System Theory of the Ecological Niche

Abstract

The ecological niche is a confused concept, not easily distinguished from related ideas like habitat or environment. Attempts at formal niche theory have been only partially successful because they have not recognized that the niche is a systems concept and needs to be expressed in the formalism of mathematical general system theory. This is done here. The systems nature of Grinnell's and Elton's niches is demonstrated. Then a sketch of basic system theory is presented, defining input and output environments, and state transition, response, and modeling functions. The latter is used to distinguish a living from a nonliving system. Fundamental niche is defined as the set of all input time functions which allows system viability over some specified time interval; the formulation is a straightforward restriction of the standard definition for a system from system theory. Partial niches are examined in relation to the fundamental niche formulation, showing how the system theory approach lends clarity. The realized niche is also defined readily in relation to the fundamental niche definition. Then, a recent development, the extended niche, is introduced. Unlike previous niche concepts, the extended niche incorporates indirect causal factors, a major departure from both classical and contemporary concepts. The extended niche is consistent with other niche concepts, however, which are merely restrictions of it to direct input variables. The extended niche is identical to the recent input environs from environmental system theory. There is also an output environ in the latter theory, and both are illustrated for a simple compartment model. The environ concept is shown to serve the original purposes of the niche better than the niche itself. To illustrate the span of the new system theory of the niche and its extension to environs, adaptation, succession, competitive exclusion, coevolution, niche (and environ) overlap, output niche, and dominance and control are each discussed within the new framework. By theory, systems and their environments are a priori mutually consistent, obviating the need for a concept like adaptation as a mechanism to bring about such consistency. Succession is the process of generating environ and realized niche dynamics in ecosystem development, leading to more exclusive organizations in mature stages. Competitive exclusion cannot be settled with a call to system theory; nothing in theory either requires or precludes it. On the other hand, whole ecosystem coevolution appears to be mandated by transitive extrapolation of interaction consistency throughout systems as a basic system property. Environs, like niches, are amenable to overlap measures, and hence can be implemented through empirical studies. The output environ leads to the concept of output niche, and the distinction between input and output niches provides an additional perspective on differences between Grinnell's and Elton's original niche concepts. Finally, control (dominance) is measured by comparison of input and output environs, and a new property of nontransitive control in ecosystems is developed and illustrated. The system theory formulation of the niche concept is consistent with prior concepts, only more unified, utilitarian, and expandable in new directions.