The energy pyramid is a model in ecology that describes the flow of resources through an ecosystem from the lower trophic levels to higher trophic levels. An example of a full energy flow in an ecosystem would begin with the autotrophs that take energy from the sun. Herbivores then feed on the autotrophs and change the energy from the plant into energy that they can use. Carnivores subsequently feed on the herbivores and, finally, other carnivores prey on the carnivores. In each case, energy is passed on from one trophic level to the next trophic level and each time some energy is lost as heat into the environment. This is due to the fact that each organism must use some energy that they received from other organisms in order to survive. This flow of resources through the metabolic network defines what we call the energy pyramid. The efficiency with which energy or biomass is transferred from one trophic level to the next is called the ecological efficiency. The percentage of energy at one step of a food chain that is available for consumption by the next step is called food chain efficiency.