Can assimilated energy be simply lost from a trophic level? I was reading Environmental Studies By B. S. Chauhan and there a diagram of a model of transfer of energy. ![enter image description here]( NU = Not used energy has been shown, energy that was neither stored or exported to the next level. I wonder what physiological process causes this loss of energy at all trophic levels?

In this diagram, since this is the only loss _between_ trophic levels (along with respiration and NA _within_ each trophic level), **NU** represents dead organic matter, that is not transferred along the food chain. In other words, this represents dead leaves that are not eaten by herbivores, hares that die of old age and not predation, and foxes that are killed by falling rocks. So **NU** is the amount of energy that goes directly into the decomposer community (another flow to decomposers is coming from matter that is excreted by herbivores and predators), since this diagram only includes primary producers, herbivores and predators.

**NA** , on the other hand, represents energy that is consumed by the next trophic level but not assimilated ( **N** ot **A** assimilated). So this represents faeces etc.