On the ecological meaning of the competitive exclusion principle in the context of an economic analogy

Abstract

This article examines the competitive exclusion principle, which states roughly that two sympatric species, having identical ecological niches, cannot co-exist in a stable equilibrium. The validity of the competitive exclusion principle is questioned and discussed by depicting an analogy between ecological and economic entities. It has already been pointed out that the concept of the niche should be defined independently of the principle, since otherwise the principle cannot be verified. Ecological literature has not yet provided adequate definitions of the concept of the niche; most definitions do not bother to distinguish sharply between functional and spatial characteristics of the ecosystem on the one hand, and species' characteristics on the other hand. In order to be useful in analyzing the competitive exclusion principle, the definition of the niche cannot be too narrow, since it is clear that no two organisms can concurrently occupy the same spot, or too broad, since it is well known that two sympatric species can co-exist in a stable equilibrium. Therefore, it becomes meaningful to consider the limits of co-existence and to examine those definitions of the niche and permissible competitive interactions for which the principle is valid. Roughly speaking, we suggest that investigating the validity of the competitive exclusion principle is similar to asking whether two different economic institutions, having different technologies, can co-exist stably in a given market structure. We construct a simple competitive system, in which markets are required to clear instantaneously, and the process of entry and exit of the individual components is governed by realized profits. It is shown that an unstable co-existence of different technologies results. It may be proposed that the quick adjustment process, to be carried out in the market and analogously in the ecosystem, is responsible for this result. Consequently, it is shown that a similar system, whose growth occurs only intermittently, is also unstable. This is interpreted to mean that reasons for instability should not be looked for in those aspects of the ecosystem which, in their tendency to affect ecological populations, exert forces external to the growth processitself. Stable co-existence is shown to occur when growth is intrinsically bounded. This is achieved by making production opportunities of each and every individual sensitive to ecosystem parameters. Thus, to enable stable co-existence it is not enough that population size will be regulated by relative densities. These densities should rather work in a very specific manner, which allow them to control growth and hence population size by intimately affecting the population equilibrium phenotypes.