Diel niche variation in mammalian declines in the Anthropocene

Abstract

Biodiversity is being eroded worldwide. Many human pressures are most forcefully exerted or have greatest effect during a particular period of the day. Therefore when species are physically active (their diel niche) may influence their risk of population decline. We grouped 5032 terrestrial extant mammals by their dominant activity pattern (nocturnal, crepuscular, cathemeral and diurnal), and determine variation in population decline across diel niches. We find an increased risk of population decline in diurnal (52.1% of species), compared to nocturnal (40.1% of species), crepuscular (39.1% of species) and cathemeral (43.0% of species) species, associated with the larger proportion of diurnal mammals that are primates. Those species with declining populations whose activity predominantly coincides with that of humans (cathemeral, diurnal) face an increased number of anthropogenic threats than those principally active at night, with diurnal species more likely to be declining from harvesting. Across much of the land surface habitat loss is the predominant driver of population decline, however, harvesting is a greater threat to day-active species in sub-Saharan Africa and mainland tropical Asia, associated with declines in megafauna and arboreal foragers. Deepening understanding of diel variation in anthropogenic pressures and resulting population declines will help target conservation actions.

Cathemerality: a key temporal niche

“…Across taxa, when a species is active determines when it is primarily exposed to anthropogenic pressures and so often at greatest risk of mortality as well as sub‐lethal impacts. Many anthropogenic pressures not only vary in intensity spatially but also across the 24‐h cycle (Gaston et al ., 2023), with the negative impacts of climate warming (Bonebrake, Rezende & Bozinovic, 2020), harvesting (Norris et al ., 2010; Cox et al ., 2023 b ) and disturbance (Gaynor et al ., 2018) being greater during the daytime, while over much of the land surface the nighttime is warming faster than the daytime (Cox et al ., 2020) and artificial light at night (Gaston et al ., 2013) is a distinctly nocturnal phenomenon. Further, the impacts of pressures that act most forcibly during a particular period of the diel cycle may be synergistic or additive (e.g.…”

“…Following the advent of artificial light at night humans can now be considered facultatively cathemeral, however human activity is still concentrated during the daytime. Mammals active during the day are more likely to be declining from pressures such as harvesting (Cox et al ., 2023 b ), and it is likely that it is the activity of cathemeral mammals during the day that puts them at greater risk. As a consequence, many species are shifting an increased proportion of their activity to the nighttime to avoid human activity (Gaynor et al ., 2018).…”

“…Cathemerality is an important part of this continuum because it reflects species that are capable of marked levels of activity during both the day and night (Table 1); categorisations of activity patterns will remain practically important, given that adequate data for more continuous characterisations of temporal activity are likely to remain lacking for most species. Finally, mounting anthropogenic pressures such as global warming (Cox et al ., 2020), harvesting (Norris, Michalski & Peres, 2010; Cox, Gardner & Gaston, 2023 b ) and artificial light (Kyba et al ., 2017) are often not evenly dispersed across the 24‐h cycle (Gaston, Gardner & Cox, 2023), and cathemerality may provide a behavioural advantage allowing facultative species to time their activity to when pressures are reduced.…”

A model‐based hypothesis framework to define and estimate the diel niche via the ‘Diel.Niche’ R package

“…Studies of animal diel ecology and evolution provide inference to how animal activity is shaped within the 24‐h light–dark cycle. This leads to a more complete understanding of an animal's niche (Cox et al., 2021), as well as having implications for how to conserve wild animal populations (Cox et al., 2023; Rivera et al., 2022). As non‐invasive sampling and animal‐borne sensors continue to evolve and make it easier to continuously collect data on wild animals throughout the diel cycle, we expect increasing studies focused on how animals use diel time.…”

“…A species is commonly described as such based on the majority of their activity occurring in a single distinct period of light availability (daytime, night‐time or dawn dusk respectively). Less often considered, but an important complementing diel phenotype, is cathemerality (‘through the day’; Cox & Gaston, 2023b; Cox et al., 2023), which is an even amount of activity across the entire diel period, or a large amount of activity across multiple time periods (e.g. day and night; Tattersall, 2008).…”