The feeding relationships between organisms in a community defines its trophic structure.
The transfer of food energy up the trophic levels start with primary producers such as plants and phytoplankton.
Primary consumers feed directly on primary producers, while secondary, tertiary, and quaternary consumers make up higher trophic levels of a food chain.
Most food chains are only a few links long.
Decomposers, which "feed" on organisms from all trophic levels, are not shown here.