Biologically meaningful moonlight measures and their application in ecological research

Abstract

Light availability is one of the key drivers of animal activity, and moonlight is the brightest source of natural light at night. Moon phase is commonly used but, while convenient, it can be a poor proxy for lunar illumination on the ground. While the moon phase remains effectively constant within a night, actual moonlight intensity is affected by multiple factors such as disc brightness, position of the moon, distance to the moon, angle of incidence, and cloud cover. A moonlight illumination model is presented for any given time and location, which is significantly better at predicting lunar illumination than moon phase. The model explains up to 92.2% of the variation in illumination levels with a residual standard error of 1.4%, compared to 60% explained by moon phase with a residual standard error of 22.6%. Importantly, the model not only predicts changes in mean illumination between nights but also within each night, providing greater temporal resolution of illumination estimates. An R package moonlit facilitating moonlight illumination modelling is also presented. Using a case study, it is shown that modelled moonlight intensity can be a better predictor of animal activity than moon phase. More importantly, complex patterns of activity are shown where animals focus their activity around certain illumination levels. This relationship could not be identified using moon phase alone. The model can be universally applied to a wide range of ecological and behavioural research, including existing datasets, allowing a better understanding of lunar illumination as an ecological resource. Significance statement Moon phase is often used to represent lunar illumination as an environmental niche, but it is a poor proxy for actual moonlight intensity on the ground. A model is therefore proposed to estimate lunar illumination for any given place and time. The model is shown to provide a significantly better prediction of empirically measured lunar illumination than moon phase. Importantly, it also has much higher temporal resolutions, allowing to not only detect selectiveness for light levels between nights but also within each night, which is not achievable with moon phase alone. This offers unprecedented opportunities to study complex activity patterns of nocturnal species using any time-stamped data (GPS trackers, camera traps, song meters, etc.). It can also be applied to historical datasets, as well as facilitate future research planning in a wide range of ecological and behavioural studies.

Small tube-nosed seabirds fledge on the full moon and throughout the lunar cycle

“…Average illuminated per cent of the moon (AIPM, representing moon phase) on the night of fledging and an incident moon illumination index (IMII) at the time of fledging (i.e. the final read at the burrow) were calculated by the package moonlit (see electronic supplementary material for details) [24]. AIPM at peak fledge date was plotted to assess consistency among years.…”

An anthropogenic landscape reduces the influence of climate conditions and moonlight on carnivore activity

“…Cozzi et al 2012 ; Peeva et al 2022 ). We used an alternative approach to obtain moonlight intensity on the ground based on the position of the moon, the brightness of the moon’s face, and several physical properties of light propagation, which is the proper measure of the amount of moonlight of real ecological importance (Śmielak 2023 ). Martens detect prey visually, thus increased moonlight levels may help them hunt, as is the case for other nocturnal mammals (Prugh and Golden 2014 ).…”

“…In most studies, the influence of moonlight on mammals’ activity is analysed by the lunar cycle (Cozzi et al 2012 ; Peeva et al 2022 ; Prugh and Golden 2014 ). This approach, however, does not take the amount of on-the-ground illumination throughout each night into account (Śmielak 2023 ). In our study, we used a new approach that combines the position of the moon in the sky, the brightness of the moon’s face and several physical properties of light propagation to count predicted moonlight illumination, relative to an ‘average’ full moon, for a given place and time.…”

“…In our study, we used a new approach that combines the position of the moon in the sky, the brightness of the moon’s face and several physical properties of light propagation to count predicted moonlight illumination, relative to an ‘average’ full moon, for a given place and time. Moonlight illumination was characterised using the package ‘moonlit’ to obtain the ambient intensity of moonlight on the ground (Śmielak 2023 ) and for latitude 23° E intensity of moonlight varied from 0 to 0.6. Extinction coefficient e (magnitude per air mass) was set at 0.24, corresponding to 500 m asl.…”

A framework for untangling the consequences of artificial light at night on species interactions

“…As reviewed above, natural night conditions range four orders of magnitude from starlight/new Moon of approximately 0.0006 lx to the full Moon directly overhead at approximately 0.2 lx [21]. Starlight conditions, due to the new Moon phase or the Moon being below the horizon, comprise approximately 50% of night-time hours, and thus, represent the most common night-time light condition [45]. Seymoure et al [12] translated the 2016 New world atlas of artificial night sky brightness [43] to illuminance experienced on the Earth's surface due to light pollution and found that approximately 23% of terrestrial habitats never experience new Moon lighting conditions and approximately 5% are ten times brighter and experience perpetual crescent Moon illumination.…”