Variation in species richness: Towards a unification of hypotheses

The question, “why do areas vary in species richness?” has been important throughout the history of ecology. It is difficult to answer definitively because we have so many (at least 120) plausible hypotheses. This abundance of hypotheses has led to a number of attempts to classify them. Unfortunately, richness hypotheses often defy such categorization. Instead of placing species richness hypotheses into categories, I suggest an alternative approach: to treat species richness hypotheses as violations of the assumptions of Gause’s Competitive Exclusion Principle. This is a very similar approach to the pedagogy of population genetics: evolution occurs if and only if at least one assumption of the Hardy-Weinberg principle is violated. The classification of hypotheses advocated here treats interspecific competition as a central organizing concept in community theory. However, it does not treat competition as an organizing concept in communities: indeed, the relaxation or disruption of competition is considered to be the status quo in the majority of communities.