Global Patterns of Earwig Species Richness

One of the most investigated patterns in species diversity is the so-called latitudinal gradient, that is, a decrease in species richness from the equator to the poles. However, few studies investigated this pattern in insects at a global scale because of insufficient taxonomic and biogeographical information. Using estimates of earwig species richness at country level, their latitudinal diversity gradient was modelled globally and for the two hemispheres separately after correcting for differences in country areas. Separate analyses were also conducted for mainland and island countries. All analyses clearly indicated the existence of latitudinal gradients. The most plausible explanation for the observed pattern is the so-called tropical conservatism hypothesis, which postulates (1) a tropical origin of many extant clades, (2) a longer time for cladogenesis in tropical environments thanks to their environmental stability, and (3) a limited ability of historically tropical lineages to adapt to temperate climates. Earwigs probably evolved on Gondwana and secondarily colonized the Northern Hemisphere. This colonization was hampered by both geographical and climatic factors. The Himalayan orogenesis obstructed earwig dispersal into the Palearctic region. Additionally, earwig preferences for warm/hot and humid climates hampered the colonization of temperate regions. Pleistocene glaciation further contributed to reducing diversity at northern latitudes.