Introduction

Darkness has been an integrative part of natural history of our planet for billions of years. Until recently, it has also been an integrative part of our daily lives. Now, we are one flip of a switch away from replacing darkness of the night with bright light that brings comfort and familiarity to billions of people around the globe.

Artificial lighting greatly benefits humanity; for a typical day-active species such as humans, it allows us to prolong our activities into the night and increases our feeling of safety. With the growth of the human population, increasing urbanization, access to electricity and to affordable lights, use of artificial light at night has exponentially increased in the last decades. This has transformed the nighttime environment relatively quickly, shaping the modern, urban lifestyle, but also leading to light pollution that obscures the view of the night sky and raises concerns about negative effects of night lights on our health and environment.

On Darkness, Light and Artificial Light at Night

Light pollution is caused by inadequate or excessive use of artificial lights, such as street and road lights, residential lights, decorative and security lights, lights from vehicles, boats, off-shore platforms etc. It disrupts natural light/dark cycles, and increases nocturnal illumination above natural levels in many regions of the world. In nature, where environment changes all the time, light/dark cycles are among most stable and predictable environmental changes. Hence, almost all life on Earth, including humans, evolved to use light as a cue to anticipate environmental changes and adapt their activities and biological rhythms to daily, monthly, seasonal and annual cycles of ambient light. Disruption of these cycles by artificial light at night can impact living beings from microorganisms to humans, in complex ways that we still don't understand. In addition, many species evolved to be night-active, timing their activities to exploit benefits of darkness. An impressive 30% of vertebrates and 60% of invertebrates, including half of almost 900,000 currently described species of insects, are night-active [1].

Figure 2: View of Europe at night, composite image from Suomi NPP satellite data from 2012. Resolution approx. 750m per pixel (Credits: NASA)

Nocturnal animals are adapted to dark and dimly-lit environments; they are able to navigate under faint light of stars and the moon which can easily be outshined by artificial lights or masked by urban skyglow. Due to their exceptional sensitivity to light, nocturnal animals may be particularly sensitive and negatively affected by artificial light at night. We are all familiar with attraction of insects to outdoor lights - but many more subtle impacts on movements and activities of nocturnal insects, e.g. feeding, reproduction and communication, remain hidden from our eyes.

The nighttime environment is a precious resource for most living beings on our planet; but erosion of darkness by the use of artificial lights is one of most rapidly increasing types of environmental degradation. In the second half of the 20th century artificial illumination quickly became widespread, increasing globally by 6% per year (0-20% depending on the region) [2]. Urban centers became flooded with light that reflects and scatters in the sky, extending hundreds of kilometers from its source as low-intensity skyglow. Today, as almost a quarter of the world's land experience unnaturally bright skies due to artificial lights, and this includes 88% of Europe and almost half of the US, artificial light at night is recognized as a component of global change and a biodiversity threat [3] (Figure 2). And the world keeps getting brighter: although the spread of lighting into new areas has slowed down to 2.2% over the last 5 years, the total radiance keeps increasing globally by 1.8% per year [4]. This increase in radiance coincides with the "lighting revolution", the transition to solid-state technology which aims to reduce costs and energy consumption of artificial lighting; however low costs of LEDs seem to, instead, result in greater light use - a so-called rebound effect, which was historically observed to occur in response to increased luminous efficacy [4].

Evidence is accumulating that low-level artificial light at night, as present in urban and sub-urban areas, affects a range of organisms, processes and ecosystems in their surroundings; from microorganisms, plants and animals to humans; from the flowering of plants and migrations of fish and birds to the feeding of bats and moths; from sub-alpine streams and alpine meadows to grasslands and agroecosystems. Direct effects on individual organisms are studied most, while the effects of indirect skyglow on the ecosystems are still largely unknown.

Figure 3: Illuminated road and visible skyglow in a suburban area in the US (Photo Credits: Mark Tuzman/Unsplash)

Importance of Insects for Food Production and Agriculture

Humanity depends on ecological services from natural and managed ecosystems; countless organisms are involved in these complex interactions. Managed ecosystems - agroecosystems - are immensely important for the production of food, timber and fibers, which largely depend on natural processes and wild organisms. For example, around one third of our food, including animal products, relies on crops pollinated by animals. As much as 85% of crops cultivated in Europe rely on animal pollinators for producing seeds and fruits. Not only crops, but most flowering plants - over 90% of around 250,000 species - are pollinated by animals [5]. Animal pollinators thereby sustain much of biodiversity on Earth and help maintain the integrity of most terrestrial ecosystems. Despite the common opinion that bees are the most important pollinators, food production benefits from a variety of pollinators which includes both flying and ground-dwelling arthropods such as moths, butterflies, wild bees, flies, wasps, beetles and ants, but also bats and birds. Agroecosystems therefore greatly benefit from biodiversity - a rich diversity of biological resources, species and natural habitats.

Biodiversity is therefore fundamental to agricultural production and food security. As most diverse and successful organisms on the planet, insects are largely involved and contribute to these processes. Apart from pollination, insects help maintain soil fertility and structure, enhance nutrient cycling and dung burial. They also act as natural enemies of agricultural pests, or as pest species themselves.

Agricultural areas cover 11% of the Earth's land. Spanning across large areas, they are often illuminated by artificial lights from sub-urban areas, road networks and lights from vehicles (Figure 3). Effects of such exposure on agroecosystems have not been systematically studied, however a growing amount of research indicates that streetlights can, directly and indirectly, significantly impact different levels and processes in agroecosystems - most importantly crops, their pests and natural enemies, and pollinators - ultimately affecting food production and biodiversity [6]. These impacts are often species-specific and depend on the spectral composition of the light source.

Effects of Artificial Light at Night on Insects in Agroecosystems

Both flying and ground-dwelling insects provide important ecosystem services to agriculture and can be directly and indirectly affected by artificial light at night. Aphids, commonly known as greenflies or blackflies, are among important pests of agricultural crops. They are often used in ecological studies as a model species for studying plant-insect interactions and trophic cascades. Experiments showed that artificial light at night can have bottom-up effects on aphids, mediated by plants these insects depend on for food, but also top-down effects mediated by their predators (e.g. ladybugs or slugs) or parasites. For example, exposure to artificial light at night can reduce food availability and enhance predation control by ladybeetle, a known visual predator, reducing the numbers of aphids and their parasites over multiple generations [7]. Furthermore, streetlamps can change the composition of insect and invertebrate communities roaming beneath them:

light-sensitive species can retreat to the dark areas, while visual predators become abundant and increase their activities at night, or extend their activities into the day. Day-active predators may also extend their feeding times into the night [8, 9]. Spiders stay close to the lamps and benefit from insects attracted to the lights that end up caught in their webs; while invertebrate pests such as slugs increase in abundance, attracted to artificial lights. By directly affecting their behavior and changing complex interaction networks between the species, artificial light at night can alter abundance of pests and their natural enemies present in the agroecosystems, as well as predatory control between them. These impacts may have complex consequences for agricultural production, and deserve more attention in future research.

Figure 4: Experimental field to study effects of artificial light at night in a rural setting in Westhavelland (Brandenburg), Germany

Among nocturnal insects, impacts of artificial lighting have been most studied on moths, and several different effects have been documented. As with many other insects, many species of moths are attracted to streetlights at night. Species that are not attracted become inactive under nocturnal illumination; they feed and reproduce substantially less than moths in naturally dark areas [10,11]. Moths are important night-active pollinators, but attraction to lights drives them away from the field margins and reduces interactions with the plants they pollinate. Disruption of nocturnal pollen transport and a reduced number of flower visits by moths carrying pollen was confirmed to reduce fruit development in plants [12]. Interestingly, reduced plant reproduction was evident despite the same plants receiving the same number of pollinator visits by other insects during the day. This indicates that both day- and night-active pollinators play complementary roles in plant reproduction and that disruption of nocturnal pollination may propagate to diurnal pollinators through plant-mediated interactions. That artificial light at night can disturb pollination, such an important ecosystem service, can have important consequences for biodiversity, but implications for food production remain to be determined.

Pollinators, moths and insect populations have been declining over several decades at alarming rates. The main causes of such concerning biodiversity loss are considered to be agricultural intensification leading to habitat loss and fragmentation for many species, use of pesticides and climate change; however light pollution could be an important contributor to these negative trends. In the Netherlands, moth species that show the strongest declines in numbers over time were shown to be the same species that are attracted to artificial lights at night [13]. In Germany, areas where dramatic declines in insects were recently observed are also among the most light-polluted areas in the country [6]. Given the many negative effects artificial light at night has on insects, light pollution has to be considered as a large-scale factor and a potential contributor to insect declines. This is particularly important in agroecosystems, where insects are of key importance to biodiversity and food production, and are already under substantial environmental pressure. We may need to develop nocturnal management strategies in agricultural practices to mitigate the negative effects of artificial light on insects and help conserve insect fauna in our landscapes. Recognizing the value of darkness for nature and people and actively preserving dark places, e.g. through establishing dark sky parks and reserves [14] not only provides refuges for nocturnal animals, but also rare and precious places for stargazing and appreciation of the unspoiled night sky, our common and universal heritage.

Effects of Artificial Light at Night on Crops

Plants also rely on seasonal light/ dark cycles to determine the length of their growth season, and time phenological phases during their life history, such as leaf occurrence in spring, flowering, leaf senescence and leaf loss towards winter. In road-side agricultural fields, proximity to streetlights was found to enhance growth while delaying flowering, reproduction and ultimately yield of crops such as soybean and maize [15]. Grasses grow differently when exposed to artificial light at night, they were found to grow less and also form less leaves. Urban trees close to the streetlights extend their vegetation season compared to those in darker areas: they open leaves earlier in spring, change timing of flowering, and delay and prolong leaf loss [16,17]. These subtle shifts in timing of life events can have major implications for health and survival of plants - for example, earlier leaf opening may increase the risk of damage by frost, while delayed flowering may cause a mismatch with the occurrence of key pollinators necessary for the plant's reproduction. In this way, artificial light at night may directly influence crop production in illuminated agroecosystems.

Reducing Light Pollution - Reducing Light Waste

The first step towards reducing light pollution is recognizing that light can be a pollutant. Effects of light pollution are hard to be eliminated without avoiding use of artificial light at night overall, but they can be minimized by responsible and adequate use of lighting. Any measure that reduces light waste will reduce light pollution and its effects on the environment. These include using light when it's needed, in the amount that is needed for its purpose, directing it where it is needed and using warm colors to avoid harmful short wavelengths in nocturnal illumination (Figure 5).

Figure 5: Lighting recommendations for minimizing light pollution by International Dark Sky Association IDA (darksky.org)

According to the latest estimates, artificial lighting consumes still around 20% of global electricity [2]. Streetlights are the biggest single source of light pollution, but often up to 35% of emitted light is wasted due to poor lamp design. Transition to solid-state technology is expected to reduce costs and increase the energy efficiency of artificial lighting, and in many regions of the world, LEDs are replacing traditional streetlights. However, the spectrum of commonly used white LEDs in this application is substantially different from those of traditional lamps, typically with significant amounts of short (blue) light that is known to disrupt biological rhythms in many organisms, including humans. Therefore, a shift to LEDs is expected to increase ecological impacts of light pollution.

Additionally, a rebound effect can outweigh energy savings on global and national scales.

Tuning the spectral composition of LEDs to avoid harmful short wavelengths or choosing warm colors are promising approaches to reduce adverse ecological effects of nocturnal LED lighting. Shielded and full cut-off lamps would minimize light trespass, glare and skyglow. Dynamic and adaptive solutions, such as dimmers, timers and motion sensors would further cut energy costs, reduce nocturnal illumination levels and minimize unnecessary illumination in rural and suburban areas.

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