Ecological light pollution

The most obvious change in introducing light at night is the end of darkness in general. The day/night cycle is probably the most powerful environmental behavioral signal, as almost all animals can be categorized as nocturnal or diurnal. If a nocturnal animal is only active in extreme dark, it will be unable to survive in lit areas. The most acute affects are directly next to streetlights and lit buildings, but the diffuse light of skyglow can extend out to hundreds of kilometers away from city centres.

The axial tilt of the Earth results in seasons outside of the tropics. The change in the length of the day, or photoperiod, is the key signal for seasonal behavior (e.g. mating season) in non-tropical animals and plants. The presence of light at night can result in "seasons out of time",[8] changing the behavior, thermoregulation, and hormonal functioning of affected organisms. This may result in a disconnect between body functioning and seasonality, causing disruptions to reproduction, dormancy, and migration.

The behavior of some animals (e.g. coyotes,[9] bats,[10] toads,[11] insects) is keyed to the lunar cycle. Near city centers the level of skyglow often exceeds that of the full moon,[12] so the presence of light at night can alter these behaviors, potentially reducing fitness.

In pristine areas, clouds blot out the stars and darken the night sky, resulting in the darkest possible nights. In urban and suburban areas, in contrast, clouds enhance the effect of skyglow,[12] particularly for longer wavelengths.[13] This means that the typical level of light is much higher near cities, but it also means that truly dark nights never occur in these areas.

The impact of clouds on light levels in urban and suburban ecosystems is completely reversed from what occurs in pristine areas.

The attraction of insects to artificial light is one of the most well known examples of the effect of light at night on organisms. When insects are attracted to lamps they can be killed by exhaustion or contact with the lamp itself, and they are also vulnerable to predators like bats.[5]

Insects are affected differently by the varying wavelengths of light, and many species can see ultraviolet and infrared light that is invisible to humans. Because of variances in perception, moths are more attracted to broad spectrum white and bluish light sources than they are to the yellow light emitted by low pressure sodium-vapor lamps.[14]

Insects killed by attraction to a buried light box.

The compound eye of moths results in fatal attraction to light.[15]

Dragonflies perceive horizontally polarized light as a sign of water. For this reason, sources of water are indistinguishable from asphalt roads with polarized light pollution to them. Dragonflies searching for water either to drink or in which to lay eggs often land on roads or other dark flat reflective surfaces such as cars and remain there until they die of dehydration and hyperthermia.[16]

Light pollution may hamper the mating rituals of fireflies, once they depend on their own light for courtship, resulting in decreased populations.[17][18][19]

Fireflies are charismatic (which is a rare quality amongst insects) and are easily spotted by nonexperts, providing thus good flagship species to attract public attention; good investigation models for the effects of light on nocturnal wildlife; and finally, due to their sensibility and rapid response to environmental changes, good bioindicators for artificial night lighting.[20]

Lights on tall structures can disorient migrating birds leading to fatalities. An estimated 365-988 million fatal bird collisions with buildings occur annually in North America, making human-made structures a large contributor to the decline in bird species.[21] The surface area of glass emitting artificial light at night is a major factor for fatal bird collisions with buildings, and turning off lights at night can minimize these fatalities.[22] The Fatal Light Awareness Program (FLAP) works with building owners in Toronto, Canada and other cities to reduce mortality of birds by turning out lights during migration periods.

Migratory birds confused by the World Trade Center memorial searchlights.

Similar disorientation has also been noted for bird species migrating close to offshore production and drilling facilities. Studies carried out by Nederlandse Aardolie Maatschappij b.v. (NAM) and Shell have led to development and trial of new lighting technologies in the North Sea. In early 2007, the lights were installed on the Shell production platform L15. The experiment proved a great success since the number of birds circling the platform declined by 50–90%.[56] Juvenile seabirds may also be disoriented by lights as they leave their nests and fly out to sea causing events of high mortality.[23] To minimise mortality rescue programs are conducted on many islands giving a second chance to thousands of seabird fledglings. [24]

Brazil star trails and birds in light pollution photography in Rio beach at night

jBirds flying trace and star trail near Rio de Janeiro beach at night time in light pollution

Birds migrate at night for several reasons. Save water from dehydration in hot day flying and part of the bird's navigation system works with stars in some way. With city light outshining the night sky, birds (and also about mammals) no longer navigate by stars.[25]

Ceilometers (searchlights) can be particularly deadly traps for birds,[26] as they become caught in the beam and risk exhaustion and collisions with other birds. In the worst recorded ceilometer kill-off, on October 7–8, 1954, 50,000 birds from 53 different species were killed at Warner Robins Air Force Base.[27]

Lights from seashore developments repel nesting Sea turtle mothers, and their hatchlings are fatally attracted to street and hotel lights rather than to the ocean.[28]

Artificial lighting has many negative impacts on trees and plants, particularly in fall and autumn phenology. Trees and herbaceous plants rely on the photoperiod, or the amount of time in a day where sunlight is available for photosynthesis, to help determine the changing seasons. When the hours of sunlight decrease, plants can recognize that autumn is underway and begin to make preparations for winter dormancy. For example, deciduous trees shift the colour of their leaves to maximize different wavelengths of light that are more prevalent in the fall before eventually dropping them as light becomes too scarce for photosynthesis to be worthwhile. When deciduous trees are exposed to light pollution, they mistake the artificial light for sunlight and retain their green leaves later into the autumn season. This can be dangerous for the tree, as it wastes energy trying to photosynthesize that should be preserved for winter survival. Light pollution can also cause leaf stoma to remain open into the night, which leaves the tree vulnerable to infection and disease.[29]

Similarly, light pollution in the spring can also be dangerous for trees and herbaceous plants. Artificial light causes plants to think that spring has arrived and it is time to begin producing leaves for photosynthesizing again. However, temperatures may not yet be warm enough to support the new leaf buds, and they are susceptible to frost, which can impair future leaf production. Small herbaceous plants that are exposed to artificial lighting potentially face a greater risk, as more of their body is illuminated. Therefore, only the root system is protected, and could potentially not be enough to sustain the whole plant as it tries to remain green through the fall and winter.[30]

Ecological light pollution has also critical effects on marine ecosystems. [31]

Zooplankton (e.g. Daphnia) exhibit diel vertical migration. That is, they actively change their vertical position inside of lakes throughout the day. In lakes with fish, the primary driver for their migration is light level, because small fish visually prey on them. The introduction of light through skyglow reduces the height to which they can ascend during the night.[32] Because zooplankton feed on the phytoplankton that form algae, the decrease in their predation upon phytoplankton may increase the chance of algal blooms, which can kill off the lakes' plants and lower water quality.

Light pollution impacts migration in some species of fish. For example, juvenile chinook salmon are attracted to and slowed down by artificial light. It is possible that artificial light draws them closer to the shoreline, where they face a greater risk of predation from birds and mammals. Artificial lighting also attracts a greater density of piscivorous fish, which have an advantage due to the slower movement of the juvenile fish.[33] Light pollution also has impacts on the hormonal functioning of some fish; European perch and roach both experience reductions in the production of reproductive hormones when exposed to artificial lighting in a rural environment.[34] Artificial light has also been shown to cause disruptions to fish (and zooplankton) in the high Arctic, where fishing boats with lights resulted in a lack of fish up to 200 metres below the water's surface.[35]

At the turn of the century it was discovered that human eyes contain a non-imaging photosensor that is the primary regulator of the human circadian rhythm.[36] This photosensor is particularly affected by blue light, and when it observes light the pineal gland stops the secretion of melatonin. The presence of light at night in human dwellings (or for shift workers) makes going to sleep more difficult and reduces the overall level of melatonin in the bloodstream, and exposure to a low-level incandescent bulb for 39 minutes is sufficient to suppress melatonin levels to 50%.[4][37] Because melatonin is a powerful anti-oxidant, it is hypothesized that this reduction can result in an increased risk of breast and prostate cancer.[38][39]

Other human health effects may include increased headache incidence, worker fatigue, medically defined stress, decrease in sexual function and increase in anxiety.[40][41][42][43] Likewise, animal models have been studied demonstrating unavoidable light to produce adverse effect on mood and anxiety.[44]

[45]== Effects of different wavelengths ==

The effect that artificial light has upon organisms is wavelength dependent. While human beings cannot see ultraviolet light, it is often used by entomologists to attract insects. Generally speaking, blue light is more likely to be damaging to mammals because the non-imaging photoreceptors in mammalian eyes are most sensitive in the blue region.[46] This means that if traditional vapor discharge streetlamps are replaced by white LEDs (which generally emit more of their radiation in the blue part of the spectrum), the ecological impact could be greater even if the total amount of radiated light is decreased.[47]