The latitudinal gradient of decreasing richness from tropical to extra- tropical areas is ecology’s longest recognized pattern. Nonetheless, notable exceptions to the general pattern exist, and it is well recognized that patterns may be dependent on characteristics of spatial scale and taxonomic hierarchy. We conducted an exten- sive survey of the literature and provide a synthetic assessment of the degree to which variation in patterns (positive linear, negative linear, modal, or nonsignificant) is a consequence of characteristics of scale (extent or focus) or taxon. In addition, we con- sidered latitudinal gradients with respect to generic and familial richness, as well as species evenness and diversity.We provide a classification of the over 30 hypotheses advanced to account for the latitudinal gradient, and we discuss seven hypotheses with most promise for advancing ecological, biogeographic, and evolutionary understand- ing. We conclude with a forward-looking synthesis and list of fertile areas for future research.
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