If the grass contains 100,000 kcal of energy, the energy a fox will obtain from eating a bird is 1,000 kcal.
This is defined as a unit of energy and it decreases as it moves up the food web in the ecosystem.
The organisms only derive 10 percent of the energy when the feed on others. In this case, the grass has 100,000 kcal of energy which when eaten by the bird will be 10 percent which is 10,000 kcal. When the fox eats the bird it becomes 1,000 kcal.
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