Age, Area, and the Taxonomic Structure of Marine Faunas

The distribution of species among genera is markedly uneven, with most genera species-poor and few species-rich. This structure to taxonomic diversity may arise through differential rates of speciation and/or extinction. However, ecological factors such as competition and geographic range expansion/contraction also likely contribute. Here we use a global database of extant bivalves to characterize the taxonomic structure of marine biomes and provinces. Ecological models, developed to describe the distribution of individuals among species, reveal a general form to the distribution of species among genera in modern marine faunas. The Zipf-Mandelbrot distribution is the best-fit model for most biomes and provinces, describing with two parameters the right-skewed decline in species richness across genera (gamma), and the equitability of richness among the most diverse genera (beta). Tropical faunas exhibit right-skewed diversity distributions similar in form (although more extreme) to those of temperate and polar faunas, but differ markedly in their accumulation of diverse genera. Null models show that spatial variation in taxonomic structure is not explained solely by variation in species richness. Taxon age and habitat area may both contribute to variation in taxonomic structure with older genera expected to accumulate more species and larger areas expected to support greater diversity. Both age and area are significantly correlated with the parameters of provincial diversity distributions but these relationships are non-linear and significant only when large tropical provinces, containing young and diverse faunas, are included. Within provincial faunas a positive polygonal relationship exists between genus age and species richness, yet the most diverse genera are not always the oldest. These diversity distributions in combination with age-of-first-occurrence data suggest that the taxonomic structure of regional faunas is shaped by differential speciation potentials among clades, as constrained by spatial variation in diversity accommodation space.