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Patterns of variation in species richness are some of the oldest known ecological phenomena. Centuries of research into their causes have revealed surprisingly few general insights, one of which is that the factors that control richness depend on spatial scale. At large spatial scales, processes such as dispersal, speciation and extinction are though to be most important, while biological interactions can be important at small spatial scales. The abiotic environment affects all of these processes. For example, high temperatures can promote speciation, while small-scale environmental heterogeneity can slow competitive exclusion. I am interested in controls on species richness across all spatial scales, and in understanding how processes at one scale impact processes at other scales. Accordingly, my research has spanned a vast range of scales, from field sampling plots as small as 0.016 m2 to maps of richness patterns across the entire New World. I have employed a correspondingly diverse range of approaches, as appropriate to each spatial scale. Through a combination of modeling and experimental research, my research has helped to illuminate the factors that control variation in richness, and, critically, to reveal how these controls change at different spatial scales.