The effect that artificial light has upon organisms is highly variable,[2] and ranges from beneficial (e.g. increased ability for
predatorspecies to observe
prey) to immediately fatal (e.g.
moths that are attracted to
incandescentlanterns and are killed by the heat). It is also possible for light at night to be both beneficial and damaging for a species. As an example, humans benefit from using indoor artificial light to extend the time available for
work and
play, but the light disrupts the human
circadian rhythm, and the resulting
stress is damaging to health.[3][4]
Through the various effects that
light pollution has on individual species, the
ecology of regions is affected. In the case where two species occupy an identical
niche, the population
frequency of each species may be changed by the introduction of artificial light if they are not equally affected by light at night. For example, some species of
spiders avoid lit areas, while other species willingly build webs directly on lamp posts. Since lamp posts attract many flying
insects,[5] the spiders that tolerate light gain an advantage over the spiders that avoid it, and may become more dominant in the environment as a result.[6] Changes in these species frequencies can then have
knock-on effects, as the interactions between these species and others in the ecosystem are affected and
food webs are altered. These ripple effects can eventually affect
diurnal plants and animals. As an example, changes in the activity of night active insects can change the survival rates of night blooming plants,[7] which may provide food or
shelter for diurnal animals.
The introduction of artificial light disrupts several natural light cycles that arise from the movements of the
Earth,
Moon, and
Sun, as well as from
meteorological factors.
Diurnal (solar) cycle
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.
Seasonal (solar) cycles
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.
Lunar cycles
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.
Cloud coverage
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.
Effects of light pollution on individual organisms
Terrestrial Environment
Insects
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]
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.
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]
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]
Turtles
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]
Plants
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]
Aquatic Environment
Ecological light pollution has also critical effects on marine ecosystems. [31]
Zooplankton
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.
Fish
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]
Humans
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
whiteLEDs (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]
Polarized light pollution
Artificial planar surfaces, such as
glasswindows or
asphalt reflect highly
polarized light. Many insects are attracted to polarized surfaces, because polarization is usually an indicator for water. This effect is called
polarized light pollution,[48] and although it is certainly a form of ecological photopollution, "ecological light pollution" usually refers to the impact of artificial light on organisms.
In the night, the polarization of the moonlit sky is very strongly reduced in the presence of
urbanlight pollution, because scattered urban light is not strongly polarized.[49] Since polarized moonlight is believed to be used by many animals for navigation, this screening is another negative effect of light pollution on ecology.
Prevents and Controls
To regulate and manage the problem of light pollution, it needs to establish a mature management system. Based on Zhou's studies, posing regulations such as green lighting, strengthening the propaganda and education by governors could help stop or reduce the adverse impacts of light pollution.[50]
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