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Key Words species richness, species diversity, macroecology, geographic ecology, biogeography
Abstract The latitudinal gradient of decreasing richness from tropical to extratropical 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 extensive 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 considered 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 understanding. We conclude with a forward-looking synthesis and list of fertile areas for future research.
INTRODUCTION
The oldest and one of the most fundamental patterns concerning life on earth is the increase in biological diversity from polar to equatorial regions (Brown & Lomolino 1998, Gaston 1996a, Rosenzweig 1995, Willig 2001). Indeed, a cogent statement of the gradient was articulated by von Humboldt early in the nineteenth century (Hawkins 2001). With notable exceptions, the pattern generally holds true, regardless of the biota's taxonomic affiliation (e.g., mammals, fishes, insects, and plants), geographic context (e.g., all continents and oceans), or time domain (e.g., Recent and 70 Mya). It was recognized clearly by the progenitors of evolutionary theory (Darwin 1862, Wallace 1878), and has been integral to the thinking of some of the most influential biologists of the past century (Dobzhansky 1950, Hutchinson 1959, MacArthur 1972). Indeed, a lustful preoccupation with high tropical biodiversity stimulated the conceptual development of much of community ecology. Similarly, serious concerns about the erosion of biodiversity at global, regional, and local scales have catalyzed a considerable body of research in conservation biology (Chown & Gaston 2000).
Broad recognition of the latitudinal gradient occurred in the 1800s, with an emphasis on quantification from the 1950s onward. The number of explanatory hypotheses for the gradient proliferated at the end of the twentieth century, with few attempts to falsify them in the...