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

Abstract

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°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 ecosystem function but also on ecosystem functional persistence in the Anthropocene, with pressures at night being particularly concerning.

Cathemerality: a key temporal niche

“…In mammals and lizards (Vidan et al ., 2019), the only groups thus far investigated, time‐partitioning strategies are strongly spatially structured, driven by natural cycles of light and temperature (Bennie et al ., 2014; Cox et al ., 2023 a ). Contrary to nocturnal and diurnal species whose diversities are greatest in the tropics, cathemeral richness is highest in regions with high environmental seasonality or unpredictability, notably above 50° N, and in mountainous regions (South‐Western China, Bolivia; Fig.…”

“…Cathemeral species differ from diurnal and nocturnal ones in that they directly contribute towards ecosystem functioning across the diel cycle, and as such their relative functional contribution (after controlling for functional traits, phylogeny, etc.) may be greater than those of species restricted to a single temporal niche (Cox et al ., 2023 a ). Indeed, despite diurnality being more common than cathemerality in mammals ( N = 895 versus N = 527 species, respectively), outside of the higher latitudes both diel niches contribute similar levels to mammalian functional diversity (Cox et al ., 2023 a ).…”

Diel niche variation in mammalian declines in the Anthropocene

“…Nevertheless, the high proportion of cathemeral species that are threatened in the tropics suggests that individual diel flexibility is not sufficient to avoid population decline in the Anthropocene. Unlike nocturnal and diurnal species where species richness is highest in the tropics, cathemeral richness 31 and functional diversity 10 dominates in the upper latitudes, where in the main populations are not declining. However, the upper latitudes are warming more quickly than the rest of the planet and in the future cathemeral species located there may become increasingly vulnerable 32,33 .…”

“…The time of day at which an organism is physically active (its diel niche) may influence its level of exposure to many anthropogenic pressures and so its risk of population decline. First, a species' diel niche drives variation in functional traits [8][9][10] that themselves are important determinants of extinction risk 11,12 . Second, individuals are often more vulnerable to threats when active than when hidden during periods of rest.…”

Anthropogenic changes to the nighttime environment

“…Indeed, land-use change might a priori seem likely to influence diurnal and nocturnal organisms in similar ways, through the loss or creation of suitable habitat, subject to the relative diversity of day and night-active communities associated with different land uses (Cox et al. 2023 ). However, importantly, land-use change can also result in changes in diel climate regimes.…”