Artificial light at night (ALAN): its effects on wildlife (and us)
Introduction
As urban landscapes continue to expand, natural ecosystems face increasing pressure, and biodiversity is declining at an unprecedented rate worldwide. Among these pressures, artificial light at night (ALAN) has emerged as a significant yet often overlooked environmental challenge.
Sources of artificial light include streetlights, vehicle headlights, illuminated advertisements, shop fronts, house lights, and garden lighting. Together, these sources contribute to direct illumination and skyglow, extending light pollution far beyond urban centres.
Natural Rhythms and Ecological Disruption by ALAN
All living organisms, including animals, plants, and microbes, have evolved under predictable cycles of daylight, darkness, lunar phases, and seasonal change. Until relatively recently, these natural rhythms remained remarkably stable across ecosystems.
However, rapid urbanisation has increasingly blurred the distinction between day and night, while light pollution now extends into rural and offshore environments. Simultaneously, deforestation and habitat fragmentation have reduced the availability of dark refuges that many species require for survival.
Importantly, approximately 30% of vertebrate species and 60% of invertebrate species worldwide are nocturnal (Hölker et al., 2010). Consequently, ALAN disproportionately affects a substantial proportion of global biodiversity. Insects, in particular, experience severe impacts because many species depend heavily on darkness for navigation, feeding, and reproduction.
ALAN Disrupts Wildlife Behaviour
Artificial light at night disrupts numerous biological processes, including foraging, migration, reproduction, predation, and development. As a result, species often experience reduced fitness and lower survival rates.
For example, migratory birds rely on natural light cues to navigate long distances accurately. However, artificial lighting can mask these cues, causing birds to deviate from established migration routes. In extreme cases, intense light beams can trap birds in illuminated areas, forcing them to fly continuously until exhaustion or death occurs.
Whilst this is not (yet) relevant to the UK, newly hatched sea turtles use moonlight reflected on the ocean surface to locate the sea. However, coastal lighting can confuse hatchlings and redirect them inland, away from water and suitable habitat. Consequently, many hatchlings face increased risks of dehydration, predation, and mortality.
ALAN affects insects and the food web
Artificial light also alters predator–prey relationships by changing the behaviour of both predators and their prey. Species that are naturally vulnerable during bright moonlit conditions often avoid foraging or seeking mates in illuminated environments. Therefore, artificial lighting can reduce feeding opportunities and reproductive success for many nocturnal animals.
In contrast, numerous insects, including moths, are strongly attracted to artificial light sources. These insects frequently circle lights throughout the night, expending valuable energy reserves. Consequently, many individuals die from exhaustion, while others become easy targets for predators concentrated around illuminated areas. Owens et al. (2020) identified ALAN as an important driver of insect declines, demonstrating that artificial lighting can affect insect abundance, behaviour, reproduction, and survival across a wide range of taxa.
ALAN impacts bat movement, feeding and roosting
Moreover, the ecological effects of artificial light extend beyond insects and migratory species, significantly affecting nocturnal mammals such as bats.
Artificial light at night disrupts the movement patterns and behaviour of bats, many of which rely on darkness for commuting, foraging, and accessing roosting sites. Although some bat species tolerate or even exploit illuminated areas where insects congregate, many light-sensitive species actively avoid artificial lighting. Consequently, illuminated roads, gardens, and urban developments can create barriers that fragment habitats and restrict movement between feeding and roosting locations.
Bats emergence from roosts delayed
Furthermore, prolonged exposure to artificial light may delay bats’ emergence from roosts, reducing the time available for feeding and increasing competition for resources. Reduced foraging opportunities can negatively affect body condition, reproductive success, and survival rates.
Research has shown that species such as horseshoe bats are particularly sensitive to light pollution and may abandon otherwise suitable habitats when artificial lighting levels increase. For example, Luo et al. (2021) found that artificial lighting reduced and delayed foraging activity in lesser horseshoe bats, thereby limiting access to food resources during critical periods of nocturnal activity.
Therefore, inappropriate outdoor lighting can contribute to population declines by disrupting essential ecological behaviours and reducing landscape connectivity.
These effects are particularly concerning because bats play vital ecological roles as insect predators, pollinators, and seed dispersers. Moreover, declines in nocturnal insect populations caused by ALAN may further reduce food availability for insectivorous bat species, creating both direct and indirect impacts on bat populations.
Creating wildlife-Friendly Gardens
Given the growing evidence linking ALAN to biodiversity declines, careful consideration of outdoor lighting practices is essential. Homeowners, planners, and communities can help reduce ecological impacts by limiting unnecessary illumination and directing light only where required. Furthermore, selecting wildlife-friendly lighting designs can minimise disruption while maintaining appropriate levels of safety and enjoyment.
Practical measures include using motion sensors instead of continuous lighting, shielding lights to direct illumination downward, reducing brightness levels where possible, and selecting warmer-coloured bulbs with reduced blue wavelengths. In addition, maintaining dark corridors between gardens, hedgerows, woodlands, and other habitats can support the movement of bats and other nocturnal wildlife across the landscape.
Conclusion
Ultimately, reducing light pollution represents a practical and achievable step towards protecting wildlife and preserving natural ecological processes.
By making informed choices about outdoor lighting, individuals can contribute to healthier ecosystems and support biodiversity conservation in both urban and rural environments. As evidence continues to accumulate regarding the ecological impacts of ALAN, integrating wildlife-friendly lighting practices into homes, gardens, and urban planning will become increasingly important for safeguarding biodiversity in the twenty-first century.
References
Hölker, F. et al. (2010). Light pollution as a biodiversity threat. Trends in Ecology & Evolution, 25(12), 681–682.
Owens, A. et al. (2020). Light pollution is a driver of insect declines. Biological Conservation, 241, 108259.
Luo, B. et al (2021). Artificial light reduces foraging opportunities in wild least horseshoe bats. Environmental Pollution, 288, 117765.
Further Information:
Garden Lighting: the effects on wildlife and design solutions. Speaker Dr. Davide Dominoni Glasgow University.
Dark Sky Episode 5: How are bats, birds and insects affected and what do we need to be aware of? Hudson Lighting Ltd.
