<P> A general energy flow scenario follows: </P> <Ul> <Li> Solar energy is fixed by the photoautotrophs, called primary producers, like green plants . Primary consumers absorb most of the stored energy in the plant through digestion, and transform it into the form of energy they need, such as adenosine triphosphate (ATP), through respiration . A part of the energy received by primary consumers, herbivores, is converted to body heat (an effect of respiration), which is radiated away and lost from the system . The loss of energy through body heat is far greater in warm - blooded animals, which must eat much more frequently than those that are cold - blooded . Energy loss also occurs in the expulsion of undigested food (egesta) by excretion or regurgitation . </Li> <Li> Secondary consumers, carnivores, then consume the primary consumers, although omnivores also consume primary producers . Energy that had been used by the primary consumers for growth and storage is thus absorbed into the secondary consumers through the process of digestion . As with primary consumers, secondary consumers convert this energy into a more suitable form (ATP) during respiration . Again, some energy is lost from the system, since energy which the primary consumers had used for respiration and regulation of body temperature cannot be utilized by the secondary consumers . </Li> <Li> Tertiary consumers, which may or may not be apex predators, then consume the secondary consumers, with some energy passed on and some lost, as with the lower levels of the food chain . </Li> <Li> A final link in the food chain are decomposers which break down the organic matter of the tertiary consumers (or whichever consumer is at the top of the chain) and release nutrients into the soil . They also break down plants, herbivores and carnivores that were not eaten by organisms higher on the food chain, as well as the undigested food that is excreted by herbivores and carnivores . Saprotrophic bacteria and fungi are decomposers, and play a pivotal role in the nitrogen and carbon cycles . </Li> </Ul> <Li> Solar energy is fixed by the photoautotrophs, called primary producers, like green plants . Primary consumers absorb most of the stored energy in the plant through digestion, and transform it into the form of energy they need, such as adenosine triphosphate (ATP), through respiration . A part of the energy received by primary consumers, herbivores, is converted to body heat (an effect of respiration), which is radiated away and lost from the system . The loss of energy through body heat is far greater in warm - blooded animals, which must eat much more frequently than those that are cold - blooded . Energy loss also occurs in the expulsion of undigested food (egesta) by excretion or regurgitation . </Li> <Li> Secondary consumers, carnivores, then consume the primary consumers, although omnivores also consume primary producers . Energy that had been used by the primary consumers for growth and storage is thus absorbed into the secondary consumers through the process of digestion . As with primary consumers, secondary consumers convert this energy into a more suitable form (ATP) during respiration . Again, some energy is lost from the system, since energy which the primary consumers had used for respiration and regulation of body temperature cannot be utilized by the secondary consumers . </Li>

Where does the flow of energy in an ecosystem begin