The food chain is a hierarchical network constituted by the succession of organisms that eat other organisms and may, in turn, be eaten themselves. The position an organism occupies in a food chain is indicated by its trophic level. Plants, algae, and phytoplankton constitute the lowest trophic level, whereas predators and carnivores constitute the highest trophic levels. Many humans consume food from different trophic levels, while those following vegetarian and vegan diets consume all or most of their food from the primary trophic level.