Artificial light at night causes top‐down and bottom‐up trophic effects on invertebrate populations

Crustacean photoreceptor damage and recovery: Applying a novel scanning electronic microscopy protocol in artificial light studies

Artificial light at night is a worldwide expanding form of pollution. Studies evaluating the effects of artificial light at night have often overlooked their impact on the photoreceptor, the basic functional structure of animals to absorb light. This is essential to understand the mechanisms by which this stressor may be impacting species. This study examined the photoreceptor (rhabdom) of two sandy beach crustaceans exhibiting different light tolerances at night: the amphipod Orchestoidea tuberculata and the isopod Tylos spinulosus. We developed a novel protocol to measure these species' photoreceptor areas and quantify the damage caused by artificial light at night using histological sections and scanning electron microscopy. In the isopod, a species naturally adapted to lower light intensities at night than the amphipod, the rhabdom surface was 20-times larger, and presented a tapetum, an adaptive feature found in species living in low light conditions. A brief exposure to artificial light caused 3-6 times more damage in the isopod's rhabdom. In fact, the light caused structural damage in the isopod's rhabdom but not in the amphipod's rhabdom, with no signs of recovery after 1 and 24 h. These findings suggest that the damage induced by artificial light at night on an organism's photoreceptors is more severe and persistent in species naturally adapted to lower light levels at night. Hence, this type of sensory ecological stressor may act as a novel selection pressure on these species, a concern with wide implications given the ubiquity among animals of the photoreceptor structure and its response to light.

Synergistic effects of climate and urbanisation on the diet of a globally near threatened subtropical falcon

Understanding how human activities affect wildlife is fundamental for global biodiversity conservation. Ongoing land use change and human-induced climate change, compel species to adapt their behaviour in response to shifts in their natural environments. Such responses include changes to a species' diet or trophic ecology, with implications for the wider ecosystem. This is particularly the case for predatory species or those that occupy high positions within trophic webs, such as raptors. Between 2002 and 2019, we observed 1578 feeding events of the globally near threatened and understudied, Red-necked Falcon (Falco chicquera) in Bangladesh. We explored the effects of mean monthly temperature, precipitation, temperature differences, and urban land cover on (a) mean prey weights and (b) dietary composition of 15 falcon pairs. Falcons hunted smaller prey items during months with increased temperatures and precipitation, and in more urban areas. However, during months with increased temperature differences, falcons tended to prey on larger prey items. Being specialist aerial hunters, these dietary patterns were largely driven by the probabilities of bats and birds in the diet. Falcons were more likely to prey on bats during warmer and wetter months. Furthermore, urban pairs tended to prey on bats, whereas more rural pairs tended to prey on birds. Mean monthly temperature difference, i.e., a proxy for climate change, was better at explaining the probability of bats in the falcon diet than mean monthly temperature alone. Anthropogenic dietary shifts can have deleterious effects on species with declining populations or those of conservation concern. The effects of urbanisation and human-induced climate change are expected to continue into the foreseeable future. Therefore, our findings represent a cornerstone in our understanding of how falcons respond to an increasingly human-dominated world.

LED streetlight characteristics alter the functional composition of ground-dwelling invertebrates

Artificial Light at Night (ALAN) has been identified as a primary driver of environmental change in the 21st century with key impacts on ecosystems. At the same time, developments of LED lighting systems with adjustable parameters-such as color temperature and light intensity-may provide an opportunity to mitigate the negative effects of ALAN. To test the potential effects of LED properties, we conducted a comprehensive field study over two summers at three forest sites in Switzerland. We investigated the impact of three key attributes of LED lights (color temperature, brightness, and luminaire shape) on the abundance and community structure of ground-dwelling invertebrate functional groups (predators, omnivores, and detritivores). We found a significantly increased nocturnal attraction of omnivores (+275%) and predators (+70%), but not detritivores, to ALAN, altering arthropod community composition and trophic interactions in forests. LED color temperature and luminaire shape showed minimal effects on all three functional groups, while reducing light level from 100% to 50% attracted fewer individuals in all groups with a significant effect in omnivores (-57%). In addition, we observed significant interactions of color temperatures and luminaire shapes with light intensity, with a decrease in numbers when dimming the light to 50% intensity combined with a color temperature of 3700 K for predators (-53%), with diffusing luminaire shapes for omnivores (-77%) and with standard luminaire shape for detritivores (-27%). The predator-detritivore ratio showed a significant color temperature - light level interaction, with increased numbers of predators around streetlights with 3700 K and 100% intensity, resulting in an elevated top-down pressure on detritivores. These results suggest the importance of considering combined light characteristics in future outdoor lighting designs.

Artificial light affects foraging behavior of a synanthropic bat

Artificial light at night has been considered an emerging threat to global biodiversity. However, the impacts of artificial light on foraging behavior in most wild animals remain largely unclear. Here, we aimed to assess whether artificial light affects foraging behavior in Asian parti-colored bats (Vespertilio sinensis). We manipulated the spectra of light-emitting diode (LED) lighting in a laboratory. Using video and audio recording, we monitored foraging onset, total foraging time, food consumption, freezing behavior (temporary cessation of body movement), and echolocation vocalizations in triads of bats under each lighting condition. Analyses showed that the foraging activities of experimental bats were reduced under LED light. Green, yellow, and red light had greater negative effects on bats' foraging onset, total foraging time, and food consumption than white and blue light. LED light of different spectra induced increased freezing time and echolocation vocalizations in captive bats, except for the white light. The peak wavelength of light emission correlated positively with freezing time, estimated echolocation pulse rate (the number of echolocation pulses per minute), and foraging onset, but negatively with total foraging time and food consumption. These results demonstrate that artificial light disturbs foraging behavior in Asian parti-colored bats. Our findings have implications for understanding the influencing mechanism of light pollution on bat foraging.

When night becomes day: Artificial light at night alters insect behavior under semi-natural conditions

Light is the most important Zeitgeber for temporal synchronization in nature. Artificial light at night (ALAN) disrupts the natural light-dark rhythmicity and thus negatively affects animal behavior. However, to date, ALAN research has been mostly conducted under laboratory conditions in this context. Here, we used the field cricket, Gryllus bimaculatus, to investigate the effect of ALAN on insect behavior under semi-natural conditions, i.e., under shaded natural lighting conditions, natural temperature and soundscape. Male crickets were placed individually in outdoor enclosures and exposed to ALAN conditions ranging from <0.01 to 1500 lx intensity. The crickets' stridulation behavior was recorded for 14 consecutive days and nights and their daily activity patterns were analysed. ALAN impaired the crickets' stridulation rhythm, evoking a change in the crickets' naturally synchronized daily activity period. This was manifested by a light-intensity-dependent increase in the proportion of insects demonstrating an intrinsic circadian rhythm (free-run behavior). This also resulted in a change in the population's median activity cycle period. These ALAN-induced effects occurred despite the crickets' exposure to almost natural conditions. Our findings provide further validity to our previous studies on ALAN conducted under lab conditions and establish the deleterious impacts of ALAN on animal behavioral patterns. TEASER: Artificial light at night alters cricket behavior and desynchronizes their stridulation even under near-natural conditions.

Coping with light pollution in urban environments: Patterns and challenges

Artificial light at night is a growing environmental problem that is especially pronounced in urban environments. Yet, impacts on urban wildlife have received scant attention and patterns and consequences are largely unknown. Here, I present a conceptual framework outlining the challenges species encounter when exposed to urban light pollution and how they may respond through plastic adjustments and genetic adaptation. Light pollution interferes with biological rhythms, influences behaviors, fragments habitats, and alters predation risk and resource abundance, which changes the diversity and spatiotemporal distribution of species and, hence, the structure and function of urban ecosystems. Furthermore, light pollution interacts with other urban disturbances, which can exacerbate negative effects on species. Given the rapid growth of urban areas and light pollution and the importance of healthy urban ecosystems for human wellbeing, more research is needed on the impacts of light pollution on species and the consequences for urban ecosystems.

The impacts of artificial light at night on the ecology of temperate and tropical reefs

Despite 22% of the world's coastal regions experiencing some degree of light pollution, and biologically important artificial light at night (ALAN) reaching large portions of the seafloor (greater than 75%) near coastal developments, the impacts of ALAN on temperate and tropical reefs are still relatively unknown. Because many reef species have evolved in response to low-light nocturnal environments, consistent daily, lunar, and seasonal light cycles, and distinct light spectra, these impacts are likely to be profound. Recent studies have found ALAN can decrease reproductive success of fishes, alter predation rates of invertebrates and fishes, and impact the physiology and biochemistry of reef-building corals. In this paper, we integrate knowledge of the role of natural light in temperate and tropical reefs with a synthesis of the current literature on the impacts of ALAN on reef organisms to explore potential changes at the system level in reef communities exposed to ALAN. Specifically, we identify the direct impacts of ALAN on individual organisms and flow on effects for reef communities, and present potential scenarios where ALAN could significantly alter system-level dynamics, possibly even creating novel ecosystems. Lastly, we highlight large knowledge gaps in our understanding of the overall impact of ALAN on reef systems. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Long-term exposure to experimental light affects the ground-dwelling invertebrate community, independent of light spectra

Our planet endures a progressive increase in artificial light at night (ALAN), which affects virtually all species, and thereby biodiversity. Mitigation strategies include reducing its intensity and duration, and the adjustment of light spectrum using modern light emitting diode (LED) light sources. Here, we studied ground-dwelling invertebrate (predominantly insects, arachnids, molluscs, millipedes, woodlice and worms) diversity and community composition after 3 or 4 years of continued nightly exposure (every night from sunset to sunrise) to experimental ALAN with three different spectra (white-, and green- and red-dominated light), as well as for a dark control, in natural forest-edge habitat. Diversity of pitfall-trapped ground-dwelling invertebrates, and the local contribution to beta diversity, did not differ between the dark control and illuminated sites, or between the different spectra. The invertebrate community composition, however, was significantly affected by the presence of light. Keeping lights off during single nights did show an immediate effect on the composition of trapped invertebrates compared to illuminated nights. These effects of light on species composition may impact ecosystems by cascading effects across the food web. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Part-night exposure to artificial light at night has more detrimental effects on aphid colonies than fully lit nights

Artificial light at night (ALAN) threatens natural ecosystems globally. While ALAN research is increasing, little is known about how ALAN affects plants and interactions with other organisms. We explored the effects of ALAN on plant defence and plant-insect interactions using barley (Hordeum vulgare) and the English grain aphid (Sitobion avenae). Plants were exposed to 'full' or 'part' nights of 15-20 lux ALAN, or no ALAN 'control' nights, to test the effects of ALAN on plant growth and defence. Although plant growth was only minimally affected by ALAN, aphid colony growth and aphid maturation were reduced significantly by ALAN treatments. Importantly, we found strong differences between full-night and part-night ALAN treatments. Contrary to our expectations, part ALAN had stronger negative effects on aphid colony growth than full ALAN. Defence-associated gene expression was affected in some cases by ALAN, but also positively correlated with aphid colony size, suggesting that the effects of ALAN on plant defences are indirect, and regulated via direct disruption of aphid colonies rather than via ALAN-induced upregulation of defences. Mitigating ecological side effects of ALAN is a complex problem, as reducing exposure to ALAN increased its negative impact on insect herbivores. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Skyglow relieves a crepuscular bird from visual constraints on being active

Artificial light at night significantly alters the predictability of the natural light cycles that most animals use as an essential Zeitgeber for daily activity. Direct light has well-documented local impacts on activity patterns of diurnal and nocturnal organisms. However, artificial light at night also contributes to an indirect illumination of the night sky, called skyglow, which is rapidly increasing. The consequences of this wide-spread form of artificial night light on the behaviour of animals remain poorly understood, with only a few studies performed under controlled (laboratory) conditions. Using animal-borne activity loggers, we investigated daily and seasonal flight activity of a free-living crepuscular bird species in response to nocturnal light conditions at sites differing dramatically in exposure to skyglow. We find that flight activity of European Nightjars (Caprimulgus europaeus) during moonless periods of the night is four times higher in Belgium (high skyglow exposure) than in sub-tropical Africa and two times higher than in Mongolia (near-pristine skies). Moreover, clouds darken the sky under natural conditions, but skyglow can strongly increase local sky brightness on overcast nights. As a result, we find that nightjars' response to cloud cover is reversed between Belgium and sub-tropical Africa and between Belgium and Mongolia. This supports the hypothesis that cloudy nights reduce individual flight activity in a pristine environment, but increase it when the sky is artificially lit. Our study shows that in the absence of direct light pollution, anthropogenic changes in sky brightness relieve nightjars from visual constraints on being active. Individuals adapt daily activities to artificial night-sky brightness, allowing them more time to fly than conspecifics living under natural light cycles. This modification of the nocturnal timescape likely affects behavioural processes of most crepuscular and nocturnal species, but its implications for population dynamics and interspecific interactions remain to be investigated.

Dim artificial light at night alters gene expression rhythms and growth in a key seagrass species (Posidonia oceanica)

Artificial light at night (ALAN) is a globally spreading anthropogenic stressor, affecting more than 20% of coastal habitats. The alteration of the natural light/darkness cycle is expected to impact the physiology of organisms by acting on the complex circuits termed as circadian rhythms. Our understanding of the impact of ALAN on marine organisms is lagging behind that of terrestrial ones, and effects on marine primary producers are almost unexplored. Here, we investigated the molecular and physiological response of the Mediterranean seagrass, Posidonia oceanica (L.) Delile, as model to evaluate the effect of ALAN on seagrass populations established in shallow waters, by taking advantage of a decreasing gradient of dim nocturnal light intensity (from < 0.01 to 4 lx) along the NW Mediterranean coastline. We first monitored the fluctuations of putative circadian-clock genes over a period of 24 h along the ALAN gradient. We then investigated whether key physiological processes, known to be synchronized with day length by the circadian rhythm, were also affected by ALAN. ALAN influenced the light signalling at dusk/night in P. oceanica, including that of shorter blue wavelengths, through the ELF3-LUX1-ZTL regulatory network, and suggested that the daily perturbation of internal clock orthologs in seagrass might have caused the recruitment of PoSEND33 and PoPSBS genes to mitigate the repercussions of a nocturnal stress on photosynthesis during the day. A long-lasting impairment of gene fluctuations in sites characterised by ALAN could explain the reduced growth of the seagrass leaves when these were transferred into controlled conditions and without lighting during the night. Our results highlight the potential contribution of ALAN to the global loss of seagrass meadows, posing questions about key interactions with a variety of other human-related stressors in urban areas, in order to develop more efficient strategies to globally preserve these coastal foundation species.

Artificial Light at Night (ALAN) causes size-dependent effects on intertidal fish decision-making

Artificial Light at Night (ALAN) alters cycles of day and night, potentially modifying species' behavior. We assessed whether exposure to ALAN influences decision-making (directional swimming) in an intertidal rockfish (Girella laevisifrons) from the Southeastern Pacific. Using a Y-maze, we examined if exposure to ALAN or natural day/night conditions for one week affected the number of visits and time spent in three Y-maze compartments: dark and lit arms ("safe" and "risky" conditions, respectively) and a neutral "non-decision" area. The results showed that fish maintained in natural day/night conditions visited and spent more time in the dark arm, regardless of size. Instead, fish exposed to ALAN visited and spent more time in the non-decision area and their response was size-dependent. Hence, prior ALAN exposure seemed to disorient or reduce the ability of rock fish to choose dark conditions, deemed the safest for small fish facing predators or other potential threats.

Field experimental evidence of sandy beach community changes in response to artificial light at night (ALAN)

Artificial light at night (ALAN) is a pervasive but still under-recognized driver of global change. In coastal settings, a large majority of the studies assessing ALAN impacts has focused on individual species, even though it is unclear whether results gathered from single species can be used to predict community-wide responses. Similarly, these studies often treat species as single life-stage entities, ignoring the variation associated with distinct life stages. This study addresses both limitations by focusing on the effects of ALAN on a sandy beach community consisting of species with distinct early- and late-life stages. Our hypothesis was that ALAN alters community structure and these changes are mediated by individual species and also by their ontogenetic stages. A field experiment was conducted in a sandy beach of north-central Chile using an artificial LED system. Samples were collected at different night hours (8-levels in total) across the intertidal (9-levels) over several days in November and January (austral spring and summer seasons). The abundance of adults of all species was significantly lower in ALAN treatments. Early stages of isopods showed the same pattern, but the opposite was observed for the early stages of the other two species. Clear differences were detected in the zonation of these species during natural darkness versus those exposed to ALAN, with some adult-juvenile differences in this response. These results support our hypothesis and document a series of changes affecting differentially both early and late life stages of these species, and ultimately, the structure of the entire community. Although the effects described correspond to short-term responses, more persistent effects are likely to occur if ALAN sources become established as permanent features in sandy beaches. The worldwide growth of ALAN suggests that the scope of its effect will continue to grow and represents a concern for sandy beach systems.

Under the influence of light: How light pollution disrupts personality and metabolism in hermit crabs

Anthropogenic disturbances are known to cause significant physiological and behavioural changes in animals and, thus, are the critical focus of numerous studies. Light pollution is an increasingly recognised source of disturbance that has the potential to impact animal physiology and behaviour. Here, we investigate the effect of constant light on a personality trait and metabolic rate in the European hermit crab Pagurus bernhardus. We used Bayesian mixed models to estimate average behavioural change (i.e. sample mean level behavioural plasticity) and between- and within-individual variation in boldness in response to laboratory light. Hermit crabs experiencing constant light were consistently less bold and had a higher metabolic rate than those kept under a standard laboratory light regime (12:12 h light/dark). However, there was no effect of light on individual consistency in behaviour. As boldness is associated with coping with risk, hermit crabs exposed to light pollution at night may experience increased perceived predation risk, adjusting their behaviour to compensate for the increased conspicuousness. However, reduced boldness could lead to lower rates of foraging and this, in combination with elevated metabolic rate, has the potential for a reduction in energy balance.

Effects of artificial light at night on the feeding behaviour of three marine benthic grazers from the Adriatic Sea are species-specific and mostly short-lived

Artificial light at night (ALAN) has the potential to change the day-night activity of marine benthic grazers, and can therefore alter the top-down control they exert on macroalgal communities. In laboratory experiments, we investigated the influence of three realistic ALAN regimes on food consumption and feeding rhythmicity in the sea urchins Arbacia lixula and Paracentrotus lividus as well as in the snail Cerithium spp. from the Adriatic Sea. Food consumption was assessed in assays with algal pellets, while feeding rhythms were documented with 24 h time-lapse photography. Both was done in ALAN-acclimated and in non-acclimated animals. We observed temporary and potential long-term changes in the feeding rhythms of Cerithium spp. and Paracentrotus lividus, respectively, but found no lasting influence of ALAN on consumption rates. Effects were weaker when ALAN was applied only part-night, which suggests a possible mitigation measure to reduce the impact of nighttime lighting on coastal ecosystems.

Artificial light at night may decrease predation risk for terrestrial insects

Artificial light at night (ALAN) is thought to be detrimental for terrestrial insect populations. While there exists evidence for lower abundance under ALAN, underlying mechanisms remain unclear. One mechanism by which ALAN may contribute to insect declines may be through facilitating increased predation. We investigated this by experimentally manipulating insect-substitute abundance under differential levels of light. We used insect-containing birdfeed placed at varying distances from streetlights as a proxy for terrestrial insects, inspecting the rate of predation before and after dusk (when streetlights are, respectively, off and on). We found that there was a significantly greater effect of increasing distance on predation after dusk, suggesting that predation was actually reduced by greater levels of artificial light. This may occur because ALAN also increases the vulnerability of insectivores to their own predators. Implications for foraging behaviour and alternative explanations are discussed.

Lighting up our waterways: Impacts of a current mitigation strategy on riparian bats

Increasing levels of artificial light at night (ALAN) are a major threat to global biodiversity and can have negative impacts on a wide variety of organisms and their ecosystems. Nocturnal species such as bats are highly vulnerable to the detrimental effects of ALAN. A variety of lighting management strategies have been adopted to minimise the impacts of ALAN on wildlife, however relatively little is known about their effectiveness. Using an experimental approach, we provide the first evidence of negative impacts of part-night lighting (PNL) strategies on bats. Feeding activity of Myotis spp. was reduced along rivers exposed to PNL despite no reduction in overall bat activity. We also provide the first evidence of negative effects of PNL on both feeding and activity for Pipistrellus pipistrellus which has previously been recorded feeding under artificial light. Despite having considerable energy-saving benefits, we outline the potential negative impacts of PNL schemes for bats in riparian habitats. PNL are unlikely to provide desired conservation outcomes for bats, and can potentially fragment important foraging habitats leading to a breakdown of functional connectivity across the landscape. We highlight the potential dichotomy for strategies which attempt to simultaneously address climate change and biodiversity loss and recommend alternative management strategies to limit the impacts of ALAN on biodiversity.

Long-term effects of artificial nighttime lighting and trophic complexity on plant biomass and foliar carbon and nitrogen in a grassland community

The introduction of artificial nighttime lighting due to human settlements and transport networks is increasingly altering the timing, intensity, and spectra of natural light regimes worldwide. Much of the research on the impacts of nighttime light pollution on organisms has focused on animal species. Little is known about the impacts of daylength extension due to outdoor lighting technologies on wild plant communities, despite the fact that plant growth and development are under photoperiodic control. In a five-year field experiment, artificial ecosystems ("mesocosms") of grassland communities both alone or in combination with invertebrate herbivores and predators were exposed to light treatments that simulated street lighting technologies (low-pressure sodium, and light-emitting diode [LED]-based white lighting), at ground-level illuminance. Most of the plant species in the mesocosms did not exhibit changes in biomass accumulation after 5 years of exposure to the light treatments. However, the white LED treatment had a significant negative effect on biomass production in the herbaceous species Lotus pedunculatus. Likewise, the interaction between the white LED treatment and the presence of herbivores significantly reduced the mean shoot/root ratio of the grass species Holcus lanatus. Artificial nighttime lighting had no effect on the foliar carbon or nitrogen in most of the grassland species. Nevertheless, the white LED treatment significantly increased the leaf nitrogen content in Lotus corniculatus in the presence of herbivores. Long-term exposure to artificial light at night had no general effects on plant biomass responses in experimental grassland communities. However, species-specific and negative effects of cool white LED lighting at ground-level illuminance on biomass production and allocation in mixed plant communities are suggested by our findings. Further studies on the impacts of light pollution on biomass accumulation in plant communities are required as these effects could be mediated by different factors, including herbivory, competition, and soil nutrient availability.

Spectrum of artificial light at night drives impact of a diurnal species in insect food web

Artificial light at night (ALAN) has become a profound form of global anthropogenic environmental change differing in from natural light regimes in intensity, duration, distribution and spectra. It is clear that ALAN impacts individual organisms, however, population level effects, particularly of spectral changes, remain poorly understood. Here we exposed experimental multigenerational aphid-parasitoid communities in the field to seven different light spectra at night ranging from 385 to 630 nm and compared responses to a natural day-night light regime. We found that while aphid population growth was initially unaffected by ALAN, parasitoid efficiency declined under most ALAN spectra, leading to reduced top-down control and higher aphid densities. These results differ from those previously found for white light, showing a strong impact on species' daytime performance. This highlights the importance of ALAN spectra when considering their environmental impact. ALAN can have large impacts on the wider ecological community by influencing diurnal species.

The Matthew effect: Common species become more common and rare ones become more rare in response to artificial light at night

Artificial light at night (ALAN) has been and still is rapidly spreading and has become an important component of global change. Although numerous studies have tested its potential biological and ecological impacts on animals, very few studies have tested whether it affects alien and native plants differently. Furthermore, common plant species, and particularly common alien species, are often found to benefit more from additional resources than rare native and rare alien species. Whether this is also the case with regard to increasing light due to ALAN is still unknown. Here, we tested how ALAN affected the performance of common and rare alien and native plant species in Germany directly, and indirectly via flying insects. We grew five common alien, six rare alien, five common native, and four rare native plant species under four combinations of two ALAN (no ALAN vs. ALAN) and two insect-exclusion (no exclusion vs. exclusion) treatments, and compared their biomass production. We found that common plant species, irrespective of their origin, produced significantly more biomass than rare species and that this was particularly true under ALAN. Furthermore, alien species tended to show a slightly stronger positive response to ALAN than native species did (p = .079). Our study shows that common plant species benefited more from ALAN than rare ones. This might lead to competitive exclusion of rare species, which could have cascading impacts on other trophic levels and thus have important community-wide consequences when ALAN becomes more widespread. In addition, the slightly more positive response of alien species indicates that ALAN might increase the risk of alien plant invasions.

Artificial light at night (ALAN) causes variable dose-responses in a sandy beach isopod

Artificial Light at Night (ALAN) is expanding worldwide, and the study of its influence remains limited mainly to documenting impacts, overlooking the variation in key characteristics of the artificial light such as its intensity. The potential dose-response of fitness-related traits to different light intensities has not been assessed in sandy beach organisms. Hence, this study explored dose-responses to ALAN by exposing the intertidal sandy beach isopod Tylos spinulosus to a range of light intensities at night: 0 (control), 20, 40, 60, 80 and 100 lx. We quantified the response of this species at the molecular (RNA:DNA ratios), physiological (absorption efficiency) and organismal (growth rate) levels. Linear and non-linear regressions were used to explore the relationship between light intensity and the isopod response. The regressions showed that increasing light intensity caused an overall ~ threefold decline in RNA:DNA ratios and a ~ threefold increase in absorption efficiency, with strong dose-dependent effects. For both response variables, non-linear regressions also identified likely thresholds at 80 lx (RNA:DNA) and 40 lx (absorption efficiency). By contrast, isopod growth rates were unrelated (unaltered) by the increase in light intensity at night. We suggest that ALAN is detrimental for the condition of the isopods, likely by reducing the activity and feeding of these nocturnal organisms, and that the isopods compensate this by absorbing nutrients more efficiently in order to maintain growth levels.

Monitoring Light Pollution with an Unmanned Aerial Vehicle: A Case Study Comparing RGB Images and Night Ground Brightness

There are several tools and methods to quantify light pollution due to direct or reflected light emitted towards the sky. Unmanned aerial vehicles (UAV) are still rarely used in light pollution studies. In this study, a digital camera and a sky quality meter mounted on a UAV have been used to study the relationship between indices computed on night images and night ground brightness (NGB) measured by an optical device pointed downward towards the ground. Both measurements were taken simultaneously during flights at an altitude of 70 and 100 m, and with varying exposure time. NGB correlated significantly both with the brightness index (−0.49 ÷ −0.56) and with red (−0.52 ÷ −0.58) and green band indices (−0.42 ÷ −0.58). A linear regression model based on the luminous intensity index was able to estimate observed NGB with an RMSE varying between 0.21 and 0.46 mpsas. Multispectral analysis applied to images taken at 70 m showed that increasing exposure time might cause a saturation of the colors of the image, especially in the red band, that worsens the correlation between image indices and NGB. Our study suggests that the combined use of low cost devices such as UAV and a sky quality meter can be used for assessing hotspot areas of light pollution originating from the surface.

A Systematic Review for Establishing Relevant Environmental Parameters for Urban Lighting: Translating Research into Practice

The application of lighting technologies developed in the 20th century has increased the brightness and changed the spectral composition of nocturnal night-time habitats and night skies across urban, peri-urban, rural, and pristine landscapes, and subsequently, researchers have observed the disturbance of biological rhythms of flora and fauna. To reduce these impacts, it is essential to translate relevant knowledge about the potential adverse effects of artificial light at night (ALAN) from research into applicable urban lighting practice. Therefore, the aim of this paper is to identify and report, via a systematic review, the effects of exposure to different physical properties of artificial light sources on various organism groups, including plants, arthropods, insects, spiders, fish, amphibians, reptiles, birds, and non-human mammals (including bats, rodents, and primates). PRISMA 2020 guidelines were used to identify a total of 1417 studies from Web of Science and PubMed. In 216 studies, diverse behavioral and physiological responses were observed across taxa when organisms were exposed to ALAN. The studies showed that the responses were dependent on high illuminance levels, duration of light exposure, and unnatural color spectra at night and also highlighted where research gaps remain in the domains of ALAN research and urban lighting practice. To avoid misinterpretation, and to define a common language, key terminologies and definitions connected to natural and artificial light have been provided. Furthermore, the adverse impacts of ALAN urgently need to be better researched, understood, and managed for the development of future lighting guidelines and standards to optimize sustainable design applications that preserve night-time environment(s) and their inhabiting flora and fauna.

Landscape composition drives the impacts of artificial light at night on insectivorous bats

Among the most prevalent sources of biodiversity declines, Artificial Light At Night (ALAN) is an emerging threat to global biodiversity. Much knowledge has already been gained to reduce impacts. However, the spatial variation of ALAN effects on biodiversity in interaction with landscape composition remains little studied, though it is of the utmost importance to identify lightscapes most in need of action. Several studies have shown that, at local scale, tree cover can intensify positive or negative effects of ALAN on biodiversity, but none have - at landscape scale - studied a wider range of landscape compositions around lit sites. We hypothesized that the magnitude of ALAN effects will depend on landscape composition and species' tolerance to light. Taking the case of insectivorous bats because of their varying sensitivity to ALAN, we investigated the species-specific activity response to ALAN. Bat activity was recorded along a gradient of light radiance. We ensured a large variability in landscape composition around 253 sampling sites. Among the 13 bat taxa studied, radiance decreased the activity of two groups of the slow-flying gleaner guild (Myotis and Plecotus spp.) and one species of the aerial-hawking guild (Pipistrellus pipistrellus), and increased the activity of two species of the aerial-hawking guild (Pipistrellus kuhlii and Pipistrellus pygmaeus). Among these five effects, the magnitude of four of them was driven by landscape composition. For five other species, ALAN effects were only detectable in particular landscape compositions, making the main effect of radiance undetectable without account for interactions with landscape. Specifically, effects were strongest in non-urban habitats, for both guilds. Results highlight the importance to prioritize ALAN reduction efforts in non-urban habitats, and how important is to account for landscape composition when studying ALAN effects on bats to avoid missing effects.

Artificial Light at Night Impacts the Litter Layer Invertebrate Community With No Cascading Effects on Litter Breakdown

Artificial light at night (ALAN) can impact the trophic structure of assemblages of ground-dwelling invertebrates, and changes in such assemblages can affect decomposition in terrestrial systems due to the various functional roles of these invertebrates, including microbial grazing, comminution of litter, and predation of other invertebrates, that can directly or indirectly affect plant-litter breakdown. Despite this, we are unaware of any studies that have evaluated the effects of ALAN on the breakdown of plant litter in a terrestrial ecosystem. We sought to answer whether ALAN affects litter breakdown via its effects on a community of ground-dwelling arthropods using two field experiments. In one experiment, we manipulated the presence of ALAN and the size classes of soil invertebrates that could enter mesh bags containing plant litter (litterbags). We found that the rate of plant-litter breakdown increased with the mesh size of litterbags but was unaffected by presence of ALAN. In a second field experiment carried out to examine the effects of ALAN on the trophic structure of litter-layer invertebrate communities, while controlling for potential effects of ALAN on vegetation, we again found that ALAN did not affect litter breakdown despite the fact that ALAN increased the abundances of secondary and tertiary consumers. Our finding that larger assemblages of ground-dwelling secondary and tertiary consumer invertebrates under ALAN did not slow litter breakdown through increased top-down control of detritivores suggests ALAN may disrupt predator-prey interactions in litter-layer communities.

Artificial light at night disrupts species interactions and changes insect communities

Artificial light at night (ALAN) is globally increasing, posing a threat to biodiversity. The impact of nocturnal illumination on individual insects has been relatively well documented. Recent studies show that ALAN also impacts species interactions, including intra-specific communication, trophic interactions and plant-pollinator interactions, with cascading effects in the ecosystem and impacts on ecosystem functioning that extend beyond nocturnal communities and illuminated areas. Reduced population sizes and changes in community composition because of exposure to ALAN have been reported but the understanding of the impacts of ALAN on insect communities is currently limited to few groups and ecosystems. The theoretical framework on how ALAN impacts insect communities and populations is poorly developed, limiting our understanding and the formulation of relevant hypotheses.

Artificial night light alters ecosystem services provided by biotic components

The global catastrophe of natural biodiversity and ecosystem services are expedited with the growing human population. Repercussions of artificial light at night ALAN are much wider, as it varies from unicellular to higher organism. Subsequently, hastened pollution and over exploitation of natural resources accelerate the expeditious transformation of climatic phenomenon and further cause global biodiversity losses. Moreover, it has a crucial role in global biodiversity and ecosystem services losses via influencing the ecosystem biodiversity by modulating abundance, number and aggregation at every levels as from individual to biome levels. Along with these affects, it disturbs the population, genetics and landscape structures by interfering inter- and intra-species interactions and landscape formation processes. Furthermore, alterations in normal light/dark (diurnal) signalling disrupt the stable physiological, biochemical, and molecular processes and modulate the regulating, cultural and provisioning ecosystem services and ultimately disorganize the stable ecosystem structure and functions. Moreover, ALAN reshapes the abiotic component of the ecosystem, and as a key component of global warming via producing greenhouse gases via emitting light. By taking together the above facts, this review highlights the impact of ALAN on the ecosystem and its living and non-living components, emphasizing to the terrestrial and aquatic ecosystem. Further, we summarize the means of minimizing strategies of ALAN in the environment, which are very crucial to reduce the further spread of night light contamination in the environment and can be useful to minimize the drastic impacts on the ecosystem.

Disruptive effect of artificial light at night on leaf litter consumption, growth and activity of freshwater shredders

Artificial light at night (ALAN) is a globally widespread phenomenon potentially affecting ecosystem processes, such as leaf litter breakdown, which is a source of organic matter in fresh waters. Here, we conducted a long-term experiment to test the effects of ALAN (2 lx) differing in spectral composition: white LEDs and high pressure sodium lamps (HPS) on leaf consumption, growth and activity of two macroinvertebrate species of shredders: Gammarus jazdzewskii and Dikerogammarus villosus (Crustacea, Amphipoda), compared to the undisturbed light-dark cycle. We also tested if the nocturnal illumination would influence the algal community colonising leaves, which is an important component of the leaf-shredder diet. We found that LED light increased the consumption of leaves by both species, which was nearly twice as high as in other treatments, and supressed the growth rate of G. jazdzewskii, whereas the growth of D. villosus was not affected by either light type. Moreover, D. villosus reduced its activity when exposed to ALAN of both types. As ALAN-induced changes in shredder growth and consumption were not associated with their increased activity or decreased food quality, we suggest that LED light may be a source of physiological stress for shredders, raising their energy expenditure, which was compensated by increased food intake. We have shown that LED illumination induces greater effects on wildlife than alternative, narrow wavelength spectrum light sources, such as HPS lamps, and may potentially alter the litter breakdown in aquatic ecosystems. It may accelerate the turnover of leaves by shredders, but on the other hand, it may negatively affect the fitness of macroinvertebrates and thus disturb the leaf processing over a longer term.

The impact of artificial light at night on taxonomic, functional and phylogenetic bird species communities in a large geographical range: A modelling approach

Artificial light at night (ALAN) is currently recognised as an important environmental disturbance that influences habitats, fitness and behaviour of numerous organisms. However, its effect on bird community distribution on a large spatial scale still remains unclear. Therefore, I decided to use a predictive approach to test an assumption that artificial nightlight, as one of 73 predictors, determines taxonomic, functional and phylogenetic levels of an avian community. In order to safeguard inference from any inconsistency, I used not one but four indices describing functional diversity, two measures showing phylogenetic species richness, and one reflecting taxonomic diversity. For all these measures of species communities I developed two sets of Random Forest models: one set included ALAN as an additional predictor, while the other did not. Following cross validation tests as well as an independent evaluation of models, I demonstrated that artificial night light improved the performance of predictive models. Taxonomic species richness decreased linearly along with increasing artificial luminescence. Moreover, functional diversity showed a unimodal relation to ALAN, which meant that most niches were occupied on a moderate level of artificial lighting. Finally, phylogenetic diversity was under the highest pressure of ALAN, because even a minimal amount of artificial night lighting radically reduced this measure of biodiversity. On the basis of predictive maps, I also found that models which did not include urbanisation processes showed high values of avian biodiversity in regions where in fact they were low. Thus, I conclude that ALAN as a human footprint can play a key role when analysing the distribution of bird communities on large spatial scales.

The diversity of photosensitivity and its implications for light pollution

Artificial light at night (ALAN) is a pervasive anthropogenic pollutant, emanating from urban and suburban developments and reaching nearly all ecosystems from dense forests to coastlines. One proposed strategy for attenuating the consequences of ALAN is to modify its spectral composition to forms that are less disruptive for photosensory systems. However, ALAN is a complicated pollutant to manage due to the extensive variation in photosensory mechanisms and the diverse ways these mechanisms manifest in biological and ecological contexts. Here, we highlight the diversity in photosensitivity across taxa and the implications of this diversity in predicting biological responses to different forms of night lighting. We curated this paper to be broadly accessible and inform current decisions about the spectrum of electric lights used outdoors. We advocate that efforts to mitigate light pollution should consider the unique ways species perceive ALAN, as well as how diverse responses to ALAN scale up to produce diverse ecological outcomes.

Effects of artificial light on growth, development, and dispersal of two North American fireflies (Coleoptera: Lampyridae)

Holometabolous insects exhibit complex life cycles in which both morphology and ecological niche change dramatically during development. In the larval stage, many insects have soft, slow-moving bodies and poor vision, limiting their ability to respond to environmental threats. Artificial light at night (ALAN) is an environmental perturbation known to severely impact the fitness of adult insects by disrupting both temporal and spatial orientation. The impact of ALAN on earlier life stages, however, is largely unknown. We conducted a series of laboratory experiments to investigate how two distinct forms of ALAN affect the development and movement of immature Photuris sp. and Photinus obscurellus fireflies. Although long-term exposure to dim light at night (dLAN), akin to urban skyglow, did not impact overall survivorship or duration of egg, larval, and pupal stages in either species, it did accelerate weight gain in early-instar Photuris larvae. Late-instar Photuris exposed to point sources of ALAN at the start of their nightly foraging period were also significantly more likely to burrow beneath the soil surface, rather than disperse across it. ALAN may therefore impede dispersal of firefly larvae away from illuminated areas, which could have downstream consequences for the reproductive fitness of adults.

Impact of artificial light at night on diurnal plant-pollinator interactions

Artificial light at night has rapidly spread around the globe over the last decades. Evidence is increasing that it has adverse effects on the behavior, physiology, and survival of animals and plants with consequences for species interactions and ecosystem functioning. For example, artificial light at night disrupts plant-pollinator interactions at night and this can have consequences for the plant reproductive output. By experimentally illuminating natural plant-pollinator communities during the night using commercial street-lamps we tested whether light at night can also change interactions of a plant-pollinator community during daytime. Here we show that artificial light at night can alter diurnal plant-pollinator interactions, but the direction of the change depends on the plant species. We conclude that the effect of artificial light at night on plant-pollinator interactions is not limited to the night, but can also propagate to the daytime with so far unknown consequences for the pollinator community and the diurnal pollination function and services they provide.

Bats seek refuge in cluttered environment when exposed to white and red lights at night

BACKGROUND

Artificial light at night is recognized as an increasing threat to biodiversity. However, information on the way highly mobile taxa such as bats spatially respond to light is limited. Following the hypothesis of a behavioural adaptation to the perceived risks of predation, we hypothesised that bats should avoid lit areas by shifting their flight route to less exposed conditions.

METHODS

Using 3D acoustic localization at four experimentally illuminated sites, we studied how the distance to streetlights emitting white and red light affected the Probability of bats Flying Inside the Forest (PFIF) versus along the forest edge.

RESULTS

We show that open-, edge-, and narrow-space foraging bats strongly change flight patterns by increasing PFIF when getting closer to white and red streetlights placed in the forest edge. These behavioural changes occurred mainly on the streetlight side where light was directed.

CONCLUSIONS

The results show that bats cope with light exposure by actively seeking refuge in cluttered environment, potentially due to involved predation risks. This is a clear indication that bats make use of landscape structures when reacting to light, and shows the potential of vegetation and streetlight orientation in mitigating effects of light. The study nevertheless calls for preserving darkness as the most efficient way.

Pollination and fruit infestation under artificial light at night:light colour matters

Rapid human population growth and associated urbanization lead to increased artificial illumination of the environment. By changing the natural light–dark cycle, artificial lighting can affect the functioning of natural ecosystems. Many plants rely on insects in order to reproduce but these insects are known to be disturbed by artificial light. Therefore, plant–insect interactions may be affected when exposed to artificial illumination. These effects can potentially be reduced by using different light spectra than white light. We studied the effect of artificial lighting on plant–insect interactions in the Silene latifolia–Hadena bicruris system using a field set-up with four different light treatments: red, green, white and a dark control. We compared the proportion of fertilized flowers and fertilized ovules as well as the infestation of fruits by Hadena bicruris, a pollinating seed predator. We found no difference in the proportion of fertilized flowers among the treatments. The proportion of fruits infested by H. bicruris was however significantly higher under green and white light and a significantly lower proportion of fertilized ovules was found under green light. We show that artificial light with different colours impacts plant–insect interactions differently, with direct consequences for plant fitness.

Artificial light at night at the terrestrial-aquatic interface: Effects on predators and fluxes of insect prey

The outcomes of species interactions–such as those between predators and prey–increasingly depend on environmental conditions that are modified by human activities. Light is among the most fundamental environmental parameters, and humans have dramatically altered natural light regimes across much of the globe through the addition of artificial light at night (ALAN). The consequences for species interactions, communities and ecosystems are just beginning to be understood. Here we present findings from a replicated field experiment that simulated over-the-water lighting in the littoral zone of a small lake. We evaluated responses by emergent aquatic insects and terrestrial invertebrate communities, and riparian predators (tetragnathid spiders). On average ALAN plots had 51% more spiders than control plots that were not illuminated. Mean individual spider body mass was greater in ALAN plots relative to controls, an effect that was strongly sex-dependent; mean male body mass was 34% greater in ALAN plots while female body mass was 176% greater. The average number of prey items captured in spider webs was 139% greater on ALAN mesocosms, an effect attributed to emergent aquatic insects. Non-metric multidimensional scaling and a multiple response permutation procedure revealed significantly different invertebrate communities captured in pan traps positioned in ALAN plots and controls. Control plots had taxonomic-diversity values (as H’) that were 58% greater than ALAN plots, and communities that were 83% more-even. We attribute these differences to the aquatic family Caenidae which was the dominant family across both light treatments, but was 818% more abundant in ALAN plots. Our findings show that when ALAN is located in close proximity to freshwater it can concentrate fluxes of emergent aquatic insects, and that terrestrial predators in the littoral zone can compound this effect and intercept resource flows, preventing them from entering the terrestrial realm.

Interactions between artificial light at night, soil moisture, and plant density affect the growth of a perennial wildflower

Artificial light at night (ALAN) has been shown to alter aspects of plant growth, but we are not aware of any studies that have examined whether the effects of ALAN on plants depend upon the backdrop of variation in other abiotic factors that plants encounter in field populations. We conducted a field experiment to investigate whether ALAN affects the growth and anti-herbivore defenses of common milkweed, Asclepias syriaca, and whether the effects of ALAN are influenced by plant density or soil moisture content. Artificial light at night, soil moisture, and plant density were manipulated according to a split-plot factorial design. Although increasing soil moisture by watering had no significant effects on latex exudation, attributes of plant growth generally responded positively to watering. The basal stem diameter (BSD) and height of plants were affected by ALAN × soil moisture interactions. For both of these variables, the positive effects of ALAN were greater for plants that were not watered than for plants that were. Basal stem diameter was also affected by an ALAN × plant density interaction, and the positive effect of ALAN on BSD was greater in the low-density treatment than in the high-density treatment. Our results demonstrate that the effects of ALAN on plant growth can be altered by soil moisture and plant density. Consequently, the effects of ALAN on plants in nature may not be consistent with existing frameworks that do not account for critical abiotic variables such as water availability or biotic interactions between plants such as competition.

National Scale Spatial Variation in Artificial Light at Night

The disruption to natural light regimes caused by outdoor artificial nighttime lighting has significant impacts on human health and the natural world. Artificial light at night takes two forms, light emissions and skyglow (caused by the scattering of light by water, dust and gas molecules in the atmosphere). Key to determining where the biological impacts from each form are likely to be experienced is understanding their spatial occurrence, and how this varies with other landscape factors. To examine this, we used data from the Visible Infrared Imaging Radiometer Suite (VIIRS) day/night band and the World Atlas of Artificial Night Sky Brightness, to determine covariation in (a) light emissions, and (b) skyglow, with human population density, landcover, protected areas and roads in Britain. We demonstrate that, although artificial light at night increases with human density, the amount of light per person decreases with increasing urbanization (with per capita median direct emissions three times greater in rural than urban populations, and per capita median skyglow eleven times greater). There was significant variation in artificial light at night within different landcover types, emphasizing that light pollution is not a solely urban issue. Further, half of English National Parks have higher levels of skyglow than light emissions, indicating their failure to buffer biodiversity from pressures that artificial lighting poses. The higher per capita emissions in rural than urban areas provide different challenges and opportunities for mitigating the negative human health and environmental impacts of light pollution.

A question of size and fear: competition and predation risk perception among frugivores and predators

Mammalian spatial and temporal activity patterns can vary depending on foraging behavior or the perception of predation or competition risk among species. These behaviors may in turn be altered by human influences such as defaunation. Herein, we evaluate whether frugivores avoid areas with high visitation rates by potential predators or competitors, and whether this avoidance changes in areas with different degrees of defaunation. We installed 189 cameras under fruit trees in six areas of the Atlantic Forest, Brazil, that differ in the abundance of top predators and large frugivores. Small predators and small frugivores were more frequent at night while large frugivores were more frequent during the day, but small frugivores visited and spent less time at fruiting trees on brighter nights, unlike large predators and large frugivores. Small frugivores also were less frequent in areas with high visitation by large frugivores and more frequent in highly defaunated areas. Our results suggest that the dynamics among mammalian functional groups varied according to diel patterns, potential competitors, and defaunation. We highlight the importance of understanding how species interactions are changing in areas exposed to strong human impacts to mitigate the indirect effects of defaunation.

Effects of Artificial Light at Night (ALAN) on European Hedgehog Activity at Supplementary Feeding Stations

: Artificial light at night (ALAN) can have negative consequences for a wide range of taxa. However, the effects on nocturnal mammals other than bats are poorly understood. A citizen science camera trapping experiment was therefore used to assess the effect of ALAN on the activity of European hedgehogs (Erinaceus europaeus) at supplementary feeding stations in UK gardens. A crossover design was implemented at 33 gardens with two treatments-artificial light and darkness-each of which lasted for one week. The order of treatment depended on the existing lighting regime at the feeding station: dark treatments were applied first at dark feeding stations, whereas light treatments were used first where the station was already illuminated. Although temporal changes in activity patterns in response to the treatments were noted in some individuals, the direction of the effects was not consistent. Similarly, there was no overall impact of ALAN on the presence or feeding activities of hedgehogs in gardens where supplementary feeding stations were present. These findings are somewhat reassuring insofar as they demonstrate no net negative effect on a species thought to be in decline, in scenarios where the animals are already habituated to supplementary feeding. However, further research is needed to examine long-term effects and the effects of lighting on hedgehog prey, reproductive success and predation risk.

Artificial light at night alters the sexual behaviour and fertilisation success of the common toad

Artificial Light At Night (ALAN) is an emerging pollution, that dramatically keeps on increasing worldwide due to urbanisation and transport infrastructure development. In 2016, it nearly affected 23% of the Earth's surface. To date, all terrestrial and aquatic ecosystems have been affected. The disruption of natural light cycles due to ALAN is particularly expected for nocturnal species, which require dark periods to forage, move, and reproduce. Apart from chiropterans, amphibians contain the largest proportion of nocturnal species among vertebrates exhibiting an unfavourable conservation status in most parts of the world and living in ALAN polluted areas. Despite the growing number of studies on this subject, our knowledge on the direct influence of nocturnal lighting on amphibians is still scarce. To better understand the consequences of ALAN on the breeding component of amphibian fitness, we experimentally exposed male breeding common toads (Bufo bufo) to ecologically relevant light intensities of 0.01 (control), 0.1 or 5 lux for 12 days. At mating, exposed males took longer than controls to form an amplexus, i.e. to pair with a female, and broke amplexus before egg laying, while controls never did. These behavioural changes were associated with fitness alteration. The fertilisation rate of 5 lux-exposed males was reduced by 25%. Salivary testosterone, which is usually correlated with reproductive behaviours, was not altered by ALAN. Our study demonstrates that ALAN can affect the breeding behaviour of anuran species and reduce one component of their fitness. Given the growing importance of ALAN, more work is needed to understand its long-term consequences on the behaviour and physiology of individuals. It appears essential to identify deleterious effects for animal populations and propose appropriate management solutions in an increasingly brighter world.

Methods for Assessment and Monitoring of Light Pollution around Ecologically Sensitive Sites

Since the introduction of electric lighting over a century ago, and particularly in the decades following the Second World War, indications of artificial light on the nighttime Earth as seen from Earth orbit have increased at a rate exceeding that of world population growth during the same period. Modification of the natural photic environment at night is a clear and imminent consequence of the proliferation of anthropogenic light at night into outdoor spaces, and with this unprecedented change comes a host of known and suspected ecological consequences. In the past two decades, the conservation community has gradually come to view light pollution as a threat requiring the development of best management practices. Establishing those practices demands a means of quantifying the problem, identifying polluting sources, and monitoring the evolution of their impacts through time. The proliferation of solid-state lighting and the changes to source spectral power distribution it has brought relative to legacy lighting technologies add the complication of color to the overall situation. In this paper, I describe the challenge of quantifying light pollution threats to ecologically-sensitive sites in the context of efforts to conserve natural nighttime darkness, assess the current state of the art in detection and imaging technology as applied to this realm, review some recent innovations, and consider future prospects for imaging approaches to provide substantial support for darkness conservation initiatives around the world.