The relationship between any system and its surrounding environment can be described in one of three ways. In an isolated system, neither matter nor energy is exchanged with its environment. In a closed system, energy, but not matter, is exchanged. In an open system, both matter and energy are exchanged between the system and its surrounding environment. Any ecosystem is an example of an open system. Energy can enter the system in the form of sunlight, for example, and leave in the form of heat. Matter can enter the system in many ways. Rain falls upon it and leaves by evaporation or streamflow, or animals migrate into the system and leave in the form of decay products.