What does the energy in an energy pyramid represent?

Question

Suppose the energy at the second trophic level as given in an energy pyramid is 100 kcal per square metre. It could mean either of the two following things: 1. 100 kcal is the energy this primary consumer has stored in its flesh - that is, the energy left over from the amount consumed, after assimilation and respiration. This would be the net productivity of that consumer, or the energy available to the secondary consumer which feeds on this. 2. 100 kcal is the energy this primary consumer consumes. It will utilise a part of this in respiration, lose some amount in assimilation, so that a small amount is left over which is then stored in it as mass, or flesh - which would be its net productivity. Similarly, the first trophic level, the autotrophs' energy could represent either the total amount of food they make (the GPP), that is, the energy akin to the second case above, or the energy stored in them and available to the next trophic level (the NPP), like in the case 1 above. So which of these is correct - what do we represent as the energy of a trophic level in an energy pyramid? I think it should be the first case, the stored energies, since after all these are what you will measure as the energy of any trophic level - the *stored* energy at that level.

Answer 1

First of all, this is an interesting question, a fine question! It is important to get the definitions right; and many times a description glosses over them. In fact, I had to check my old textbook to be sure you were right.

And indeed you are correct; it is number 1. The energy listed for each trophic level is the stored energy. Likewise, for biomass, the dry wt. listed for each trophic level is that stored in each level; and for number of individuals, it is the number of them at each level, not the number of the prey they consume.

Source: Campbell, Neil A. _Biology_ 4th ed. pp. 1151 (energy in each trophic level for 1,000,000 J of sunlight) and 1152.