This phenomenon of conservation is explained by what we call the first law of thermodynamics—sometimes referred to as the law of energy conservation. The law State, energy cannot be created or destroyed. Energy can be described as the ability to do work, where work is the movement of the matter when a force is applied to it. A closed system is a system in which no matter or energy is allowed to enter or leave the first law of thermodynamics tells us that the amount of energy within an ecosystem is constant. It doesn't change. An open system, on the other hand, allows stuff to come in and go out. Since most systems are not closed, the laws energy conservation can be rephrased to say that the change in the internal energy of the system is equal to the difference between the amount of energy coming in, minus the amount of energy going out. In other words, the amount of energy in the system can change, but only if it comes from another system or goes to another system. At any rate, systems, whether they're open or closed, do not create or destroy energy. Rather energy can enter from one system and leave to another.