The Adverse Ecological Impacts of Light Pollution: LEDs' Role in Mitigation

Humans have produced artificial light at night (ALAN) for millennia, and for almost 150 years, this light has been powered by electricity. The global scale of light pollution continues to grow to this day, with large-scale encroachment into regions of high biodiversity [1]. This continuing rise has led to an increasing focus of research into the ways in which ALAN can have negative impacts both on ecological systems and on human health and wellbeing [2]. Today, ALAN is viewed as an important "driver of environmental change" alongside the likes of climate change and habitat degradation. The scale of concern is so great that ALAN has been implicated in the widely reported declines in insects, dubbed "insectageddon" by the media [3].

In general, ALAN affects ecosystems through two pathways. Direct emissions from lights can be perceived by organisms or can alter the lightscape of their ecosystem. Meanwhile, light can be reflected back from the atmosphere, altering ambient light levels at night in a process known as "sky-glow".

Light Pollution and Ecosystems

Organisms can have their perception of the natural cycles of light and dark disrupted by light pollution (including both direct emissions and sky-glow), or they can be directly impacted by interaction with light sources, which can alter their behavior. Two well-known examples illustrate how these interactions can impact both nocturnal and diurnal organisms: the attraction of night-flying moths to light sources, and the early onset of the "dawn chorus" of songbirds. The reason why moths exhibit "flight to light" behavior remains uncertain, but all currently-prevailing theories relate to the ways in which moths could make use of natural sources of (emitted or reflected) light at night. Regardless of the mechanism, research has shown this phenomenon impacts moths in multiple ways [4], including by impeding both feeding [5] and reproduction [6]. In the case of day-active songbirds, the territorial singing that comprises the dawn chorus takes place at both dawn and dusk, when ambient light conditions are changing. Singing behavior may also vary between seasons, to coincide with the timing of the breeding season, which is often guided by changes in day length. ALAN can mask both of these signals, effectively by extending the period perceived as daylight; this leads both to earlier singing at dawn on a daily basis [7] and to earlier singing within the year [8].

As well as directly affecting organisms, ALAN can also have indirect or cascading effects by disrupting the interactions between species that underpin crucial ecological processes. For example, many fruiting plants rely on animals to disperse their seeds effectively: animals eat the fruit, move onwards, and pass out the seed in a new location where it may germinate, away from the competition of its parent. However, by discouraging fruit-eating bats from foraging, ALAN reduces the proportion of fruits that are harvested, and therefore the proportion of seeds that may be dispersed [9]. Similarly, many plants rely on insect visitors for pollination (the process by which seeds are fertilized), and nocturnal moths provide a substantial part of this pollination service [4]. However, around street lights, the behavior of moths changes so that they carry less pollen from fewer plant species [10], leading plants to set fewer seeds, even in species that are mostly pollinated by day-flying bees [11].

Impacts of ALAN have been demonstrated throughout the natural world: in vertebrate and invertebrate animals, plants, and microbes alike. The complex webs of interactions between species that tie ecosystems together also provide pathways for these negative impacts to indirectly affect more species, and even the assembly of whole ecological communities [12].

Mitigation of Light Pollution

Although levels of concern about the ecological impacts of light pollution have grown substantially [2], reducing these impacts poses a substantial problem. Put simply, ALAN is perceived to be highly desirable for human nighttime safety and security (despite recent concerns about health and wellbeing [2]), putting it into stark contrast with other pollutant drivers of environmental change (e.g. noise or air pollution), which are considered to be undesirable for both humans and ecosystems. As a consequence, much of the recent research in this field has focused, not on the simple effects on ecosystems of ALAN when compared to natural darkness, but on the relative effects of different types of ALAN. An understanding of which lighting systems are least disruptive to the ecology of nocturnal and diurnal organisms will allow us to adopt best practices that mitigate the negative effects of light pollution on biodiversity and human health alike.

In particular, there is a growing focus surrounding the adoption of novel technologies in street lighting, and the potential for these to mitigate - or indeed, worsen - the impacts of ALAN. The increasing use of part-night lighting is one such focus, and several recent studies have suggested that its uptake may be beneficial to ecosystems by allowing a period of full natural darkness, even if this period is somewhat curtailed [13,14]. Most prominent, however, is a focus on the introduction of LED street lights, which have been enthusiastically adopted due to features including greater energy efficiency, long life-span, and "whiter" emissions spectra, benefitting human color perception [15]. Initially, this transition was viewed as unfavorable for biodiversity, even being labeled as a major emerging threat to urban ecosystems [15], because early commercially-available LED street lights mainly emitted light across a broader range of wavelengths than incumbent technologies such as high-pressure sodium, and in particular, contained a greater proportion of blue wavelengths to which many organisms are particularly sensitized [16]. More recently, there has been a growing acceptance that as well as dangers, LED technology also holds considerable potential for developing mitigation strategies, because of its great flexibility. In particular, it has been proposed that LEDs provide opportunities to tune the emissions of lights to specific, customized spectra which are least ecologically damaging [17], and that they are well-suited to adoption of variable lighting strategies that include periods of dimming or switching off [18].

Figure 2: Differences between the properties of LEDs and existing street lighting technologies may provide both threats and opportunities to ecosystems

Figure 2 shows the differences between the spectral composition of two lamp types (LEDs and high‐pressure sodium) and change in light intensity at different distances from the light (when mounted at 4 m height) are shown. LEDs emit a greater proportion of light at shorter blue wavelengths (to which nocturnal wildlife may be particularly sensitive) than HPS lamps. However, HPS lamps scatter light over greater distances, meaning they may affect ecosystems further away. To measure spectral composition, irradiance was measured in the laboratory for one light of each type (in its fixture) in μW/cm² per nm. Intensity is shown as the percentage of the irradiance measured at the wavelength of peak emission. For change in light intensity, measurements of light intensity were taken at ground in the field, at night, beneath a light of each type. Five measurements of light intensity were taken using a handheld light meter (Holdpeak HC‐881C, Holdpeak, Hong Kong), at intervals of 5m between 0m (i.e., directly under the light) and 20m from the light. The mean recorded light intensity (lux) across those five measurements is shown; values plotted on the x‐axis were measured at 0 lux across all readings (this figure is reproduced from [13] under a CC-BY-3.0 license).

Current Evidence for Impacts of Switching to LED Street Lights

Costs

Research focusing on the costs and opportunities surrounding adoption of LED lights has already revealed substantial complexity in the responses of ecosystems to this new technology, especially when compared to the impacts of ALAN from other types of light source.

A key risk associated with transitioning to LED lighting is that, because LEDs are more versatile and cheaper to run than existing technologies, their adoption directly leads to the use of ALAN over greater geographic areas and at higher radiance [19], thereby exposing a larger number of ecosystems to the disruptive influence of light pollution, and to a larger extent. In addition to simply increasing the rate at which additional lighting is installed (because of savings made elsewhere by increased energy efficiency) [19], LEDs may also encourage the use of lighting in novel settings, such as for solar-powered ornamental garden lighting, where species may be exposed to ALAN in environments where they previously experienced natural darkness, leading to direct impacts on their behavior [20]. LEDs may also contribute to increasing sky-glow, because they typically contain a greater proportion of short wavelengths [19]. However, with careful design (e.g. proper shielding), LEDs can enable lighting installations that are less wasteful and produce lower sky-glow: for example, a switch to LEDs from a mixture of metal halide and high-pressure sodium lights for aesthetic lighting of a church in Slovenia reduced waste light by a factor of at least 30 [21].

Beyond the potential impacts of increasing ALAN as a whole, some studies have identified ways in which switching to LED from other lighting technologies directly increases disruption to the ecosystem. Periphyton is the layer of algae and other photosynthetic microorganisms that grows underwater, coating the surface of objects that project above the sediment, including larger plants, and forms an important component of marine and freshwater food webs. The total quantity (or biomass) of periphyton decreases by over 60 % following the introduction of white LEDs (4000 K color temperature), but is unaffected by high-pressure sodium lights, probably because the blue light content of the LEDs, but not the HPS lights, is sufficient to disrupt perception of day-night cycles [22]. At the opposite end of the food chain, LEDs can increase the night-time activity and/or hunting success of visual predators (such as ground-beetles), leading to greater predation, and consequently reduced abundance, of herbivores such as slugs and aphids [23]. The wider impacts of such "top-down effects" on different species as they cascade through a food web can be difficult to predict.

Opportunities

By contrast, the effects of LEDs on the growth and flowering of terrestrial plants may be lower than other light sources [23]; an effect which may also be linked to the spectral composition of LEDs, because blue wavelengths are less important to plant photosynthesis. In the same way that effects on predators can lead to top-down effects in ecosystems, effects on plants (and other primary producers) can lead to cascading "bottom-up effects" which are equally unpredictable. Therefore, LEDs may cause less severe bottom-up effects than other lighting technologies in some settings.

Indeed, several studies appear to show that LEDs are less disruptive - or at least, no more disruptive - to certain elements of ecosystems than the incumbent lighting technologies. Commercially available LED street lights are significantly less attractive to nocturnal insects (including flies, beetles, moths, and other groups) than metal halide lights, and no more attractive than high-pressure sodium (HPS) lights [24]. This effect is the same when comparing domestic lights, with LEDs being less attractive than traditional tungsten filament bulbs and novel compact fluorescent bulbs, regardless of color temperature [25]. Similarly, LEDs had no more impact upon seed set in a nocturnally pollinated plant than HPS lights [13]. Many bat species are also sensitive to ALAN: some (such as the greater and lesser horseshoe bats, Rhinolophus ferrumequinum and R. hipposideros) will avoid light sources, whereas others (such as the common pipistrelle Pipistrellus pipistrellus) are tolerant to lights and may even preferentially hunt in their vicinity (probably responding to the high attracted densities of their insect prey) [26]. LEDs caused less disruption to the activity of both light averse and light-tolerant groups than mercury vapor lights in some studies [27] (but not all [28]), although there was no change to the activity of light-tolerant bats following a switch from low-pressure sodium lights to LEDs [29]. On the whole, these results do not support the widely held expectation that blue-rich LED lighting will increase the disruptive effects of ALAN on insects and other nocturnal wildlife, but more research is necessary.

Figure 3: An increasing number of studies have examined differences between the ecological impacts of commercially available LEDs and existing street lighting technologies. Here, lighting rigs (L: high-pressure sodium; R: commercially-available LED) used in a study of impacts on nocturnal pollination by moths [13] are shown

Most studies to date have pertained to the comparison between the current field of commercially available LEDs and existing lighting technologies. However, some research has also begun to explore the effects of the flexible options available with LEDs. Top-down effects caused by increased nighttime activity of spiders and beetles can be partially reduced both by 50% dimming of LEDs and by manipulating their emissions spectra to reduce the content of blue light [30]. Both of these strategies have independently been shown to have benefits in other systems. Dimming LEDs also reduces impacts on the activity of some bat species (though not all) [31,32]. Red LEDs cause less disruption to mating behaviors and growth of immature stages in various moth species than either green or white LEDs (though even red LEDs caused some disruption compared to natural darkness) [6,33,34]. Most encouragingly, LEDs that had been specifically customized to reduce the attraction of nocturnal insects did indeed attract fewer insects than commercially available LEDs and compact fluorescent lamps, even though the color temperatures were comparable [17]. These results illustrate the great potential for LEDs to enable lighting design that is sensitive to, and actively avoids, the disruption of nocturnal ecosystems.

Unanswered Questions

Whilst much of the early research into the possibilities of LED technology to mitigate ecological impacts is promising, there is much that remains to be investigated before optimal lighting systems can be confidently designed. It is notable that studies comparing the effects of LEDs to existing light types are highly idiosyncratic with regards to which technology forms the comparison group. This variation probably reflects regional variation in incumbent lighting technologies, with researchers tending to draw comparisons between LEDs and the prevailing local technology (e.g. low- and high-pressure sodium lights, in many studies from the UK [13,24,29]). Since the conclusions of similar studies are sometimes different depending on the baseline lighting type (e.g. LEDs disrupt activity of light-tolerant bats less than mercury vapor lights, but no difference to low-pressure sodium [27,29]), there is a need for further research to establish how the impacts of LEDs on a wide variety of species and ecosystem processes differ from the full range of existing lighting technologies.

The scale of the impacts of ALAN may also vary regionally, and is likely to be more acute in regions of higher biodiversity. For example, a far greater proportion of moths transport pollen in Mediterranean regions [35,36] than in the UK and northern Europe [37], where much of the research into the effects of ALAN on nocturnal pollination has taken place [10,13], so cascading ecosystem-level impacts could be larger in the Mediterranean. Understanding the degree to which such variation exists, and how it alters the impacts of ALAN on ecosystems, may enable tailored, context-specific mitigation to be implemented in different regions.

Looking further to the future, there has, to date, been extremely little research into the potential ecological effects of the many novel lighting strategies that are made more feasible by the rise of LEDs. Dimming LEDs by 50 % reduced top-down effects on ecosystems, especially when paired with part-night lighting [30] and reduced impacts on the activity of some (but not all) bat species [31,32]. No further studies have yet been conducted into the broader implications of dimming, whilst no studies at all have to date investigated the ecological effects of motion-sensitive street/footpath lighting, even though this technology is already in use in some locations.

Conclusions

There is great potential for LEDs to enable lighting design that is sensitive to, and actively avoids, the disruption of nocturnal ecosystems, in combination with existing benefits including improved energy efficiency. Current evidence suggests that even the currently available blue-rich LEDs may be less harmful than initially feared, though some studies have found detrimental effects. However, dimming LEDs or tuning their emissions spectra to avoid the wavelengths that nocturnal organisms have high sensitivity to are both viable options to reduce impacts, especially if paired with other, more general, mitigation strategies such as part-night lighting. Other possibilities (e.g. motion-sensitive street lighting) remain to be rigorously tested. In some cases, research in this field has benefitted considerably by active partnership with the lighting industry, and there is a clear and important role for further industry involvement in developing, testing, and ultimately promoting environmentally friendly lighting solutions.

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