Pyramids of energy
A graph showing the amount of energy passing through each trophic level is a pyramid of energy. It is given as kilojoules per square metre per year (kJ m−2 yr−1).
Only about 10% of the energy of one trophic level is passed on to the level above it; about 90% is lost to the environment. So an energy pyramid always shows less energy at each higher level, and an energy pyramid is never inverted. It is also because of this loss of energy that food chains usually have fewer than five links.
- Tertiary consumers (hawk): 10 J
- Secondary consumers (frog): 100 J
- Primary consumers (insect): 1000 J
- Producers (plant): 10000 J
Figure 15.13 — a pyramid of energy.
Working with the 10% rule
Plants trap 10000 J.
Insects receive 10% of 10000 J = 1000 J.
Frogs receive 10% of 1000 J = 100 J.
The hawk receives 10% of 100 J = 10 J — a thousandth of what the plants trapped.
Why energy can never be inverted
Numbers and biomass are snapshots, taken at one moment, so a fast-breeding level can look small. Energy is a flow measured over a whole year, and no level can pass on more energy than it received — energy cannot be created. Each level receives only a tenth of the level below, so each bar must be smaller than the one beneath it. With 10 J left at the fourth level, there is simply too little to support a fifth.
10% up, 90% lost
A pyramid of energy (kJ m−2 yr−1) is always upright: only about 10% of each level's energy reaches the next.