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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.

Here's a link to it: http://dropcanvas.com/8r3h5

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?