Artificial Light Increases Local Predator Abundance, Predation Rates, and Herbivory

Does artificial light at night alter moth community composition?

Ecological studies investigating the effects of artificial light at night (ALAN) have primarily focused on single or a few species, and seldom on community-level dynamics. As ALAN is a potential cause of insect and biodiversity declines, community-level perspectives are essential. We empirically tested the hypothesis that moth species differentially respond to ALAN and that these responses can cause shifts in community composition. We sampled moths from prairie fragments in Colorado, USA. We tested whether local light sources, sky glow, site area and/or vegetation affected moth community diversity. We found that increased sky glow decreased moth abundance and species richness and shifted community composition. Increased sky glow shifted moth community composition when light and bait traps were combined; notably this result appears to be driven entirely by moths sampled at bait traps, which is an unbiased sampling technique. Our results show that ALAN has significant effects on moth communities and that local light sources have contrasting effects on moth community composition compared to sky glow. It is imperative that we better understand the contrasting effects of types of ALAN to comprehend the overall impacts of light pollution on biodiversity declines. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Spectral composition of light-emitting diodes impacts aquatic and terrestrial invertebrate communities with potential implications for cross-ecosystem subsidies

Resource exchanges in the form of invertebrate fluxes are a key component of aquatic-terrestrial habitat coupling, but this interface is susceptible to human activities, including the imposition of artificial light at night. To better understand the effects of spectral composition of light-emitting diodes (LEDs)-a technology that is rapidly supplanting other lighting types-on emergent aquatic insects and terrestrial insects, we experimentally added LED fixtures that emit different light spectra to the littoral zone and adjacent riparian habitat of a pond. We installed four replicate LED treatments of different wavelengths (410, 530 and 630 nm), neutral white (4000 k) and a dark control, and sampled invertebrates in both terrestrial and over-water littoral traps. Invertebrate communities differed among light treatments and between habitats, as did total insect biomass and mean individual insect size. Proportional allochthonous biomass was greater in the riparian habitat and among some light treatments, demonstrating an asymmetrical effect of differently coloured LEDs on aquatic-terrestrial resource exchanges. Overall, our findings demonstrate that variation in wavelength from LEDs may impact the flux of resources between systems, as well as the communities of insects that are attracted to particular spectra of LED lighting, with probable implications for consumers. This article is part of the theme issue 'Light pollution in complex ecological systems'.

Uncovering the Effects of COVID-19 Mask Wearing on Bird Flight Initiation Distance in Urbanized Areas in the Southern Philippines

The COVID-19 pandemic has caused significant changes in public and human activities worldwide, including using masks and reducing human interaction. These changes have also affected wildlife behavior, especially in urban areas. However, there is limited understanding of the impact of COVID-19-related human activities, mainly mask wearing, on the behavior of urban bird species. This case is intriguing in the Philippines, where COVID-19 restrictions and mask wearing have been more prolonged than in other countries. We studied two common urban bird species (Geopelia striata and Passer montanus) in Southcentral Mindanao, Philippines, to assess their response to mask wearing by examining their alert distance (AD) and flight initiation distance (FID). We found that birds had a reduced FID to mask wearing, but only significantly in G. striata (Zebra Doves) and not in P. montanus (Eurasian tree sparrow). The effect of the variables related to urbanization on FID was contrasting. For example, ambient noise increased bird vigilance while proximity to roads reduced bird FID in urbanized areas, but their effects were weaker compared to mask wearing. We conclude that mask wearing during the COVID-19 pandemic is a significant environmental element that alters bird escape responses in urban areas, and the effects may be species-specific.

Artificial light at night increases top-down pressure on caterpillars: experimental evidence from a light-naive forest

Artificial light at night (ALAN) is a globally widespread and expanding form of anthropogenic change that impacts arthropod biodiversity. ALAN alters interspecific interactions between arthropods, including predation and parasitism. Despite their ecological importance as prey and hosts, the impact of ALAN on larval arthropod stages, such as caterpillars, is poorly understood. We examined the hypothesis that ALAN increases top-down pressure on caterpillars from arthropod predators and parasitoids. We experimentally illuminated study plots with moderate levels (10-15 lux) of LED lighting at light-naive Hubbard Brook Experimental Forest, New Hampshire. We measured and compared between experimental and control plots: (i) predation on clay caterpillars, and (ii) abundance of arthropod predators and parasitoids. We found that predation rates on clay caterpillars and abundance of arthropod predators and parasitoids were significantly higher on ALAN treatment plots relative to control plots. These results suggest that moderate levels of ALAN increase top-down pressure on caterpillars. We did not test mechanisms, but sampling data indicates that increased abundance of predators near lights may play a role. This study highlights the importance of examining the effects of ALAN on both adult and larval life stages and suggests potential consequences of ALAN on arthropod populations and communities.

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.

Artificial light at night promotes bottom-up changes in a woodland food chain

Artificial light at night (ALAN) is a recognised disruptor of biological function and ecological communities. Despite increasing research effort, we know little regarding the effect of ALAN on woody plants, including trees, or its indirect effects on their colonising invertebrates. These effects have the potential to disrupt woodland food webs by decreasing the productivity of invertebrates and their secretions, including honeydew and lerps, with cascading effects on other fauna. Here, we cultivated juvenile river red gums (Eucalyptus camaldulensis) for 40 weeks under experimentally manipulated light (ALAN) or naturally dark (control) conditions. To assess direct impacts on tree growth, we took multiple measures of growth at four time periods, and also measured physiological function, biomass and investment in semi-mature trees. To assess experimentally the direct and indirect (tree-mediated) impacts of ALAN on invertebrates, from 19 weeks onwards, we matched and mismatched trees with their original ALAN environments. We colonised trees with a common herbivore of E. camaldulensis, the red gum lerp psyllid (Glycaspis nr. brimblecombei) and then measured the effects of current and historic tree lighting treatment on the psyllid life cycle. Our data revealed direct effects of ALAN on tree morphology: E. camaldulensis trees exposed to ALAN shifted biomass allocation away from roots and into leaves and increased specific leaf area. However, while the intensity of ALAN was sufficient to promote photosynthesis (net carbon gain) at night, this did not translate into variation in tree water status or photosystem adaptation to dim night-time light for ALAN-exposed trees. We found some evidence that ALAN had broad-scale community effects-psyllid nymphs colonising ALAN trees produced more lerps-but we found no other direct or indirect impacts of ALAN on the psyllid life cycle. Our results suggest that trees exposed to ALAN may share morphological responses with trees under dim daylight conditions. Further, ALAN may have significant 'bottom-up' effects on Eucalyptus woodland food webs through both trees and herbivores, which may impact higher trophic levels including woodland birds, mammals and invertebrates.

The morphological effects of artificial light at night on amphibian predators and prey are masked at the community level

Artificial light at night (ALAN) is a pervasive pollutant that influences wildlife at both the individual and community level. In this study, we tested the individual-level effects of ALAN on three species of tadpole prey and their newt predators by measuring prey pigmentation and predator and prey mass. Then we evaluated whether the individual-level effects of ALAN on pigmentation and mass had cascading community-level effects by assessing the outcome of predator-prey interactions. We found that spring peepers exposed to ALAN were significantly darker than those reared under control conditions. Additionally, wood frogs reared in ALAN conditions were significantly smaller than those reared in control conditions. In contrast, Eastern newts collected earlier in the spring that were exposed to ALAN were significantly larger than controls while those collected later in the spring were not affected by ALAN, suggesting phenological differences in the effect of ALAN. To understand how changes in pigmentation and size due to ALAN influence predation rates, we ran predation assays in both ALAN-polluted and ALAN-free outdoor environments. After the predation assay, the size disparity in wood frogs reared in ALAN was eliminated such that there was no longer a treatment difference in wood frog size, likely due to size-selective predation. This demonstrates the beneficial nature of predators' selective pressure on prey populations. Lastly, despite individual-level effects of ALAN on pigmentation and mass, we did not detect cascading community-level effects on predation rates. Overall, this study highlights important species-level distinctions in the effects of ALAN. It also emphasizes the need to incorporate ecological complexity to understand the net impact of ALAN.

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.

Predator-Prey Relationship between Urban Bats and Insects Impacted by Both Artificial Light at Night and Spatial Clutter

Predators respond to the increase of prey by aggregation in space or foraging more often. However, foraging habitat suitability limits predators' responses. For nocturnal insectivorous bats, artificial light at night (ALAN) can trigger insect prey aggregation. It is not clear how ALAN might affect predator-prey relationships in the urban setting, where urban bats could have adapted to the city, and novel spatial complexity introduced by man-made objects might alter foraging habitat suitability. We strategically selected sites to represent different levels of ALAN and spatial complexity. We recorded bat commuting and foraging activities and collected aerial insects to examine how ALAN and spatial complexity affected bat-insect relationships. We found that insect biomass was positively correlated with ALAN, but was not affected by spatial complexity. Large-sized big brown bats and hoary bats positively responded to change of prey in open sites whereas small-sized eastern red bats and silver-haired bats positively responded in cluttered sites, suggesting that the impact of ALAN could vary when ALAN is coupled with urban spatial complexity. Our study demonstrates that foraging habitat suitability can alter which species might benefit from ALAN. Predator-prey relationships in cities are complex, but general ecological principles still apply in novel urban ecosystems.

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.

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.

The complexity of global change and its effects on insects

Global change includes multiple overlapping and interacting drivers: 1) climate change, 2) land use change, 3) novel chemicals, and 4) the increased global transport of organisms. Recent studies have documented the complex and counterintuitive effects of these drivers on the behavior, life histories, distributions, and abundances of insects. This complexity arises from the indeterminacy of indirect, non-additive and combined effects. While there is wide consensus that global change is reorganizing communities, the available data are limited. As the pace of anthropogenic changes outstrips our ability to document its impacts, ongoing change may lead to increasingly unpredictable outcomes. This complexity and uncertainty argue for renewed efforts to address the fundamental drivers of global change.

Effects of artificial light at night and predator cues on foraging and predator avoidance in the keystone inshore mollusc Concholepas concholepas

The growth of Artificial Light At Night (ALAN) is potentially having widespread effects on terrestrial and coastal habitats. In this study we addressed both the individual effects of ALAN, as well as its combined effect with predation risk on the behaviour of Concholepas concholepas, a fishery resource and a keystone species in the southeastern Pacific coast. We measured the influence of ALAN and predation risk on this mollusc's feeding rate, use of refuge for light and crawling out of water behaviour. These behavioural responses were studied using light intensities that mimicked levels that had been recorded in coastal habitat exposed to ALAN. Cues were from two species known to prey on C. concholepas during its early ontogeny: the crab Acanthocyclus hassleri and the seastar Heliaster helianthus. The feeding rates of C. concholepas were 3-4 times higher in darkness and in the absence of predator cues. In contrast, ALAN-exposed C. concholepas showed lower feeding activity and were more likely to be in a refuge than those exposed to control conditions. In the presence of olfactory predator cues, and regardless of light treatment, C. concholepas tended to crawl-out of the waterline. We provide evidence to support the hypothesis that exposure to either ALAN or predation risk can alter the feeding behaviour of C. concholepas. However, predator cue recognition in C. concholepas was not affected by ALAN in situations where ALAN and predator cues were both present: C. concholepas continued to forage when predation risk was low, i.e., in darkness and away from predator cues. Whilst this response means that ALAN may not lead to increased predation mortality in C. concholepas, it will reduce feeding activity in this naturally nocturnal species in the absence of dark refugia. Such results may have implications for the long-term health, productivity and sustainability of this keystone species.

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.