The core of systems ecology is the notion that interactions amongst organisms, and between organisms and their environment, can be quantified. The interactions themselves are invisible, but one way of quantifying them is to relate them to the flow of some measurable, necessary factor to and from the organisms. Energy transfers have frequently been used to map the interactions within an ecosystem: this approach is used in Chapters 9 and 10. Any element which is taken up by plants and animals can be used in a similar way. In this chapter we concentrate on the movement of two key elements – nitrogen and phosphorus – not as energy carriers, but as building blocks essential for the growth of any organism. It is useful to examine the pathways of nitrogen and phosphorus against the background of the carbon cycle, since for the major biological portion of their cycles they are found in organic form (that is, as part of carbon-based molecules). This property is shared by sulphur, which was not studied in detail at Nylsvley, but is included here for completeness. Some details of other elements, such as calcium, magnesium, potassium and sodium, are also included.

Among the key processes regulating the passage of N, P and S through the ecosystem is the process whereby they are liberated from their carbon bondage, and become available for uptake by organisms. The other essential elements (such as potassium, magnesium and calcium) are less intimately associated with carbon.