Since time immemorial, the Earth has been subject to the alternation of day and night, and living creatures have adapted to this circumstance. This rhythm and the seasonal changes in the length of the day in our latitudes are important sources of information for many organisms. When that changes as a result of the use of artificial light during phases of natural darkness, impacts on vital functions are inevitable.
Mammals, birds, insects, amphibians, reptiles, fish, plants and the structure and function of ecosystems can be negatively affected by artificial light.
Besides changes to physiological processes, there are also behavioural changes, which can relate to such aspects as attraction and rejection and to disturbed orientation.
Reproduction, development, communication, foraging, predator-prey relationships and radius of action are affected as a result.
The effects of light pollution range from changes in the mix of species within communities to the extinction of isolated populations, particularly in the case of sedentary, specialised and endangered species.
Illumination or spotlighting can cause bats to abandon their roost. Or the lack of darkness will prevent them from flying out to look for food. The lack of food, in turn, can have negative effects on the rate of reproduction.
Artificial light affects the predator-prey relationship in various ways: While some dwarf bat species and the Common Bat hunt insects attracted by the light, horseshoe bats and mouse-eared bats are disturbed by the light and frightened away.
Migratory bats that cover long distances between their summer and winter quarters, such as the Nathusius' pipistrelle, are attracted by light installations during migration. Deviation from the optimum route costs energy and weakens the bats. Unfortunately, too little is still known about their exact migration routes and times. Nathusius' pipistrelles often hibernate in Tyrol; they come in autumn (September) and move to their summer quarters in spring (March, April).
Illuminated areas are avoided by small and large mammals; they fragment and diminish their habitats.
Artificial light disrupts mammals' phases of rest and inhibits melatonin synthesis so that the animals are weakened in the long term. This is particularly relevant in winter, as disturbances result in increased energy requirements and the animals' reserves may not last until spring.
At luminous intensities as low as 0.3 lux (max. full moon brightness), bird species such as robins, blackbirds, great tits and blue tits start their territorial singing earlier in the day and season. This results in earlier breeding, foraging and development, which in turn can affect the birds' fitness and life expectancy.
About two thirds of migratory birds fly at night to take advantage of darkness for protection from predators and also to save time, energy and water. The main migration periods are from August to November and from February to May.
Point light sources and large illuminated areas can lead to disorientation and considerable energy losses. In the worst case, birds collide with structures and are killed.
The higher the structure and the more exposed the site, the more likely it is that the illuminated facility will become a trap for migratory birds. Bird migration can be concentrated on the margins of the Alps, mountain passes and in some valleys, and illuminated ruins, castles, mountain refuges, summit crosses, ski slopes and other lighting installations in such locations are therefore critical. In Tyrol, a significant share of the migrating birds pass through the Oberland.
Bird trap – Post Office Tower:
The effect of illumination on a 160 meter high-rise building in Bonn was investigated for a year. More than a thousand birds representing 29 different species were drawn to the light. About 200 of them were killed when they collided with the structure and many more were injured.
Further observations carried out showed that more than 90 percent of all birds displayed disturbed behaviour on entering the cone of light, including circular flight, loss, change or reversal of direction, and changes of speed.
Haupt H, Schillemeit U (2011) Lichtanlagen bringen Zugvögel vom Kurs ab. Natur und Landschaft. 43(6):165-170.
Bird trap – Tribute in Light:
The Tribute in Light is an annual light installation dedicated to the memory of the victims of the 9/11 terrorist attacks in New York. Powerful xenon searchlights directed up into the sky create columns of light that are visible at a distance of up to 100 km.
In a seven-year period of observation, it was found that large flocks of migratory birds were drawn to the light even when visibility was good. The density of migratory birds in the area of the light installation was about 20 times higher than in the surrounding area.
Abnormal behaviour was observed, such as reduced speed, circling and more frequent bird calls. When the light installation was switched off, the behavioural problems immediately stopped.
Van Doren BM, Horton KG, Dokter AM, Klinck H, Susan B, Elbin SB, Farnsworth A (2017) High-intensity urban light installation dramatically alters nocturnal bird migration. PNAS. 11175–11180. doi: 10.1073/pnas.1708574114
Why many moths and other insects are attracted to artificial light at night is not exactly known. It is assumed that they navigate with the help of light from the sky, and artificial light impairs orientation. For beetles and hymenoptera, there is evidence that they use stars and the moon as aids to navigation.
Once trapped in a cone of light, the animals flutter around until they die of exhaustion or burn in the luminaire. In this way, billions of insects are removed from their habitat and can no longer fulfil their functions there.
Many nocturnal insects see their environment in colour; the sensitivity of their facetted eyes is enormous. While humans are completely colour-blind in dim starlight, moths such as the elephant hawk moth can still see colours. Nocturnal insects can see colours ranging from ultraviolet to infrared, with maximum sensitivity in the short-wave spectral range.
It has been shown in a study that high-pressure sodium lamps (1850 kelvin) attract the most nocturnal insects. The least attraction is generated by warm white LEDs (2700 kelvin). The test results are significant, especially for moths and dipterous insects.
But why high-pressure sodium lamps have a greater attraction for nocturnal insects than neutral white LEDs (4200 kelvin) and warm white LEDs is still unknown. Their spectrum and the warm-white light colour would suggest otherwise.
Metal halide lamps have an even higher attraction for nocturnal insects than high-pressure sodium lamps and, for reasons of insect protection, they should not be used.
Huemer P. et al., 2011: Anlockwirkung moderner Leuchtmittel auf nachtaktive Insekten. Unpublished study of the Office of the Tyrolean Ombudsman for the Environment and Tiroler Landesmuseen Betriebs GmbH, Innsbruck.
Moths are important plant pollinators and a source of food for bats, amphibians and nocturnal mammals. Birds also eat large moths and caterpillars.
The loss of moths as pollinators and a source of food due to habitat loss, intensive agriculture and light pollution has far-reaching ecological effects.
Decreasing populations and a decline in the numbers of species and the distribution of moths have already been observed in some European countries.
Nocturnal insects are important pollinators:
It was found in a study that flowers cultivated under artificial lighting received 62 percent fewer visits from nocturnal pollinators than those growing at dark reference sites.
The loss in terms of pollination by nocturnal insects was not compensated by daytime pollinators, as the case of the cabbage thistle (Cirsium oleraceum) showed. The reduced number of nocturnal pollinators resulted in a 13 percent decrease in the number of fruits per plant.
Artificial light thus clearly influenced the sexual propagation of the plants.
Knop E, Zoller L, Ryser R, Gerpe Ch, Hörler M, Fontaine C (2017) Artificial light at night as a new threat to pollination. Nature. 548:206-209. doi:10.1038/nature23288
When the days become shorter in winter, some insects enter into a diapause (state of arrested development). Moth caterpillars pupate, for example. If daylight is prolonged through the use of artificial light, such insects are prevented from entering this phase of hibernation, and their chances of surviving the winter are reduced.
Glow worms glow in the dark in a process known as bioluminescence. The light in their bodies is the result of a chemical reaction in which a bioluminescent substrate is oxidised by an enzyme. 98 percent of the energy is converted into light and only 2 percent heat is generated. Bioluminescence generally serves to ward off enemies or to attract prey and mating partners.
There are three native species of what are known as glow worms: the common glow worm, the Central European firefly and the short-winged firefly or lesser glow worm. In the case of the Central European firefly, the males, females and larvae can all glow, whereas only the female and the larvae of the common glow worm glow. In the case of the short-winged firefly, the males and females cannot fly and glow only weakly.
Adult glow worms can be observed on warm nights from June to July. The flightless females of the common glow worm and the Central European firefly use their light signal to indicate their readiness to mate to flying males.
Female fireflies do not start to glow until a certain degree of darkness has set in. Artificial light can reduce the duration of glow and thus prevent successful reproduction. Artificial light also limits the range of the signals.
Male fireflies are attracted by low light intensities, especially in the orange to green wavelength range, but they are repelled by intensive lighting. Female glow worms in an area within the range of street lighting, for example, sometimes wait in vain for a partner.
Algae perform photosynthesis and need sunlight to live. This is why algae are mainly found close to the surface of water. In the shelter of the night, animal microorganisms, also called zooplankton, rise to the surface of the water and feed on algae there. During the day this zooplankton is to be found at deeper water levels.
Prevention of this vertical migration in standing waters by urban light smog or illuminated riverside paths can disrupt the food chain in lakes and lead to frequent algae bloom and thus to deterioration in the quality of the water.
Amphibians often avoid daylight to protect themselves from dehydration and predators. An example of their predominantly nocturnal activity can be heard in summer in the form of the frogs’ evenings chorus.
Because of their permeable skins and their amphibious habitat requirements, these animals are sensitive and particularly endangered. Amphibians are protected animals according to the Tyrolean Nature Conservation Law (TNSchVO 2006). All species are included in the Red List of endangered Austrian amphibians.
Amphibians' eyes can adapt to very low levels of light. The common frog can even see colours in the dark. The common toad needs only a fraction of the light of the starry sky to catch its prey at night and for visual orientation only requires the illuminance of the brightest visible star (Sirius 0.00001 lux).
In the presence of illumination, some amphibians emerge later from their hiding places and thus have less time to find food. When frogs and toads hunt insects attracted by artificial light, they run the risk of becoming victims of predators or road traffic themselves. In addition, their visual system is overtaxed; some frogs take hours to get used to the darkness again.
Some species of frogs mate only at very low levels of light or do not emit their calls under in the presence of light. Migration to their spawning waters is also made more difficult and reproduction is adversely affected.
Some fish species are attracted by artificial light, others are deterred by it. Some react with increasing activity, while others become less active. The migration behaviour and distribution of fish can be affected by lighting located close to the water.
The rainbow trout, for example, feeds mainly on dark nights, and not in artificially illuminated areas or during a full moon.
The fact that some fish species are attracted by artificial light is exploited in commercial fishing, but predatory fish have also discovered the phenomenon to their advantage. In a positive context, the attraction effect is used to guide the fish onto fish ladders on dams.
Artificial light at night also affects the hormonal balance of fish, which in turn influences growth and reproduction.
Artificial light sources located near beaches can cause problems of orientation for freshly hatched turtles especially – often with fatal results.
They hatch out at night when the temperatures are lower. When they emerge from the sand, they have to find their way straight to the sea. For that purpose they make use of local light stimuli; the hatchlings move in the direction of the lighter surface of the sea lower down the beach, in which the light of the celestial bodies is reflected.
Artificial light used to illuminate hotels, residential buildings and streets can disorient the hatchlings so that they move inland, where they are run over by vehicles or die of exhaustion.
The higher plants of our latitudes are active in the summer months and dormant in the winter. Shorter days in autumn stimulate bud growth in trees and shrubs. Longer days in spring and summer cause many wild plants to form blossoms.
Plant growth and development are linked to the natural length of the day (= photoperiod). The photoperiod is a very important stimulus for life processes in plants.
Cascading effect:
Scientists discovered that amber light and, to a lesser extent, white light suppress the flowering of the greater birdsfoot trefoil (Lotus pedunculatus).
When pea aphids (Acyrthosiphon pisum) switched from sucking the plant juices from the shoots to the flowers in August, the number of animals in the amber light boxes in the field trial decreased significantly - presumably due to the fact that fewer inflorescences were present.
Bennie J, Davies TW, Cruse D, Inger R, Gaston KJ (2015) Cascading effects of artificial light at night: resource-mediated control of herbivores in a grassland ecosystem. Phil. Trans. R. Soc. B. 370:20140131. dx.doi.org/10.1098/rstb.2014.0131
Numerous families of plants such as orchids and carnations are dependant on pollination by nocturnal insects. Where there are fewer insects, the plant population is also at risk. The most frequently studied butterfly-flower interactions relate to the owlet and hawk moth families.
