As was made clear in the previous chapter, the ecosystem is the basic functional unit with which we must deal since it includes both the organisms and the nonliving environment, each influencing the properties of the other necessary for maintenance of life as we have it on the earth. By considering the ecosystem first we are in a very real sense beginning our study of ecology with the gross anatomy and physiology of nature, much as a beginning medical student might begin his study with the gross anatomy and physiology of the human body. Once a clear image of overall structure and function has been obtained, component parts and functions, such as populations, the cyclic behavior of nitrogen, or plant productivity can be considered.