Global patterns of moss diversity: taxonomic and molecular inferences

A latitudinal gradient of decreasing species diversity from the tropics toward the poles is considered to be one of the few truly general patterns in biogeography and ecology (Rosenzweig, 1995). As a result, a voluminous body of literature has accumulated over the last 30 years, dedicated to explaining the basis of the latitudinal gradient (Rohde, 1992). Substantial progress has been made toward cataloging moss diversity through floristic treatments of many tropical areas. During this effort, bryologists have repeatedly noted that there appear to be high levels of species diversity in the tropics (e.g., Argent, 1979; Gradstein & Pocs, 1989; Frahm, 2003). Frahm (2003) indicated that about two-thirds of all bryophyte species (mosses, hornworts, and liverworts) occur in the tropics. To date, however, no formal analyses of global distribution patterns have been undertaken to test the hypothesis that mosses are in fact more diverse in tropical than higher latitude regions. Crosby & al. (2000) recognized 12,800 moss species in their worldwide checklist. The number of species increases as new taxa are described and decreases as species are synonymized during the course of revisionary work (Touw, 1974). Given these contradictory trends, the current number is probably a pretty good, albeit rough, estimate for the total number of mosses. The statistical analysis of broad-scale trends in biodiversity must be undertaken with care. In addition to complicating factors such as uneven sampling intensity, differing sample area sizes, and non-comparable habitats and levels of habitat heterogeneity among areas, species themselves are unequal in “biodiversity value”. Some, for example, are closely related to congeners while others are more isolated; some species contain high levels of genetic diversity while others are relatively invariant. For these reasons, some have advocated alternative metrics for quantifying biodiversity that do not rely on subjective and sometimes arbitrary definitions of what constitutes a species (Faith, 1992, 1994; Vane-Wright & al., 1991; Nixon & Wheeler, 1992; Humphries & al., 1995). The primary purpose of this study was to test the hypothesis that mosses are more diverse near the equator than at higher latitudes. Data were compiled from regional checklists to estimate species richness (“taxonomic diversity”), and these were compared to geographic patterns in molecular diversity derived from nucleotide sequences from the plastid rps4 gene and an intron in the mitochondrial nad5 gene. Two estimates of molecular diversity were compared to the species richness data: nucleotide diversity, which represents standing levels of biodiversity at the DNA level, and phylogenetic diversity (PD; Faith, 1992, 1994), which provides a phylogenetic (i.e., historical) estimate of molecular biodiversity.