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, such as plants. Some of this energy is lost when producers use sunlight 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. Autotrophs must continue to capture the sun's energy for ecosystems to persist. At present, humans use 40% of net primary productivity, leaving only 60% for non-agricultural organisms.