Human alteration of natural light cycles: causes and ecological consequences

Abstract

Artificial light at night is profoundly altering natural light cycles, particularly as perceived by many organisms, over extensive areas of the globe. This alteration comprises the introduction of light at night at places and times at which it has not previously occurred, and with different spectral signatures. Given the long geological periods for which light cycles have previously been consistent, this constitutes a novel environmental pressure, and one for which there is evidence for biological effects that span from molecular to community level. Here we provide a synthesis of understanding of the form and extent of this alteration, some of the key consequences for terrestrial and aquatic ecosystems, interactions and synergies with other anthropogenic pressures on the environment, major uncertainties, and future prospects and management options. This constitutes a compelling example of the need for a thoroughly interdisciplinary approach to understanding and managing the impact of one particular anthropogenic pressure. The former requires insights that span molecular biology to ecosystem ecology, and the latter contributions of biologists, policy makers and engineers.

Fig. 1

Change in illumination at the Earth’s surface with solar (positive) and lunar altitude (negative) above the horizon; typical illumination levels of artificial light at night (ALAN); and levels at which nighttime lighting has been observed to have biological effects [arrows; Sharma et al. (1997) (a), Zubidat et al. (2007) (b), Johnson (1979) (c), Stone et al. (2009) (d), Kuijper et al. (2008) (e), Riley et al. (2012) (f), Bedrosian et al. (2011) (g), Miller (2006) and Kramer and Birney (2001) (h), Falkenberg and Clarke (1998) and Clarke et al. (1996) (h), Santos et al. (2010) (j), Dauchy et al. (1997) and Cos et al. (2006) (k), Bachleitner et al. (2007) (l), Evans et al. (2007a) (m), Larsen and Pedersen (1982) and Dice (1945) (n), Dice (1945) (o–q); studies of levels at which nighttime lighting has biological effects are from Gaston et al. (, Table 3)]. Main figure modified from Beier (2006), with additional data from Kurtze (1974); Rich and Longcore (2006) and Gaston et al. (2013). SS Sunset, CT civil twilight, NT nautical twilight, AT astronomical twilight

Fig. 2a–g

Global distributions of natural and artificial light. a–c Modelled yearly levels of daylight, moonlight and twilight in total hours, respectively, following equations in Meeus (2008), d cloud cover [composite of 12 monthly cloud fraction images for 2012 (Stockli 2013)], e mean annual lightning flash rate [flashes km⁻² year⁻¹ for 2012; Lightning Imaging Sensor/Optical Transient Detector gridded lightning climatology data set (NASA 2012)], f fire [sum of 12 cloud-corrected fire pixel images for 2012; NASA Land Processes Distributed Active Archive Center (2013)], and g artificial nighttime lighting {represented as digital number in 2010 [US Defense Meteorological Satellite Program (DMSP)/Operational Linescan System (OLS) 2012]}

Fig. 3

A raw nighttime stable lights image (2012) from the US DMSP/OLS (in Behrmann equal-area projection at a resolution of 810 × 810 m). Digital number indicates light intensity with 0 representing darkness and 63 indicating the brightest pixels. Histograms show data from this image a without and b with 0 digital number values. c Grey graphs show the mean digital number for particular columns/rows (latitudes/longitudes)

Fig. 4

The spectral radiance of a daylight, b incandescent, c low-pressure sodium, d light-emitting diode, e mercury vapour, and f fluorescent lighting. Daylight spectra from pveducation.org and artificial light spectra from http://ngdc.noaa.gov/eog/night_sat/spectra.html

Fig. 5a–d

The effects of ALAN on animals. a Loggerhead turtle hatchlings crawl towards artificial light when it is turned on, and the ocean when it is turned off (Salmon et al. 1995). b The effect of high-pressure sodium street lighting on the abundance of invertebrates within trophic groups. Bars represent the total number of individuals in each group collected from pitfall traps under lights (open bars) and between lights (grey bars) (Davies et al. 2012). c The influence of light intensity on the suppression of pineal melatonin content after 30 min of exposure. Bars indicate mean pineal melatonin content (for each group n = 7). *p < 0.001 (Brainard et al. 1984). d The effect of artificial night light on paternity gain for adult and yearling blue tits Cyanistes caeruleus occupying edge territories; data are point estimates and 95 % confidence intervals from a generalised linear mixed model in which age and territory category are fixed factors and male identity and season are random intercepts; numbers show sample sizes (Kempenaers et al. 2010)

Fig. 6a–d

Actual temperature, and day length calculated in two different ways for 2007 at Penryn Campus, Cornwall. a Temperature (°C) recorded on a minute-by-minute basis, b mean daily temperature, c estimated day length based on latitude and the angle of the sun, and d estimated day length based on minute-by-minute energy measurements (kW m⁻²; beginning of day was identified as the time when values increased from 0 to >0 and end when they decreased from >0 to 0. Day length was then assumed to be the time between these two points). Data courtesy of K. Anderson

Fig. 7

Routes by which influences of ALAN on interspecific interactions have consequences for community structure and ecosystem function, process and services