Gradients in regional diversity of freshwater taxa

Opinion about latitudinal diversity gradients in freshwater fauna has varied over past decades. Global data have been compiled for diversity of Ephemeroptera, Plecoptera, and Trichoptera at the site scale, but no assessment of global variability in regional diversity has been done for any taxon except fish. Global variation cannot be inferred from site-scale data because the relationship between site and regional diversity is not necessarily linear. We have assembled global data on regional diversity for 7 freshwater taxa. Here, we plot diversity against regional area, develop diversity-area regressions, and then plot the residuals against latitude to investigate latitudinal diversity gradients. The existence and directions of gradients vary substantially among taxa. For Ephemeroptera and Plecoptera, significant latitudinal gradients exist, and diversity is greater at higher latitudes. For Trichoptera and Caudata, latitudinal gradients are not apparent, although taxa within the Caudata show distinct patterns. For Odonata, Osteichthyes, and Anura, latitudinal gradients are highly significant, and diversity is greatest at low latitudes. A clear distinction between the life cycles of tropically diverse taxa and of other taxa (excluding fish) is a long terrestrial phase in the tropically diverse taxa (e.g., Odonata). We discuss reasons for the gradients and this contrast in ecological and evolutionary contexts (e.g., habitat complexity and cross-habitat adaptation).