Trophic efficiency refers to the way in which energy is passed around the levels of a food web. At the bottom level of a food chain, or its lowest "trophic level," autotrophs such as plants or cyanobacteria gather sunlight and synthesize proteins. Primary consumers representing the next-highest trophic level eat the producers and gain energy. Secondary consumers then eat the primaries and extract energy from them.
Some energy is lost at each level of a food chain. The degree to which energy is passed upward is the trophic efficiency of the system. As a rule, only around 10 percent of the energy present at a trophic level is eligible for transfer to a higher level through consumption. This inefficiency imposes restrictions on the growth of higher levels of an ecological pyramid. If an ecosystem's entire energy budget is derived from the production carried out on its lowest level, higher levels necessarily have less energy available to them and can support less biomass than levels below.
Producers have a finite amount of energy available to them. Approximately half of the captured energy is expended in growth and reproduction, with half of what remains being lost to the ecosystem due to producers dying without being consumed. Herbivores are also subject to this inefficiency, but with lower starting figures, and the same is even more true of predators and scavengers. At each level, the total energy budget for net production is only 10 percent of the previous level's budget.