Previous Year Question

Energy flow in an ecosystem is Unidirectional. Energy received from the Sun is captured by Producers and then passes to Primary Consumers, Secondary Consumers and higher trophic levels.

However, the entire amount of energy is not transferred from one Trophic Level to the next. Organisms use a large proportion of available energy for respiration, movement, growth, reproduction and other metabolic activities, while considerable energy is dissipated into the environment as Heat.

Consequently, the energy available to the next trophic level is lower than that available at the preceding trophic level.

This is commonly explained by Lindeman's 10% Law. On average, only about 10% of the energy available at one trophic level is transferred to the next trophic level.

For example, if Producers contain 10,000 J of available energy, approximately:

Producers — 10,000 J
Primary Consumers — 1,000 J
Secondary Consumers — 100 J
Tertiary Consumers — 10 J

Because energy progressively decreases at successive trophic levels, the Pyramid of Energy is always upright, and the number of trophic levels in a normal food chain is also limited.

It should also be remembered that Energy is not recycled. Nutrients may cycle through an ecosystem, but Energy is progressively dissipated during each trophic transfer.