Competitive Exclusion: The exclusion principle is recast in the context of a generalized scheme for interspecific...

Two species, A and B, interact when their niches alpha and beta intersect in space and time. The composite element AB may be either more, less, or equally effective in regulation. It is suggested that the first condition exists when the overlap between alpha and beta is small; this leads, on a large scale, to complex biocenotic phenomena in which components of a tremendously diversified biota may coexist. The coupling AB is likely, on the other hand, to be antagonistic to optimization of regulation by the whole community; in this case, it is postulated, one of the components (the less effective regulator acting alone) is purged. This is competitive exclusion, and it develops when alpha and beta intersect greatly. Between these extremes of cooperation and competition lies an area of niche intersection in which AB is not much better or worse than A or B alone. This case gives rise to passive coexistence of A and B. These are the extremes; even a cursory consideration of the permutational possibilities for niche intersection gives an immediate concept of the overwhelming variety of possible interspecific interactions and a glimpse of the dangers inherent in overemphasizing a single aspect. It is therefore advocated that the principle of competitive exclusion be regarded as only a small segment of a broad class of interspecific phenomena (9).