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Environment & Wildlife



Fatal Attraction: Insects trapped by streetlight.

Artificial night lighting is one of the most pervasive – and yet under recognised – causes of environmental pollution.

from The National Trust's State of Nature Report 2019

We are in the middle of the sixth mass extinction, where the decline in species on our planet is 100 to 1,000 times the natural background rate. Unlike previous extinction events, this one is caused by human activity.

Many reports including the Government's own "UK Biodiversity Indicators" show that there has been a sharp decline in insect numbers in recent decades, with a 30% drop in insect pollinators between 1980 and 2017 and a 64% decline in the 'Priority' species from 1970 to 2018.[1] This probably won't come as a surprise if you're old enough to remember when car windscreens, after a long road journey in the 1960s or '70s, would be covered in squashed bugs. Take the same journey today & there'll hardly be any.

The sharp decline in insect numbers in particular, can be put down to a combination of factors:- habitat loss, pesticide use, invasive species and climate change. There is also significant evidence that Artificial Light At Night (ALAN) is an additional a key driver in the Insect Apocalypse, with studies showing that ALAN affects insect development, movement, foraging and reproductive success, as well as predation.[2]

Practically all life on earth has evolved under the influence of the daily 24-hour cycle of light and dark, and the changing seasons. It stands to reason then that our biological clock is affected by these cycles, and that the rapid recent increase in ALAN, affects both flora & fauna.

There are numerous examples of the effects of ALAN in scientific studies, covering a wide range of animals and plant species. The ALAN database,[3] for example, has details of over 600 research papers covering everything from slugs to the siberian hamster.

Here's a brief summary of some of the effects...



More than 60% of invertebrates (insects, spiders, crabs, snails etc) are nocturnal,[4] making them especially susceptible to the effects of ALAN.

Why are insects important? Well, they are responsible for pollination of about 80% of UK plants, including a large number of crops. They also provide other ecological benefits[5] including:-

  • Breaking down & decomposing of organic matter, feeding on dead plant tissues, dead animals & excrement.
  • Biological/Pest control – Ladybirds eat aphids etc… Nematoids & beetles kill slugs…
  • Food for animals – numerous UK birds, mammals & fish live on insects including all 17 of our native bat species.
  • Environmental indicators – eg butterflies are often used as an indicator of environment quality – as are caddisflies on waterways.

Many invertebrates depend on the natural rhythms of day–night and on seasonal and lunar changes in light levels, to trigger vital stages in their life-cycles such as egg-laying, emergence and hibernation. Some even complete their lifecycle within a lunar cycle of 28 days, where the presence or absence of moonlight triggers the beginning and end of each lifecycle.[6] Several studies have shown than ALAN affects the development of insects, by altering the timing of these life-cycle development stages.[2]

Many insects are attracted to blue and ultaviolet light. This affects nocturnal flying insects in several ways, where they can become trapped around lighting, literally 'like a moth to the flame':-

  • Lighting can attract insects from 500m away or more, leading to a population reduction in their habitat & loss of plant pollination etc. Reduction in the local population also has a knock-on effect on other species that feed on the insects, and can fragment the species population.
  • About a third of all the insects trapped in the light from a street lamp, will die of exhaustion or predation.[6] Those that survive will often be too weak to forage, reducing nocturnal pollination rates.[6] Some of those that do survive will rest on nearby surfaces & have higher risk of daytime predation, and their weakened state will also affect mating etc.
  • Movement and migration of insects is inhibited, where a row of regular lighting (eg. street lights) can act as a barrier.[2]

Aquatic insects are also affected by ALAN. Riverflies, which provide food for fish and birds are attracted to light in their adult stage, but as larvae their movement is inhibited by light.[2,4]

For the astronomer, it's also interesting to note that African Dung Beetles make use of polarised light from the Milky Way to help orientate themselves... Dragonflies also use light to orientate themselves when flying, which can be disturbed by ALAN. They also make use of polarised light to detect water surfaces when laying eggs, and can mistake other surfaces, such as red cars, oil spills & solar PV panels for water.

It's not just direct light that causes problems. Skyglow light pollution can delay the start of nocturnal insect feeding times, as insects rely on the levels of light to tell them when to feed. Night time flower opening on the other hand is triggered by changes in temperature, so the two are desynchronised, leading to a drop in the rate of plant pollination and weaker, less fit insects.[2]

Nocturnal insect reproduction is also affected by the level of ambient light at night, either delaying or eliminating the window of time during which night-active insects engage in courtship and mating. Some moths only reproduce when illumination from the Moon is below First Quarter and Fireflies rely on low-light levels in order to mate.[2]

Part-night switching of lights has been shown to help night-time pollination.[7]



Aside from the impact of ALAN on insects as a food source, some migratory birds have been shown to use the night sky to as a compass by the apparent rotation of the stars around the North Star, Polaris.[8]

In the US, light from cities is known to affect migration, with a significant number of bird strikes occuring on tall buildings. In the UK, this doesn't seem to be such a big problem, although studies in Europe indicate that high illuminated buildings do attract migrating birds and are responsible for thousands of deaths each year. Aircarft red warning lights on these tall buildings aren't effective at deterring birds, unless they are in beacon mode, flashing at intervals at least 3 seconds apart.[4]

More relevant perhaps is that Light Pollution has been shown to affect the immune response of birds. In some countries house sparrows in light polluted environments have been shown to be more susceptable to the West Nile Virus,[9] though no cases have yet been observed in the UK.

When birds day-night rhythm is disturbed by ALAN, it has stress-related effects. Many bird species like robins, blackbirds, coal tits and blue-tits, when affected by light pollution, begin their dawn chorus earlier in the year and earlier in the morning. The consequence is earlier breeding, foraging and development. This altered behaviour is attributed to changes in the hormone balance, with impairments in terms of their fitness and life expectancy likely.[4]



About 69% of mammals are nocturnally active and a third of them are bats. In the UK the more common bat species have increased in recent years,[1] where light pollution favours species like the common pipistrelle, which normally feed at dusk. One study showed that they were light-opportunistic, feeding off the buffet of insects which street lights attract; whereas some light-averse rarer species showed reduced activity.[10]

Bats are also known to abandon roosts which are brightly illuminated - one more reason why historic buildings shouldn't be illuminated indiscriminately. They are also sensitive to different colours & in some instances red street lights have been used to enable bats to cross roads in woodland,[11] where light from 'normal' street lights would have acted as a barrier to their movement.

Other mammals such as small rodents like mice and hares also limit their activity and foraging at high illumination levels.[4]

The amount of light also affects some mammals reproduction. In pigs, for example, the mating seasons are regulated largley by biological changes caused by the photoperiod - the length of day.[12]

There has also been quite a lot of concern over the impact of 'blue' light on our melatonin hormone levels and circadian rhythm - our body clock. This isn't unique to humans, as melatonin is also found in birds, fish and other mammals, but it is only part of the story. Research by the US Dept of Energy shows that most of our exposure to this 'blue' light comes from lighting in our homes and workplaces, rather than from ALAN. Circadian rhythms are also affected by the kind of light we experience in the mornings, not just 'blue' light at night time.



Fish are adversely affected by ALAN in urban areas where riverbank and bridge lighting spill onto the water. Even with low levels of illumination their behaviour is disturbed at night, when they would normally be dormant.[13]

Fish abundance & behaviour has also been shown to be affected by light from the riverbank, with fish attracted by the light leading to increased predation.[14] In Nigeria fishermen also use lights at night to attract fish to their boats...



In the UK ALAN has been shown to inhibit the growth of common frog tadpoles increasing the risk of predation.[15] The breeding cycle of Common Toads have also been shown to be adversely affected by light.[16]



Plants react to the natural length of the day, their growth and development being linked to it. Shorter days in autumn, for example, stimulate bud growth in woody plants for the next season. Longer days in spring and summer induce flowering in many wild plants of our latitudes.

So plants are also affected by an increase in ALAN. For example, it is known that trees or individual branches illuminated by street lights lose their foliage later, leading to frost damage and weakening of the tree. If plants are constantly exposed to light, they become more susceptible to disease. Artificial lighting at night, for example, leads to increased sensitivity to ground-level ozone and consequently to leaf damage.[4]

In a UK study, bud opening in trees was linked with artificial lighting with trees bursting into bud more than a week earlier — a magnitude similar to that predicted for 2°C of global warming due to climate change.[17]

[1] DEFRA's "UK Biodiversity Indicators 2020" https://www.gov.uk/government/statistics/biodiversity-indicators-for-the-uk
[2] SSRN paper "Light Pollution Is A Driver Of Insect Declines" (2019) https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3378835
[3] http://alandb.darksky.org/
[4] Austrian Government – "Austrian Guidelines For Outdoor Lighting" (2019) https://www.land-oberoesterreich.gv.at/files/publikationen/us_Leitfaden_Guidelines_Outdoor_lighting_english.pdf
[5] The Guiardian – "Eight Things Insects – Those Unsung Heroes – Do For Us"(2015) https://www.theguardian.com/commentisfree/2015/aug/04/insects-uk-species-protect
[6] Buglife (The Invertebrate Conservation Trust) - "A Review Of The Impact Of Artificial Light On Invertebrates" (2011) https://cdn.buglife.org.uk/2019/08/A-Review-of-the-Impact-of-Artificial-Light-on-Invertebrates-docx_0.pdf
[7] University of York – Street Light Switch-Off Benefits Night-Time Pollinators (2019) https://www.york.ac.uk/news-andevents/news/2019/research/street-light-switch-off-night-time-pollinators/
[8] Journal of Experimental Biology (2001) https://jeb.biologists.org/content/204/22/3855
[9] New Scientist - "Light pollution's effects on birds may help to spread West Nile virus" https://www.newscientist.com/article/2210827-light-pollutions-effects-on-birds-may-help-to-spread-west-nile-virus/
[10] The Royal Society – "Effects of dimming light-emitting diode street lights on light-opportunistic and light-averse bats in suburban habitats" (2018) https://royalsocietypublishing.org/doi/full/10.1098/rsos.180205
[11] "Bat-friendly street lights for Worcestershire crossing" https://www.bbc.co.uk/news/uk-england-hereford-worcester-49534621
[12] National Library of Medicine "Influence of light and photoperiodicity on pig prolificacy" https://pubmed.ncbi.nlm.nih.gov/3910824/
[13] DW Documentary - "The Disappearing Darkness" at around the 34min mark: https://youtu.be/9NyQgHGF1NM?t=1994
[14] Journal of Applied Ecology (2013) "Potential effects of artificial light associated withanthropogenic infrastructure on the abundance andforaging behaviour of estuary-associated fishes" https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1111/1365-2664.12024
[15] DEFRA "The biodiversity impacts of street lighting" (2015) http://sciencesearch.defra.gov.uk/Document.aspx?Document=13201_Thebiodiversityimpactsofstreetlighting.pdf
[16] Oxford Academic - Conservation Physiology (2020) "Artificial light at night disturbs the activity and energy allocation of the common toad during the breeding period" https://academic.oup.com/conphys/article/7/1/coz002/5307659
[17] Nature (2018) "The dark side of light: how artificial lighting is harming the natural world" https://www.nature.com/articles/d41586-018-00665-7