I would say energy myself, and this is because energy plays a key role in all of these. With trophic levels energy is being transferred amongst the levels. Food chain's is a map of how the energy is passed along the chain. Food web is a web of how energy is transferred, and this is why I would say energy is a keyword.
A food chain is a simple linear sequence of transfer of food and energy when one organism is consumed by another organism. A food chain includes tropic levels according to the feeding organisms. In an energy pyramid representing a food chain the energy from the bottom to the top of the trophic level will decrease. A food web is an interconnection of two or more food chains.
According to the above explanation this can be said that the food web includes all the other words.
