The four realms and their processes can be studied at global, through continental and regional, to local scales . The processes of the four realms interact in the life layer to produce the environments of the earth's surface. A helpful way to understand the processes that shape the environments of the life layer is to study them as systems . All flow systems have a power source . Open flow systems have inputs and outputs, while closed flow systems do not.
Cycles are closed matter flow systems.
In a cycle, a fixed amount of material is continually recirculated through a series of pathways or loops. Feedback in a flow system occurs when the flow in one pathway affects the flow in another. Positive feedback increases flow while negative feedback reduces it. Negative feedback in a flow system tends to produce stability or equilibrium . Time cycles are periodic changes in system flow rates that occur over periods ranging from hours to millions of years.