Green plants form the base of food chains. In that role, they are called primary producers by ecologists. Green plants use sunlight in photosynthesis, initially to split water into hydrogen and oxygen, a reaction that requires energy. “Reduced” hydrogen is incorporated into energy-containing substances that drive the biochemistry of plant growth and produce energy-storing substances such as sugars, which can also be linked together to form cellulose, a major plant structural material. Oxygen is a waste product of the photosynthetic process. Each of the food chain steps is called a trophic level. Green plants (trophic level one) become the food of herbivores (trophic level two), also known as primary consumers. Herbivores consume plants for their growth and reproduction, and themselves become the prey of carnivores (trophic level three), or secondary consumers. Lower carnivores become the prey of higher carnivores (trophic level four and higher) in the food chain, which may have more levels ending in “top” carnivores. Dead material, including whole organisms, fecal material, and sloughed-off fragments such as hair and skin, become detritus, which is energy-containing material available to decomposers—bacteria and fungi. The preponderance of detritus is plant remains, either whole plants or plant parts such as twigs, leaves, flowers, dead seeds, and roots. Bacteria and fungi are the principal organisms that use detritus as a food source.
As a generalization, only about 10% of the energy of each trophic level is transferred to the next level. About 90% is lost in the effort expended in finding food, digestion, growth, reproduction, and other work.
Inadequately shown in the figure is the reality that animal species are seldom confined to a specific trophic level. Herbivores, for example, are opportunistic carnivores.