Light clutter in Orlando, Florida. Photo: Oksana Hlodan.
Life on Earth has evolved for eons with predictable daily, monthly, and annual patterns of light and dark. The physiology and ecology of species, the interactions between species, and functioning of ecosystems is governed in part by light. In modern times, humans have developed and deployed extensive outdoor and indoor electrical lighting. The outline of these lights is now visible from space.1 By disrupting natural patterns of darkness, artificial light acts as a pollutant, with significant and adverse impacts to ecosystems.
Figure: Earth at Night
A composite image of the Earth at night in 1994-95. This image is actually a composite of hundreds of pictures acquired by three of the four DMSP satellites, which operate in low-altitude polar orbits and have the unique capability to detect low levels of visible-near infrared (VNIR) radiance at night. Image: NASA/Goddard Space Flight Center, ?Scientific Visualization Studio.
Is light pollution a modern problem?
Humans have long known that lights can be used to influence the behavior of other species. For example:
- Early humans used the light of fire to keep predators at bay.
- The philosopher Aristotle (384 BC-322 BC) observed that moths were attracted to a flame.2
- In modern times, lighthouses and light-ships confused birds during their nocturnal migration, sometimes resulting in collision and demise. As early as the 1880s, scientists remarked, “the destruction of birds by light-houses on the coast of the United States must amount to many thousands annually.”3
- Many observations have been made of birds killed in lighted areas, including the first street lights,6 tall lighted structures, ceilometers (vertical lights used to measure the altitude of clouds at airports), and television and radio towers (see review in Gauthreaux et al.7).
- Mortality of sea turtle hatchlings disoriented by lights has been recorded at least since the mid-1900s and has motivated research and mitigation as well.4,5
The constellation Orion, imaged at left from dark skies, and at right from the metropolis of Orem, UT, with a population of about half a million people. Photos taken 8 January 2009 by Jeremy Stanley.
Types of light pollution
- Ecological light pollution has been identified as “artificial light that alters the natural patterns of light and dark in ecosystems.”8
- Astronomical light pollution, which is not the same as ecological light pollution, is light that interferes with the view of the night sky.
- The general term “light pollution” can also refer to light that is aesthetically disruptive.
This time exposure photo of New York City at night, with the Empire State Building at center, shows sky glow, one form of light pollution. Photo taken in October 2004 by Charliebrown7034.
- Light pollution includes sky glow, the reflected light in the sky from artificial lights.
- Direct glare is a light that is directly visible to an observer at night.
- Illumination is the amount of light energy incident on a particular area.
- Luminance is the brightness of a light source against its surroundings.
- Spectrum comprises the different wavelengths in any particular light.
How does light pollution affect wildlife?
Loggerhead turtle hatchlings in Broward County, Florida making it to the ocean just after sunrise. Artificial lighting on nesting beaches disrupts the ability of marine turtle hatchlings to find the sea, an effect termed “hatchling disorientation.” Photo: Mary Wozny, Broward County Sea Turtle Conservation Program; Sea Turtle Image Library.
Light is necessary for some species to become familiar with their environments. It can also serve to extend the activity period of species that are mainly active in the daytime. Known as the “night light niche,”9 the increased activity time may benefit these species, but it comes at the expense of other species with which they compete, or on which they prey. Unfortunately, light can also cause disorientation or act as an unnatural stimulus. Sea turtle hatchlings are disoriented by lights along the beaches where females lay their eggs4,5 (see review in Salmon10). The attraction of insects to lights is another example of this phenomenon, which results in the deaths of billions of insects each summer in Germany alone.11
Many reproductive activities are synchronized by light, or they require specific lighting conditions. Fireflies use lights to attract mates—a communication that artificial lights make less visible.12 The spawning of corals is highly synchronized with lunar cycles,13 which is a method of “predator swamping” that maximizes the survival of the larvae. Other species, such as some insects and frogs, only mate in the darkest part of the night, presumably to minimize the risk of predation.14
Circadian rhythms and physiology (functions)
Animals set their internal clocks based on light cues and light pollution can have weighty consequences. Alterations of the natural pattern of light disrupts “circadian” cycles—a roughly twenty-four-hour cycle in the biochemical, physiological, or behavioral processes of organisms. In many species, exposure to light at night suppresses the production of the hormone melatonin. Light in the shorter wavelengths (e.g., blue) is biologically active, and it is most effective at triggering such physiological changes in animals.15 In humans, decreased concentration of melatonin is associated with increased risk of breast and prostate cancer.16,17 For species that are normally active at night, such as salamanders,18 sugar gliders,19 and flying squirrels,20 encountering light can cause them to delay their nightly activities such as foraging. Light at night can also influence the timing of physiological changes associated with migration.21
Interactions between species are affected by lighting conditions. For example, some lizards are successful in establishing themselves in non-native environments because they have the ability to exploit artificial night lighting successfully.22 Experimental research has shown the decline of one native gecko species in Hawaii when it was out-competed by another species in the presence of clustered insect distributions caused by lights.23 Species can show strong preferences for activity during different light conditions. We now know that changing the lighting could increase or decrease competition between species.
As a general rule, additional light allows predators to find their prey. Some prey species, therefore, evolved to be active only during the dark of night to avoid predators. This pattern is seen across many groups of organisms, including seals that capture juvenile salmon under artificial lights,24 nocturnal rodents that are exposed to predators in various environments,25 and snakes that reduce activity during the full moon when they would be vulnerable to predation from owls.26 The exceptions to this rule are species that seek safety from predators in numbers, such as flocks of birds27 or schools of fish.28 In these instances, additional light may benefit the prey species because it heightens their communal awareness of their environment.
How can light pollution be addressed?
Light pollution is simultaneously extremely simple and extremely difficult to remediate. It is simple because once a light is switched off or redirected, the pollutant is gone from the system with no expensive clean-up effort. Because remediation requires changing the behavior of billions of people and overcoming their various prejudices and attitudes about darkness and artificial lighting at night, it is extraordinarily difficult to control. Local and regional ordinances can educate the public, and such regulations have been shown to address this challenge effectively.10 Efforts to mitigate the effects of light pollution on species and habitats should consider five elements of lighting:
1. Need Is the light needed? The choice not to light may be appropriate in many circumstances—especially in parks or wilderness areas where visitors are prepared for the darkness. In addition, under many circumstances, removing existing lights is an option because they were not needed.
2. Direction All light should be directed where it is needed, and any light escaping in other directions should be eliminated. To reduce sky glow this means using lights that are “full cut-off,” which is defined as a light that emits virtually no light upward and very little light in the 10° angle below the horizon. Depending on where the light is located, additional shielding may be necessary to keep light from spilling into sensitive habitats such as a wetland or forest. Even lights that are directed downward may still cause adverse effects for ecosystems.
3. Intensity Users should only install lights as bright as needed for a particular situation because the influence of a light correlates with its intensity. If an existing light is shielded properly, often less light is just as effective because it is all going where it is desired. For natural areas, intensity should be kept low so that the contrasts between lit and unlit areas are minimized. This increases overall visibility by allowing the human eye to keep some of its adaptation to the dark. When lights are very bright, the eye adapts to this brightness and all else appears as dark shadows. When illumination is closer to ambient conditions, the eye is actually able to see more that is not directly illuminated by the light.29
4. Duration Not every light needs to be on from dusk to dawn. Lighting can be minimized by setting the fixture to turn off after a certain hour (the Dutch government does this with some of its street lights,30), or by putting the light on a motion sensor so that it is only on when needed. Good practices such as turning lights out when they are not needed could go a long way to minimizing light pollution on private property, not to mention the benefits of reducing pollution from energy production and saving money.
5. Spectrum Although all light has some effects on wildlife and habitats, certain spectra are more damaging. Full spectrum light, which has blue and ultraviolet wavelengths, should not be used. Even though such lights allow people to see color at night, the presence of the blue light sends an environmental signal that it is daytime. Ultraviolet light is highly attractive to insects and it should be avoided as well. Longer wavelengths such as yellow and red appear to have fewer impacts in general, although even longer wavelengths were shown to disrupt foraging of beach mice18 and the orientation of some salamanders.31 In the laboratory, some migratory birds were unable to orient under red lights. The research suggested that green should be used on offshore oil platforms to make it safer for these birds to migrate.32
Acknowledgment: This review draws on a previously published paper by the authors in “Frontiers in Ecology and the Environment.”8
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