Since Lithium has one valence electron that means it needs to lose it to form a stable structure. That means it's valence is equal to one. Oxygen has six atoms, in order to form a stable octet structure it needs to accept 2 electrons, therefore it's valence is 2.
Therefore two Lithium atoms bond with one oxygen atom to form Li2O
The disadvantages of the pyramid of productivity as a representation: 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.
It would be pictorial data cause historian's use historical pictures.