One of the most dramatic and important consequences of biological regulation in the community as a whole is the phenomenon generally known as ecological succession, but better described by the pharse, ecosystem development. When a cultivated field is abandoned in the eastern part of north s, america, for example, the forest that originally occupied the site returns only after a series of temporary communities have preceded it. The successive stages may be entirely different in structure and function from the forest that eventually develops on the site. In fact, we may think of such temporary communities as developmental stages analogous to the life history stages through which many organisms pass before reaching adulthood. Capacity for self development constitutes an important property that distinguishes systems with major biological components fro systems that are primarily physical. Models of ecological systems that fail to include short term developmental and longer term ecolutionary processes will fall short of the mark. In other words, when dealing with ecosystems we must include developmental parameters in addition to parameters derived from physical laws (such as laws of thermodynamics).

     To look at the situation in another way we can say that change with time in ecosystem structure and function results from an interaction of physical farces impinging from without (recall the discussion of the concept of ‘’forcing function’’) and developmental processes generated within the system. For convenience we may speak of a sequence of changes primarily due to the former as allogenic syccession (allo = outside, genic = relating to) and internally generated sequences as outogenic succession (auto = self – propelling ) or outogenic development. As we shall see, allogenic processes dominate some ecosystem and autogenic processes others. But first let us consider autogenic development as a uniqe feature of most ecosystems.