In ecosystems, the direction of energy flow is from the sun to producers (plants), then to primary consumers (herbivores), secondary consumers (carnivores), and so on through the food chain or web. This flow of energy is unidirectional, as energy is continually transferred from one organism to another and eventually lost as heat. The sun is the ultimate source of energy in ecosystems, as it provides the light and heat needed for photosynthesis to occur.
