The energy that is passed on between the trophic levels is lost considerably. It has long been assumed that the efficiency between trophic levels is around 10%. This is true on average, but more detailed investigations have shown that the efficiency is between 2% and 24% (Pauly and Christensen 1995).

The losses are caused by

1. Large part of the plant biomass is not consumed, that it dies beforehand. It feeds the decomposers (bacteria, fungi and other detritivorous animals).
2. Not all of the biomass consumed is digestible and available for incorporation into the biomass. This part is also available to the detritus.
3. Part is lost in the form of respiratory heat.

Source:

Townsend, Colin R. 2009. ecology. 2nd ed. 2009. Berlin, Heidelberg: Springer Berlin Heidelberg: Imprint: Springer Spektrum. S. 439

Pauly, D. & Christensen, V. (1995) Primary production required to sustain global fisheries. Nature, 374, 255-257.