Thirty years of study to demonstrate, quantify, and explain the latitudinal gradient of species richness in mammals of the New World have produced two results; a surety that such a gradient exists and a lack of consensus as to what causes the inverse relationship between species richness and latitude. If the effects of continental area are removed, the latitudinal gradient remains strong and proves to be universal for the New World. This gradient in richness occurs not only at the species level, but also at a series of macrotaxonomic levels (generic, familial, and ordinal), which represents a distribution of ecological types, or bauplans. What causes these phenomena? I assert that none of the specific explanations that have been proposed (e.g., competition or spatial heterogeneity) alone can account for the latitudinal pattern. A universal explanation is likely to be more general; I propose that there is a shift in the impact of abiotic and biotic factors that limit species along the gradient from the poles to the equator. This shift, in turn, produces a change in factors that influence species richness from one that limits the number of species in polar regions to one that limits the physical or niche space of species in the tropics. Ultimately, these phenomena produce the latitudinal gradient (as well as the Rapoport's rule phenomena). Effects of abiotic and biotic factors are different for species and bauplans; species are impacted heavily by both, whereas bauplans are influenced much more strongly by abiotic factors.

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