The energy flows through an ecosystem in a pyramid of energy. This pyramid has the autotrophs or producers at the bottom of the pyramid, with primary, secondary, tertiary consumers going in order to the top of the pyramid. The rule of 10's states that each trophic or feeding level has only 10 percent of the energy of the previous level. That is because producers capture sunlight directly and convert it to the chemical energy of glucose. A primary consumer or herbivore, eats a producer. It had to "spend" some of its energy in its search for food, to maintain its own metabolism and a great amount is lost as heat energy. Therefore, as each level progresses to a top consumer, the amount of energy available at each level is a fraction of the previous level.