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How Wildlife Struggles with the Dark Side of Light

Artificial light has thrust us out of the dark ages, giving us the security and freedom to travel, work and socialise at night. Simultaneously, it is silently plunging the natural world into a chaos that ironically, we cannot see.

by Myles Pinkney

With the daily and seasonal rhythms of light providing an essential trigger for animals to forage, breed, migrate, sleep and evade predators, it is literally hard-wired into their survival systems. Gareth Wilmer(1). reported Professor Oscar Corcho from the Universidad Politécnica de Madrid in Spain saying that ‘the negative consequences of light pollution are as unknown by the population as those of smoking in the 80s.’

Sánchez de Miguel et al(2) report that humans have caused a staggering 49% increase in light pollution in the last 25 years. And that’s just light detectable by satellite. This means many people have never seen the milky way, with natural darkness fast becoming a rarity. Whilst we sleep, light pollution is having untold and unknown impacts on wildlife. With research in its infancy, we are only just beginning to unlock the secrets of this hidden pollutant.

Impacts of Light Pollution on Body Clocks

Circadian rhythm is a natural, internal process that regulates the sleep–wake cycle roughly every 24 hours. Light pollution is affecting the internal body clock in a number of animals, particularly birds. A study by Thomas Raap et al in Nature(3) reveals artificial light is making Great Tits leave their nests earlier believing the sun has already risen. Of course with less sleep this could affect their fitness and ability to breed.

Blackbirds too are among those affected. Nordt and Klenke published a study in PLoS ONE(4) that looked into the effect of artificial light and noise pollution on blackbirds. They found that urban birds began their dawn chorus as much as 5 hours earlier than their rural counterparts. Using more energy by being awake for longer could have dramatic consequences that we just don’t understand yet.

Impacts of Light Pollution on Anatomy

Light isn’t just affecting body clocks, but anatomy too. An alarming article by Dominoni et al in PLoS ONE(5) discovered that in the second year of a two-year study, European blackbirds (Turdus merula) constantly exposed to light roughly equivalent to a full moon at night showed ‘no sign of reproductive activity.’ Both testicular size and testosterone concentration in the blood remained at baseline levels for the entire reproductive season. The revelation that ‘chronic low intensities of light at night can dramatically affect the reproductive system’ has worrying implications for urban birdlife across the planet.

Impacts of Light Pollution on Migration

Migrating birds also face disruption as light pollution impacts their internal night-time compass. A study by Adams et al in Environmental Evidence(6) reveals that young birds use the stars to identify northern and southern constellations in order to navigate. Of course, constellations that they learnt may look very different under heavy levels of light pollution. Young birds have underdeveloped vision, and according to Heswall et al in Ecology(7), this makes them even more susceptible to disorientation by artificial light.

Impacts of Light Pollution on Grounding Birds

Seabirds such as the petrel and shearwater nest in burrows and are mainly nocturnal in their breeding places. This makes them particularly sensitive to light when they make their first flights from the coast to the sea. In a well-documented but bizarre and little understood phenomenon known as ‘grounding,’ artificial light makes them fall on urban areas, where they are then unable to fly away. Chevillon et al in Global Ecology and Conservation(8) report that 40,000 petrels were ‘grounded’ by artificial light between 1996-2021 on the Reunion Island near Madagascar. Luckily for them a local rescue campaign managed to rescue 88% of these birds, but across many other coasts, grounded birds are not so lucky and die from injuries, predation or starvation.

Light Pollution Causes Disorientated Turtles

It’s not just seabirds that are affected along our coastlines. The dramatic impact that artificial light has on turtles has been recognised as early as 1911 and is documented in Australia’s National Light Pollution Guidelines for Wildlife.(9) Sea turtles mainly hatch at night, making their way to the sea by recognising the moonlit horizon. It is a well-recorded and saddening scene where turtle hatchlings become disorientated and head towards the artificial light. Thums et al study in the Royal Society Open Science(10) report that as a consequence, they often fall victim to the wheels of passing traffic, die from exhaustion, dehydration or end up as a predator’s lunch.

Impacts of Light Pollution on Lesser Horseshoe Bats

Some species are affected in the opposite way to turtles and birds. A study by Zeale et al in Global Change Biology(11) found that slower-flying species of bats like the rare and declining Lesser Horseshoe will actively avoid an artificially lit hedgerow and seek the dark. Considering there are just 15,000 Lesser Horseshoe bats in England and Wales, this makes dark corridors imperative for them to feed and breed.

Impacts of Light Pollution on the Common Toad

Another animal in decline is the common toad, which soon may not be able to live up to its name. They have seen a dramatic 68% decline in the last 30 years. Learning that they are negatively affected by our lights during their nocturnal wanderings is of great concern. Even with low artificial light levels, a recent study by Touzot et al in Conservation Pysiology(12) revealed that male toad activity and movement is reduced by more than half when they should be migrating towards breeding ponds. This is a worrying statistic if it proves to reduce the reproduction rate of an already struggling species.

Impacts of Light Pollution on Glow Worms

Our lights even disturb the smallest of animals. During the summer nights, you may have been lucky enough to glimpse the wonderful green-orange bioluminescent light that female glow worms emit to attract males. A study published in Insects by Van den Broeck found an alarming fact; female glow worms suffer from a lower chance of attracting a male when exposed to artificial streetlight, which could inevitably lead to population decline. Normally females would stop glowing after one night, having successfully attracted a male glow worm. Under the streetlights, females continued to glow for as long as 15 nights, unable to find a pairing.

What Are the Solutions to Light Pollution?

Now that we have heard all of the doom and gloom, how do we resolve it? With the global rise in energy costs, using less light has glaringly obvious benefits that make their way to our pockets. We ideally should only use outdoor lighting when absolutely necessary with the light directed ground-wards or shielded. Using non-reflective and dark coloured surfaces is a good way to prevent light spill. While research into the impacts of different coloured light on wildlife is still in its early days, blue light is proven to have the most significant negative effect on animals. According to Jägerbrand et al’s study in Sustainability(14) the use of amber LEDs is thought to be the least damaging. But we still shouldn’t fall into the trap of thinking this is a solution. It’s just a step in the right direction. Zeale et als study on bats in Global Change Biology(15) showed that particular species avoided hunting in white light areas but were unaffected by red light. More research is needed to see if we can deliberately use different colour light according to what species are in need of a helping hand in a particular area. Timed lights that switch-off and dim are also strategies we can use to ensure we are giving nocturnal species that much needed period of natural darkness to hunt and feed undisturbed.

Light Pollution Solutions in Action

According to Dr Christopher Kyba, creator of the Radiance Light Trends App and physicist tracking the increase in light pollution, “the problem is that in every large city there are hundreds of thousands or even millions of people making lighting decisions.” Green et al, published in The Astronomy and Astrophysics Review(16), reveal how in 1958 the city of Flagstaff in Arizona introduced legislation prohibiting the use of searchlights. In 1989 they introduced further restrictions on how much light new developments could emit, along with incentives for shielded lighting. Kyba states that “this has had a major impact on the skyglow from Flagstaff.” Between 1989 and 2015, they not only achieved their target of limiting skyglow increase, but actually reduced sky brightness by 10%. This is remarkable considering how artificial light keeps increasing year after year. Flagstaff is a city of 70,000 people, but has a remarkable 1/10th the artificial light levels of a city its size and population in the state of Wyoming. Dr Jeffrey Hall is the Executive Director of Lowell Observatory and Chair of the American Astronomical Society’s Committee on Light Pollution, Radio Interference, and Space Debris. He states that “preserving the night sky has become part of the local culture. There are two major observatories right on the city limits, but it's gone well beyond astronomy and is now part of what people value about living in the area.”

While planning and development is somewhat dull it is a vital tool for protecting wildlife. The Gorgon Project is one of the world's largest natural gas plants on Barrow Island in Western Australia, but is also built next to an important flatback turtle nesting site. From an early phase in the design, the project was careful to reduce the impact of light on hatchlings as can be seen in Australia’s National Light Pollution Guidelines for Wildlife(17).

Directional and shielded lighting was used, in addition to mounting the lights as low to the ground as practical. Louvre lighting reduced glare, coupled with the use of black-out blinds and timed lighting. The layout of buildings was also designed to prevent the majority of windows facing the beaches. On top of that, car parks were positioned to prevent headlights lighting up the dunes. If that wasn’t enough, the site annually reviews its lighting plan before nesting season.

Halfway across the globe in Florida, people face the same challenges to prevent light pollution affecting turtles. Rachel Tighe, Lighting Project Manager at Sea Turtle Conservancy, works on a dedicated campaign with estate agents that provides information folders to new coastal property owners. She says that this “informs them about lighting disturbances, information about wildlife friendly lighting, native vegetation, how to find and purchase turtle friendly lighting, and other ways they can help the sea turtles in their community.” The charity also holds workshops to educate building professionals about turtle-friendly lighting. This level of community engagement is a key way to begin to switch off our seemingly endless

dazzle of lights.

These examples prove that the final responsibility doesn't just lie with us. Legislation and measures to limit light pollution should also come from the councillors, politicians and developers who run our towns and cities. Community projects are also a brilliant way to unite people over a sense of achievement from our actions.

Engagement and Conclusion

So how can you get involved as an individual? Stargazing is a wonderful hobby, but also a an engagement tool to get people talking about the value of the night. Why not download ‘The Loss of the Night App,’ which requires you to look for specific stars and see if you can see them from your location? All of your observations are sent to the Globe at Night Project, where they can then be viewed by everyone. Light pollution is a hugely under-researched pollutant, with many of its effects being longer term and more subtle than with other pollutants which grab our attention and our headlines. As always, we only start to take things seriously when they affect us. The side-effects of blue light on our bodies and brains is becoming more well known, and with most of the world now bathed in artificial light throughout the night, it is clear that wildlife is also suffering untold damage. As Dr Jeffrey Hall aptly summarises, “it's the classic problem that occurs whenever we make rapid alterations to an incredibly complex system: many unexpected and unintended side effects.” So please join us and say no to badly designed and excessive light.


1. This article was originally published in Horizon, the EU Research and Innovation magazine 2. Sánchez de Miguel, A.; Bennie, J.; Rosenfeld, E.; Dzurjak, S.; Gaston, K.J. First Estimation of Global Trends in Nocturnal Power Emissions Reveals Acceleration of Light Pollution. Remote Sens. 2021, 13, 3311. 3. Raap, T., Pinxten, R. & Eens, M. Light pollution disrupts sleep in free-living animals. Sci Rep 5, 13557 (2015). 4. Nordt A, Klenke R. Sleepless in town--drivers of the temporal shift in dawn song in urban European blackbirds. PLoS One. 2013 Aug 7;8(8):e71476. doi: 10.1371/journal.pone.0071476. PMID: 23940759; PMCID: PMC3737108. 5. Long-Term Effects of Chronic Light Pollution on Seasonal Functions of European Blackbirds (Turdus merula) Dominoni DM, Quetting M, Partecke J (2013) Long-Term Effects of Chronic Light Pollution on Seasonal Functions of European Blackbirds (Turdus merula). PLOS ONE 8(12): e85069. 6. Adams, C.A., Fernández-Juricic, E., Bayne, E.M. et al. Effects of artificial light on bird movement and distribution: a systematic map. Environ Evid 10, 37 (2021). 7. Heswall A, Miller L, McNaughton EJ, Brunton-Martin AL, Cain KE, Friesen MR, Gaskett AC. 2022. Artificial light at night correlates with seabird groundings: mapping city lights near a seabird breeding hotspot. PeerJ 10:e14237 8. Leo Chevillon, Julie Tourmetz, Jérôme Dubos, Yahaïa Soulaimana-Mattoir, Clémence Hollinger, Patrick Pinet, François-Xavier Couzi, Martin Riethmuller, Matthieu Le Corre, 25 years of light-induced petrel groundings in Reunion Island: Retrospective analysis and predicted trends, Global Ecology and Conservation, Volume 38, 2022, e02232, ISSN 2351-9894, 9. ‘National Light Pollution Guidelines for Wildlife Including Marine Turtles, Seabirds and Migratory Shorebirds, Commonwealth of Australia 2020.’ 10. Thums M, Whiting SD, Reisser J, Pendoley KL, Pattiaratchi CB, Proietti M, Hetzel Y, Fisher R, Meekan MG. Artificial light on water attracts turtle hatchlings during their near shore transit. R Soc Open Sci. 2016 May 18;3(5):160142. doi: 10.1098/rsos.160142. PMID: 27293795; PMCID: PMC4892457. 11. Zeale, MRK, Stone, EL, Zeale, E, Browne, WJ, Harris, S, Jones, G. Experimentally manipulating light spectra reveals the importance of dark corridors for commuting bats. Glob Change Biol. 2018; 24: 5909– 5918. 12. Morgane Touzot, Loïc Teulier, Thierry Lengagne, Jean Secondi, Marc Théry, Paul-Antoine Libourel, Ludovic Guillard, Nathalie Mondy, Artificial light at night disturbs the activity and energy allocation of the common toad during the breeding period, Conservation Physiology, Volume 7, Issue 1, 2019, coz002, 13. Van den Broeck, M.; De Cock, R.; Van Dongen, S.; Matthysen, E. Blinded by the Light: Artificial Light Lowers Mate Attraction Success in Female Glow-Worms (Lampyris noctiluca L.). Insects 2021, 12, 734. 14. Jägerbrand, A.K.; Bouroussis, C.A. Ecological Impact of Artificial Light at Night: Effective Strategies and Measures to Deal with Protected Species and Habitats. Sustainability 2021, 13, 5991. 15. Zeale, MRK, Stone, EL, Zeale, E, Browne, WJ, Harris, S, Jones, G. Experimentally manipulating light spectra reveals the importance of dark corridors for commuting bats. Glob Change Biol. 2018; 24: 5909– 5918. 16. Green, R.F., Luginbuhl, C.B., Wainscoat, R.J. et al. The growing threat of light pollution to ground-based observatories. Astron Astrophys Rev 30, 1 (2022). 17. ‘National Light Pollution Guidelines for Wildlife Including Marine Turtles, Seabirds and Migratory Shorebirds, Commonwealth of Australia 2020.’

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