Ecological pyramids of productivity are very useful when representing the production of biomass at each trophic level, it shows you the energy going through each level of the food chain. Units used to represent this data are grams per meter2 per year or calories per meter2 per year. You'll be able to appreaciate a standard ecological pyramid when energy at the lower trophic levels are high and as you arise the pyramid, energy decreases. In a situation like this, since lower trophic levels have more energy, this means they have more population, meaning that they are able to feed organisms on the upper levels.
One important thing to consider is that as energy is transferred from one trophic level to another, only 10% of it is used to build new biomass. Therefore in a pyramid of productivity each step will be 10% the size of the previous one (100, 10, 1, 0.1, 0.01, 0.001 etc.).

Noticeable advantages you'll be able to appreciate when using pyramids of productivity are:
- There are no inverted pyramids
- The input of solar energy can be added
- It takes account of the rate of production over a period of time
However here are some disadvantages as well which are:
- The rate of biomass production of an organism is required, which involves measuring growth and reproduction through time.
- There is still the difficulty of assigning the organisms to a specific trophic level. As well as the organism in the food chains there is the problem of assigning the decomposers and detritivores to a particular trophic level.

This youtube video, may help you understand Food Chains in a better way. Enjoy!