Where the energy goes
The main source of energy for the biosphere is the Sun. Green plants and algae, the autotrophs, absorb solar energy reaching the Earth and make glucose from water and carbon dioxide. Fixing solar energy to make food in this way is called photosynthesis.
The energy made by producers passes from organism to organism along the trophic levels. At each step only about 10% of the energy a level receives passes on to the next, and about 90% is lost to the environment as heat.
Energy dissipation
The loss of energy as it flows from one trophic level to the next is called energy dissipation. Figure 15.14 shows how a cow loses energy.
| Energy from food, 100% | Share |
|---|---|
| Used in respiration | 60% |
| Lost in excreta and dead matter | 30% |
| Stored in body tissues | 10% |
Figure to be added
Because a considerable part of the energy is lost as it passes through the trophic levels, short food chains are more efficient than long ones.
A long food chain (15.15)
Plant → grasshopper → frog → snake → hawk
Four transfers: the hawk gets about 0.01% of the plant's energy.
A short food chain (15.16)
Plant → elephant → leopard
Two transfers: the leopard gets about 1% of the plant's energy.
Why eating lower down feeds more people
The same arithmetic applies to people. A field of grain eaten directly passes about 10% of its energy to us. Fed to cattle first, only about 10% of the grain's energy becomes beef, and we get 10% of that — about 1%. So the land that feeds one person on meat could feed several on grain. The cow's figures show where the rest goes: most is burnt in respiration just to keep the animal alive and warm.
Short chains waste less
About 90% of the energy is lost at each step — a cow uses 60% in respiration and loses 30% in waste — so short food chains are more efficient.