Limiting factors, competitive exclusion, and a more expansive view of species coexistence.

When I began graduate school in the 1980s, species interactions were viewed predominantly through the prism of resource competition. Species found together were largely understood to coexist because of how they segregated their use of the available spectrum of local resources. I entered this world of ideas wholeheartedly, but I was fascinated by why prey species are able to coexist with some predators and not others. Consequently, the central concept of my scientific discipline, Hutchinson’s niche, seemed to have no bearing on my ecological fascinations. Predation was understood largely as a species interaction that ameliorated the effects of resource competition among the prey, and so its main consequence was to prevent competitive exclusion (Paine 1966). In struggling to make sense of all this, I came across an article by Simon A. Levin in The American Naturalist (Levin 1970) that was a revelation to me. In it, Levin articulated many of my own misgivings about having resource competition as the sole basis for understanding species coexistence, and he offered a new vantage from which community organization could be understood. The article begins with a presentation of the ideas underlying Gause’s principle of competitive exclusion—the conceptual basis of Hutchinson’s definition of the niche (1957, “Concluding remarks,” Cold Spring Harbor Symposium of Quantitative Biology 22:415–427). The article is the epitome of scholarship, using many quotes from the relevant articles to build his argument. The motivation for the argument is Slobodkin’s 1961 analysis of Gause’s competitive exclusion principle and his evaluation of its test-