Héctor T. Arita

The diversity of sites and the distribution of species are fundamental pieces in the analysis of biogeographic and macroecological questions. A link between these two variables is the correlation between the species diversity of sites and the mean range size of species occurring there. Alternatively, one could correlate the range sizes of species and the mean species diversity within those ranges. Here we show that both approaches are mirror images of the same patterns, reflecting fundamental mathematical and biological relationships. We develop a theory and analyze data for North American mammals to interpret range-diversity plots in which the species diversity of sites and the geographic range of species can be depicted simultaneously. We show that such plots contain much more information than traditional correlative approaches do, and we demonstrate that the positions of points in the plots are determined to a large extent by the average, minimum, and maximum values of range and diversity but that the dispersion of points depends on the association among species and the similitude among sites. These generalizations can be applied to biogeographic studies of diversity and distribution and in the identification of hotspots of diversity and endemism.

Several ecological and evolutionary hypotheses have been proposed to explain the latitudinal diversity gradient (LDG), but a general model for this conspicuous pattern remains elusive. Mid‐domain effect (MDE) models generate gradients of species diversity by randomly placing the geographic ranges of species in one‐ or two‐dimensional spaces, thus excluding both evolutionary processes and the effect of contemporary climate. Traditional MDE models are statistical and static because they determine the size of ranges either randomly or based on empirical frequency distributions. Here we present a simple dynamic null model for the LDG that simulates stochastic processes of range shifts, extinction and speciation. The model predicts higher species diversity and higher extinction and speciation rates in the tropics, and a strong influence of range movements in shaping the LDG. These null expectations should be taken into consideration in studies aimed at understanding the many factors that generate latitudinal diversity gradients.

Aim To assess the hypothesis that free‐living prokaryotes show a pattern of ‘no biogeography’ by examining the scaling of soil prokaryotic diversity and by comparing it with other groups’ biogeographical patterns.Location Two sites in the tropical deciduous forest of Chamela, Jalisco, on the western coast of Mexico.Methods We examined the diversity and distribution of soil prokaryotes in two 8 × 8 m quadrats divided in such manner that we could sample at four spatial scales. Restriction fragment length polymorphisms of16S rRNAgenes were used to define operational taxonomic units (OTUs) that we used in lieu of species to assess diversity.Results We found highly structured species assemblages that allowed us to reject multiple predictions of the hypothesis that soil bacteria show ‘no biogeography’. The frequency distribution of range size (measured as the occupancy of quadrats) of OTUs followed a hollow curve similar to that of vertebrates on continents. Assemblages showed high levels of beta diversity and a non‐random nested pattern of diversity. OTU diversity scaled with area followed a power function with slopesz = 0.42 and 0.47.Main conclusions We demonstrate a non‐ubiquitous dispersal for soil prokaryotes, which suggests a complex biogeography similar to that found for terrestrial vertebrates.