Ecological light pollution refers to how artificial light affects individual living things and the overall balance of ecosystems.
Artificial light can have different effects on organisms. Some effects are helpful, such as when predators can see prey more easily. Other effects can be harmful or even deadly, like when moths are drawn to light sources and die from the heat. Artificial light at night can also be both helpful and harmful to a species. For example, humans benefit from using indoor lights to stay active longer, but this light can disrupt the body’s natural daily cycle, leading to health problems.
The way artificial light affects individual species can change the ecology of an area. When two species share the same role in an ecosystem, artificial light might change how common each species is if they react differently to light. For instance, some spiders avoid areas with light, while others build webs near lamps. Since lamps attract insects, spiders that tolerate light may outcompete those that avoid it, becoming more common. These changes can cause other effects, as interactions between species and food webs are altered. These changes can even affect plants and animals that are active during the day. For example, if night-active insects change their behavior, it can impact the survival of plants that bloom at night, which in turn may affect daytime animals that rely on them.
Adding artificial light at night is one of the most significant changes humans have made to Earth. This change is as serious as other major environmental issues, such as pollution, changes in land use, and climate change caused by rising levels of greenhouse gases.
Natural light cycles
Artificial light changes many natural light patterns that come from the movements of Earth, the Moon, and the Sun, as well as from weather conditions.
The most obvious change from adding light at night is the loss of darkness. The day/night cycle, or diurnal cycle, is a strong signal that influences the behavior of most animals, as nearly all species are either active during the day or at night. If a nocturnal animal needs complete darkness to be active, it may struggle to survive in areas with light. The strongest effects of light pollution are near streetlights and buildings, but the spread of light in the sky, called skyglow, can reach hundreds of kilometers from cities. Even blind individuals often follow the day/night cycle, with about one-quarter of fully blind people staying in sync with it.
Earth's axial tilt causes seasons outside the tropics. Changes in the length of daylight, or photoperiod, signal seasonal behaviors, such as mating, in non-tropical animals and plants. Artificial light at night can cause "seasons out of time," altering behaviors, body temperature regulation, and hormone production in affected organisms. This can create a mismatch between an organism's body functions and the actual season, leading to problems with reproduction, hibernation, and migration.
Some animals, such as coyotes, bats, toads, and insects, rely on the lunar cycle for their behavior. In cities, skyglow often is brighter than a full moon, so artificial light can change these behaviors, possibly harming their survival. Scientists have also found that artificial light at night affects the daily rhythms of nocturnal animals.
In natural areas, clouds hide stars and make nights very dark. In cities and suburbs, however, clouds increase the brightness of skyglow, especially for longer light wavelengths. This means nights near cities are much brighter, and truly dark nights rarely occur in these areas.
This also means that the night sky is becoming harder to see in some places, especially in large cities. Studies show that the night sky is becoming 10% less visible worldwide each year.
Effects on organisms
Terrestrial species are animals that live mostly or entirely on land. This group includes birds, insects, and land mammals.
Insects are often drawn to artificial light, a well-known effect of light at night. When insects gather near lamps, they may become exhausted or injured from touching the light source. They are also more likely to be caught by predators such as bats.
Insects react differently to various light colors. Many can see ultraviolet and infrared light, which humans cannot. Nocturnal insects like moths use light to help them stay at the right height while flying. They turn their bodies toward light sources. However, near artificial lights, this behavior can cause them to circle the light repeatedly, trapping them. Moths are especially attracted to white or blue light because they are sensitive to ultraviolet light.
Dragonflies use horizontally polarized light to find water. Because of this, they may mistake asphalt roads or dark, reflective surfaces like car roofs for water. They may land on these surfaces and die from dehydration or overheating.
Light pollution can harm fireflies, which use their own light to attract mates. This can lead to fewer fireflies. Fireflies are easy to spot and respond quickly to environmental changes, making them useful for studying the effects of artificial light.
Artificial light at night may contribute to the decline of amphibian populations. Many amphibians are active at night, and changes in their natural light-dark cycle can disrupt their biological rhythms. Light pollution can also affect their growth, feeding, mating, and survival strategies.
Most bats are nocturnal and are greatly affected by artificial light. These effects vary by species and situation. Light can disrupt their migration, hunting, roosting, and foraging behaviors.
Many birds migrate at night because it is cooler, which helps them avoid dehydration. However, city lights can make it harder for birds to navigate using the stars. Lights on tall buildings can confuse migrating birds, leading to collisions. In North America, between 365 and 988 million birds die each year from hitting buildings. Turning off lights during migration can help reduce these deaths. Programs like FLAP work with building owners to minimize bird deaths.
Birds near offshore oil and gas platforms may also become disoriented by lights. Experiments with new lighting technologies in the North Sea showed a 50–90% drop in birds circling platforms. Young seabirds may be attracted to lights when leaving their nests, leading to high death rates. Rescue programs on islands help save many seabirds.
Ceilometers, which are powerful lights used for measuring weather, can trap birds in their beams. In 1954, 50,000 birds from 53 species were killed near a military base.
Light pollution can cause birds to mature sexually earlier and forage more aggressively than usual. A study found that light pollution increases the time birds sing by about 50 minutes, especially in species with large eyes, open nests, and long migration ranges.
Artificial lights near beaches can confuse sea turtles. Mother turtles rely on moonlight to guide their hatchlings to the ocean, but lights from streets or hotels often lead hatchlings inland, where they die.
Light pollution harms trees and plants, especially in fall. Plants use the photoperiod, or the length of daylight, to prepare for winter. Artificial light can trick plants into staying green longer, wasting energy they need for survival. It can also keep leaf pores open at night, making plants more likely to get sick. In spring, artificial light may cause plants to grow leaves too early, risking damage if temperatures are too low.
Aquatic animals live in water, such as fish, dolphins, and turtles. Light pollution affects marine ecosystems, with about 22% of the world’s coastline exposed to artificial light at night.
Zooplankton, like Daphnia, move up and down in lakes daily. In lakes with fish, they avoid light because fish hunt them. Light pollution from skyglow limits how high zooplankton can swim at night. This can lead to more algae growth, harming water quality.
Light pollution can also affect the migration of aquatic animals.
Effects of different wavelengths
The effect of artificial light on living things depends on the color of the light. Humans cannot see ultraviolet light, but scientists who study insects often use it to attract bugs. Blue light is more likely to harm mammals because the special light-sensing cells in their eyes are most responsive to blue wavelengths. In humans, exposure to blue light, especially at night, can reduce the production of a hormone called melatonin, which may cause sleep problems and increase stress.
In nature, a process called Rayleigh scattering makes the night sky appear brighter. This happens because shorter wavelengths, like blue light, spread out more in the atmosphere than longer wavelengths, such as red or green light. If traditional streetlights that use vapor discharge are replaced with white LEDs, which produce more light in the blue range, the harm to the environment might increase even if less total light is used.
Polarized light pollution
Artificial flat surfaces, such as glass windows or asphalt, reflect highly polarized light. Polarized light pollution occurs because of the materials and technology we choose to use in construction. This pollution can be lessened by carefully selecting building materials. An example of polarized light is the light reflected by the glass of a watch. Many insects are drawn to polarized surfaces because polarization often signals the presence of water. This effect is called polarized light pollution. While it is a type of ecological light pollution, the term "ecological light pollution" usually describes how artificial light affects living things.
At night, the polarization of moonlight in the sky is greatly reduced when urban light pollution is present. This happens because light scattered from cities is not strongly polarized. Scientists believe many animals use polarized moonlight to navigate. Therefore, the reduction of polarized moonlight is another harmful effect of light pollution on ecosystems.