At each transfer of energy within a food chain, approximately 90% of the chemical energy stored in organisms of the lower level are lost, and therefore unavailable to the higher level (second law of thermodynamics). Since the total amount of energy entering the food chain is fixed by photosynthetic activities of plants, more useable energy is available to organisms occupying lower positions in the food chain than to those at higher trophic level. Expressing this concept in simpler terms, one might say, for example:
Corn 10,000 unit of energy > Beef 1000 unit of energy > Human 100 unit of energy.
By moving man one step lower in the food chain, ten times more energy becomes directly available.
CMP Bio World 