Global variation in unique and redundant mammal functional diversity across the daily cycle

Aim: Organisms primarily influence ecosystems through their functional traits when they are physically active. Following the nocturnal bottleneck, the expansion of mammals into the daytime expanded mammalian functional diversity (FD), however there is also notable overlap in trait space across diel niches leading to redundant FD. We explore how the unique and redundant contribution of each diel niche varies spatially and in relation to natural variation in light and temperature. Location: Global. Taxon: Extant mammals. Methods: Based on five major functional traits (body mass, litter size, diet breadth, foraging strata, habitat breadth) for 5033 extant terrestrial mammals, we determine biogeographical variation in nocturnal, crepuscular, cathemeral and diurnal FD. We calculate the proportion of mammalian FD that is unique to each diel niche, and the proportion that is redundant across the daily cycle. Results: The diversification of mammals into the daytime resulted in the creation of new FD (28.5% of FD is not nocturnal; Lower Quartile 17.3%; Upper Quartile 38.2%). Most of this expansion occurred at higher latitudes where uniquely cathemeral FD dominates (> 55 degrees N, 41.1% of mammalian FD; Lower Quartile 33.3%, Upper Quartile 53.6%), associated with fewer hours of biologically useful moonlight and daylight. Where there are more hours of biologically useful daylight, unique diurnal FD is common. However, more than half of non-nocturnal FD is redundant, increasing ecosystem stability as different species carry out similar functions at different times of day, and suggesting that many mammals have not diversified far from their nocturnal ancestors. Main conclusions: Over much of the land surface more than a half of FD only occurs at night, underscoring the importance of nocturnal mammals for ecosystems. Understanding diel variation in FD not only informs on community structure and eco-system function but also on ecosystem functional persistence in the Anthropocene, with pressures at night being particularly concerning.