A latitudinal gradient in the diel partitioning of species richness?

The writings of naturalists from two centuries past are brimming with accounts of the stark differences in the kinds and numbers of organisms encountered during the day and night and between the tropical and temperate zones. However, only recently have ecologists begun to systematically describe and explain the geographic variation in the diel activities of species on Earth. Examining data from 60 insect communities globally, I find that the partitioning of total species richness across three diel activity periods tracks the latitudinal gradient. In general, the proportions of diurnal and nocturnal species are highest among tropical communities and decline poleward, while cathemeral activity characterises over half of all species in communities at high latitudes. These latitudinal trends in diel partitioning at the community level broadly reflect recently documented patterns in the global distributions of vertebrate species using different activity periods. I outline six hypotheses that may account for a latitudinal gradient in the diel partitioning of species richness.