A mug of mulled wine in hand, I cross a fairy-lit bridge, and suddenly, I’m noticing the robins again. The time of year has arrived when the city takes on a night-time glow, lit by decorative strings and shapes of artificial lighting to brighten up the darkening evenings. While the warm glow can help to get us in the Christmas spirit or bridge the long winter nights before the days begin to lengthen, artificial light at night (ALAN) can have significant consequences for the other species, not to mention humans too.

More than 80% of the global population lives under light-polluted skies. Every year, the radiance of ALAN in the human-visible range increases by an average of 9.6% [1]. However, ecosystems are organised around natural light-dark cycles which have been stable over evolutionary time scales [2]. For example, corals have adapted to spawn within a few days of the bright light of a full moon [3]. Entire coral colonies release their sperm and egg cells simultaneously, creating one of the most spectacular natural phenomena: a great underwater mélange of floating gametes, ready to produce tiny coral larvae [3].

The influence of light-dark cycles on species exists naturally through diurnal (day and night), yearly (seasonal) and lunar cycles (as with corals). The combined effects of these cycles with ALAN can be complex and surprising [2]. Studies show mixed effects on both nocturnal and diurnal species. A study on the effect of lunar illumination and ALAN on the calling behaviour of the nocturnal Common Poorwill (Phalaenoptilus nuttallii) found that birds called less when artificial light was on during nights where there was also a lot of lunar illumination [4]. This could be due to the increased risk of predation during brighter nights.

Both birds and bats use the Earth’s magnetic fields for migration or orientation, and light cues are often used to calibrate their magnetic sensitivity [5]. Very bright light installations along migratory paths – such as those from oil rigs and tankers – can disorientate birds, causing them to “ground” [6]. This effect, known as “positive phototaxis,” can result in death as birds struggle to continue migration and find prey.

Fig.1:  The constellation Orion. Photographed on the left from dark skies, and on the right from Orem, Utah (population about 500,000). Photo by jpstanley. Orem, 2009 on Flickr.

Much like by using magnetic fields, we know that other species are inextricably tied to the natural rhythms of the planet – physical and environmental factors that make life on Earth possible. ALAN can affect species or their predators, causing cascade effects on the entire ecosystem. Behavioural, feeding and breeding changes can lead to unnatural variations in reproduction rates, distributions, habitat use, and birth/death rates [5]. For example, ALAN can alter diurnal patterns of activity, causing diurnal pollinators and insectivores to extend their activity times into the evening [7].

On the flip side, nocturnal insects have been shown to delay their emergence at night. Over time, these changes in activity and rest patterns can alter the development and phenology of species, with effects rippling through the food web [7]. ALAN can also block natural nocturnal light sources which can affect foraging and reproductive success. Dung beetles, for example, have been found to use the Milky Way for navigation [8]. The study showed that the beetles roll their dung balls in a straight line under a starry sky but are unable to do so in overcast conditions. (They were also able to roll just as well in a planetarium under a starry sky as they were under the Milky Way).

What can be done in cities to reduce ALAN and its negative effects? Reducing the intensity of and shielding light fixtures – especially since public lighting makes up the highest proportion of ALAN – can help [5]. Periodically turning off of powerful light sources during the night allows birds “trapped” by positive phototaxis the opportunity to leave the site [5]. Additionally, using automatic timers or motion sensors to turn off lights when they are not needed can make a significant difference. Dimming, shielding or altering the emitted light frequency of during seasons when particularly vulnerable species are most risk (such as during the courtship season of the common glowworm) can support the survival rates of these species [7].

Finally, what can you do to support the reduction of ALAN in your local community? Try to avoid outdoor decorative lighting around your home or set it on a timer. It’s particularly recommended to use warm white or amber-coloured LED lights as these are energy-saving and less disruptive to wildlife [9]. In Austria, the use of Christmas lighting consumes as much energy as 10,000 households in an entire year, in just the few weeks before Christmas [9]. You can advocate for responsible outdoor lighting in your municipality by contacting them about their outdoor lighting codes. Templates for contacting your local municipality can be found at https://darksky.org/.

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