Revisiting Rapoport’s Rule: The History of an Idea and Alternative Explanations for Latitudinal Variation in Geographic Range Size

In 1989, George Stevens described a frequently observed positive relationship between range size and latitude, which he dubbed “Rapoport’s Rule.” To varying levels of success, Stevens linked range size, latitude, species richness, climatic variation, and mass effects. The links between these phenomena appear to be tenuous, but there has been unfettered interest in testing for statistical relationships between range size and latitude. Positive relationships between range size and latitude have been repeatedly demonstrated in extant Northern Hemisphere temperate taxa and rarely for marine or Australian organisms. Limitations of methods for documenting range size–latitude correlations are discussed; successful combination of trait data with geographic distributions will engender greater support for different causal hypotheses of range size variation. Because large poleward geographic distributions drive positive correlations between range size and latitude, the historical influence of glacial advance and retreat appears to be important in shaping latitudinal variation in range size. The most compelling tests to identify the underlying processes linking latitude to range size will evaluate multiple competing hypotheses, examine historical explanations, and reject null models. The variability in the appearance of Rapoport effects should facilitate comparative analyses that identify causal explanation of what drives latitudinal gradients in range size.