is an assemblage of units, objects, or parts united by some form of regular interaction. The concept of systems is essential to the physical and biological sciences; systems are composed of different interacting units, whether at the micro (cell, plant, animal) or the macro (natural ecosystem or global climate) level. Because these units interact, a change in one unit causes changes in the others. With their interacting parts, systems tend to respond in regularized ways; their actions have patterns