With a few exceptions, all ecosystems depend on solar energy as a primary energy source. Primary productivity is the result of energy captured by producers. Some of this energy is lost when producers, such as plants, use energy for respiration. Only a portion of the energy captured by producers is passed on to consumers. Consumers also lose energy due to respiration. Note that the energy flow through an ecosystem is one way; it is not recycled. All the energy taken in by producers is ultimately lost as heat through respiration. Autotrophs must continue to capture the sunís energy for ecosystems to persist.