Ecological Orientors: Pattern and Process of Succession in Relation to Ecological Orientors

Abstract

A theoretical analysis reveals that the concept of “succession” is generally applied both to community and ecosystem phenomena, which must be kept strictly apart. “Succession” is also applied to phenomena which we call “dynamics”, “change” and “development”. Additionally, any satisfactory theory on the temporal behavior of ecological systems must account for both succession and its logical counterpart, constancy. Furthermore, it is shown that the community level is difficult to distinguish from both the population and the ecosystem level. Important community characteristics which enable us to delimit the community from other observation levels can be drawn from the analysis of semiautonomous behavior of hierarchically ordered levels. A list of aspects relating to semi-autonomy distinguishing the community both from the ecosystem and the population is presented. The confusion relating to key concepts of spatial and temporal relations in ecological systems is outlined. Different terminologies are necessary to speak about observational and theoretical entities. “Pattern” and “process” are adequate terms of an observational terminology, while “structure” and “function” are terms of theoretical reasoning. Also, discreteness or continuity of phenomena must be described in different terms. The term “process” accounts for continuous phenomena, while the term “event” is necessary to describe discrete ones.

An analysis of special problems of succession theory shows that descriptive succession theory must take into account three classes of change: changes in species composition, changes in important functional characters, and changes in type. An exhaustive causal repertoire of successional mechanisms is listed. The same mechanisms lead to either change or constancy, or to past, present, and future states of a system. Special emphasis is put on the deduction of zero-force laws. An inquiry of the reduction of community phenomena to population phenomena shows that invasion, maintenance, and extinction require different models of causality. While a causal theory for invasion is elaborated, maintenance and extinction are not yet well understood. Predictive succession theory is also not well developed. In an overview 15 successional theories are listed which claim to explain temporal community phenomena. These approaches use a wide range of mechanisms. Two theories (resource ratio hypothesis, inhibition model) explain different kinds of temporal behavior and are therefore regarded as the most advanced ones. Two examples of succession are presented in order to test the explanatory power of successional theories. In both cases a narrative explanation of succession on the level of species composition is possible a posteriori. The two cases include a great deal of surprise, and both cases do not comprise any indication of maturation, retrogression, or orderly serial development.

An analysis of the relation of successional trends to ecologically based “goal functions” in conservation and planning shows that goals of a democratic society are diverse and pluralistic. This is also the case with respect to goals of nature conservation. By analyzing an empirical sequence of primary succession in the former coal mining area of East Germany we investigate the question whether environmental goals can directly be deduced from successional trends of ecological communities. All the distinguished and described stages have a certain value in the context of conservation. Each stage is characterized by special features of contradicting value concerning erosion potential, nutrient losses, water balance, diversity of different plant and animal groups. Thus no goal functions derived from successional trends can be adopted as goals for nature conservation.