work. On average only 10% reaches the next level.
- 50%. On average only 10% of the food eaten becomes body matter available to the next level.
- It stays the same. A great deal of energy is lost at each level, so less is available at the next.
- It gets smaller at each level — correct. Yes!
- It becomes larger. Only about 10% passes on, so the energy diminishes and does not grow.