Energy enters an ecosystem primarily from the sun (for producers) and is transferred from one trophic level to the next (producers to consumers, etc.). At each transfer, a significant amount of energy is lost as heat, and this energy cannot be reused by organisms in lower trophic levels. Therefore, energy flow is unidirectional and non-cyclic. Nutrients, such as carbon, nitrogen, phosphorus, etc., are continuously cycled within the ecosystem, moving from the environment to organisms and back to the environment through processes like decomposition.