Light Pollution: Disrupting Photoperiod Animal Behavior

Light pollution is the human-made alteration of outdoor light levels from those occurring naturally. It has harmful effects on wildlife, ecosystems, energy and climate change.

Plants and animals depend on Earth's daily cycle of light and dark to govern life-sustaining behaviours such as reproduction, nourishment, sleep and protection from predators. Artificial light at night (ALAN) disrupts the world's ecosystems. Nocturnal animals sleep during the day and are active at night. ALAN radically alters their nighttime environment by turning night into day.

ALAN can also lead baby sea turtles to their demise. Sea turtles live in the ocean but hatch at night on the beach. Hatchlings find the sea by detecting the bright horizon over the ocean. Artificial lights draw them away from the ocean. In Florida alone, millions of hatchlings die this way every year.

ALAN has also been shown to have devastating effects on many bird species. Birds that migrate or hunt at night navigate by moonlight and starlight. Artificial light can cause them to wander off course and toward the dangerous nighttime landscapes of cities. Every year, millions of birds die colliding with needlessly illuminated buildings and towers. Migratory birds depend on cues from properly timed seasonal schedules. Artificial lights can cause them to migrate too early or too late and miss ideal climate conditions for nesting, foraging and other behaviours.

ALAN from cities and recent urbanisation results in significant sky glow that invades previously dark nights. Light pollution can block the assessment of photoperiodic response and blur trade-offs that evolved to enhance fitness.

ALAN has been shown to have detrimental consequences on immune function in laboratory rodents. Dim and ecologically relevant levels of ALAN impaired daily circulations of monocytes and T-cells, along with a reduction in blood monocytes and Cd68. ALAN also impairs cell-mediated immune responses in nocturnal rodents with robust pineal melatonin rhythms, including Siberian hamsters and rats.

ALAN also alters immune function in diurnal Nile grass rats. In this study, male Nile grass rats increased bactericidal capacity, pinnal swelling in a cell-mediated delayed-type hypersensitivity test, and upregulated antibody production post keyhole lymphocyte hemocyanin.

ALAN has also been shown to have an impact on body weight and metabolism. Exposure to constant light conditions elevate food intake and decrease energy expenditure compared to their counterparts with dark nights, although weight gain preceded the change in energy uptake and expenditure.

ALAN may disrupt photoperiodic time measurement and derail the onset and offset of seasonal adaptations of physiology, morphology and behaviour.

Light pollution disrupts the breeding cycles of sea turtles

Light pollution has a negative impact on sea turtles. Sea turtles use their vision and acute hearing to adapt to the night-time environment. Light pollution can disrupt their ability to search for the ocean and decrease their chances of survival.

Sea turtles nest on beaches and their hatchlings use visual clues to find the ocean. However, when the artificial light is stronger than the natural light, hatchlings become either misoriented, where they move in a circular motion or remain motionless without a clear direction, or disoriented, where they crawl in the opposite direction of the ocean and move towards the artificial lights.

The degree to which hatchlings are disturbed by artificial light depends on its intensity and wavelength. Green turtle hatchlings show positive phototaxis towards short-wave blue light with a weaker response to long-wave red light within the 350 to 540 nm wavelength range.

A study on Lanyu Island, Taiwan, found that nearly 90% of the green turtle hatchlings failed to conduct sea-finding behaviour, even under the full moon. The streetlights thus pose a serious threat to the hatchlings on this island.

A study using a portable lamp to mimic the light pollution of streetlights found that white light had a stronger impact on hatchling sea-finding behaviour than yellow light. When the lamp shield was installed on moonlit nights, more hatchlings were able to find the sea under both white and yellow lights.

It is recommended that light shields be installed on the streetlights of Lanyu Island in order to protect the sea turtle hatchlings effectively.

Light pollution affects the immune systems of birds

Light pollution can also directly impact immune function by impairing daily circulations of monocytes and T-cells, as well as reducing blood monocytes and Cd68. It can also alter immune function in diurnal Nile grass rats, increasing bactericidal capacity, pinnal swelling, and antibody production.

Additionally, light pollution can interfere with the natural trade-offs that have evolved to balance seasonal challenges, resulting in season-specific investments in immune function. For example, short days are often associated with enhanced innate immune responses when reproductive costs are minimal. Light pollution can block the assessment of photoperiodic responses, disrupting these trade-offs and negatively impacting immune function.

Overall, light pollution has been shown to have detrimental effects on the immune systems of birds, altering their natural photoperiodic responses and immune function.

Light pollution affects the reproductive cycles of birds

Light pollution has been shown to have a significant impact on the reproductive cycles of birds. Birds time their reproduction to coincide with peak food availability to feed their young, using daylight cues to breed at a similar time each year.

Research has found that light pollution can cause birds to begin nesting up to a month earlier than normal in open environments, such as grasslands or wetlands, and 18 days earlier in forested environments. This can lead to a mismatch in timing, with hungry chicks hatching before their food is readily available.

The consequences of this can be detrimental to the survival of the chicks. If food is not available when the chicks hatch, these early-season nests may be less successful at fledging at least one chick. This issue is further complicated by climate change, as rising temperatures mean that birds' food is available earlier in the year. Birds that maintain their historical breeding times may have fewer chicks survive because the food source they rely on will already be depleted by the time their chicks hatch.

Some studies have suggested that birds in areas impacted by light pollution may be better able to track climate change and adapt to the shift towards earlier food availability. However, this may be because many studies on bird reproductive cycles have been done in areas exposed to some light pollution, and so the birds' reproductive timing may actually be in response to light cues rather than climate change.

Light pollution can also affect the reproductive cycles of birds by disrupting their immune function. Immune function is often considered a proxy for survival costs in life history approaches, and optimal immune function requires a significant energetic investment for ideal function and protection against disease. Light pollution can disrupt the natural photoperiod response, which can in turn negatively impact immune function and survival.

Overall, light pollution has been shown to have significant impacts on the reproductive cycles of birds, affecting both the timing of nesting and immune function. These impacts can have detrimental effects on bird populations, which have already declined by more than 30% in recent decades.

Light pollution affects the feeding habits of insects

Insects are drawn to artificial light, which can result in their death. Insects are attracted to artificial light sources, such as streetlights, and can become trapped, leading to exhaustion and death. This can also cause a reduction in insect populations, which can have a detrimental effect on the food chain, as insects are a food source for many other animals.

Artificial light can also disrupt the feeding habits of insects by altering their daily and seasonal patterns. Insects are sensitive to changes in day length, and artificial light can interfere with their internal clocks, affecting their feeding and reproductive cycles. This can lead to a mismatch between the availability of food sources and the insects' feeding habits, resulting in reduced food intake and potential population decline.

Additionally, artificial light can affect the migration patterns of insects, causing them to alter their flight paths or migrate at the wrong time of year. This can have consequences for the insects' survival and the ecosystems they inhabit.

Overall, artificial light pollution can have significant impacts on the feeding habits of insects, disrupting their natural behaviours and affecting their populations and the ecosystems they are a part of.

Light pollution affects the reproductive cycles of insects

Insects are highly attracted to artificial light sources, which can result in them being trapped and killed, or exhausted and unable to reproduce. This can have a detrimental effect on insect populations, and the species that rely on them for food or pollination.

Artificial light can also disrupt the reproductive cycles of insects. Insects use day length as a cue for when to reproduce, and artificial light can disrupt this cue, causing insects to reproduce at the wrong time of year. This can result in insects missing out on optimal food sources, or being exposed to harsh weather conditions.

Artificial light can also affect the behaviour of insects, such as their feeding and foraging patterns. This can have knock-on effects on the wider ecosystem, as it can alter the availability of food sources for other species.

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