In an ecosystem

The ecosystem is the interaction between living beings and their physical environment. The two most important characteristics of any ecosystem are energy flow and cycling of nutrients. 

Energy flow is always unidirectional. Energy flows progressively from one trophic level to another, and cannot revert. In most of the natural ecosystems, energy comes from the Sun. The energy of the Sun is trapped by the producers, and is passed on to successive trophic levels in the form of food.

Nutrients are never lost in the ecosystem. They are recycled indefinitely. The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling.