Artificial light at night (ALAN) is closely associated with modern societies and is rapidly increasing worldwide. A dynamically growing body of literature shows that ALAN poses a serious threat to all levels of biodiversity—from genes to ecosystems. Many “unknowns” remain to be addressed however, before we fully understand the impact of ALAN on biodiversity and can design effective mitigation measures. Here, we distilled the findings of a workshop on the effects of ALAN on biodiversity at the first World Biodiversity Forum in Davos attended by several major research groups in the field from across the globe. We argue that 11 pressing research questions have to be answered to find ways to reduce the impact of ALAN on biodiversity. The questions address fundamental knowledge gaps, ranging from basic challenges on how to standardize light measurements, through the multi-level impacts on biodiversity, to opportunities and challenges for more sustainable use.

Artificial light at night (ALAN) is a relatively new and rapidly increasing global change driver. While evidence on adverse effects of ALAN for biodiversity and ecosystem functioning is increasing, little is known on the spatial extent of its effects. We therefore tested whether ALAN can affect ecosystem functioning in areas adjacent to directly illuminated areas. We exposed two phytometer species to three different treatments of ALAN (sites directly illuminated, sites adjacent to directly illuminated sites, control sites without illumination), and we measured its effect on the reproductive output of both plant species. Furthermore, in one of the two plant species, we quantified pre-dispersal seed predation and the resulting relative reproductive output. Finally, under controlled condition in the laboratory, we assessed flower visitation and oviposition of the main seed predator in relation to light intensity. There was a trend for reduced reproductive output of one of the two plant species on directly illuminated sites, but not of the other. Compared to dark control sites, seed predation was significantly increased on dark sites adjacent to illuminated sites, which resulted in a significantly reduced relative reproductive output. Finally, in the laboratory, the main seed predator flew away from the light source to interact with its host plant in the darkest area available, which might explain the results found in the field. We conclude that ALAN can also affect ecosystem functioning in areas not directly illuminated, thereby having ecological consequences at a much larger scale than previously thought. Artificial light at night (ALAN) is a relatively new global change driver, which is worldwide rapidly increasing 1. It has various ecological consequences as it can cause alterations in physiology and behaviour of organisms, thereby increasing mortality, reducing reproduction as well as altering species abundances and community composition 1-5. For example, many animals have been shown to be attracted 6,7 or repelled 8-11 by light. There is increasing evidence that ALAN has also consequences for species interactions: ALAN has been shown to alter mutualistic interactions, for example to disrupt nocturnal plant-pollinator interactions with negative consequences for the pollination service the nocturnal pollinators provide to the plants 12. However, most of the research so far has focused on the effect of ALAN on predation. 8,11,13-16. These studies show that ALAN can reduce or increase species interactions and ecosystem functioning, and that the effect often depends on light quality and quantity 5,17. However, so far, research has mostly focused on the impact ALAN has on species interactions and ecosystem functioning in directly illuminated areas 5,12,18,19 , ignoring what happens in areas in the surroundings of the illuminated area. Yet animals such as moths, might be attracted from long distances to the illuminated area potentially leading to lowered densities, interaction frequencies and ecosystem functioning in ...

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.

Migration is adaptive if survival benefits are larger than costs of residency. Many aspects of bat migration ecology such as migratory costs, stopover site use and fidelity are largely unknown. Since many migrating bats are endangered, such information is urgently needed to promote conservation. We selected the migrating Leisler's bat (Nyctalus leisleri) as model species and collected capture-recapture data in southern Switzerland year round during 6 years. We estimated seasonal survival and site fidelity with Cormack-Jolly-Seber models that accounted for the presence of transients fitted with Bayesian methods and assessed differences between sexes and seasons. Activity peaked in autumn and spring, whereas very few individuals were caught during summer. We hypothesize that the study site is a migratory stopover site used during fall and spring migration for most individuals, but there is also evidence for wintering. Additionally, we found strong clues for mating during fall. Summer survival that included two major migratory journeys was identical to winter survival in males and slightly higher in females, suggesting that the migratory journeys did not bear significant costs in terms of survival. Transience probability was in both seasons higher in males than in females. Our results suggest that, similarly to birds, Leisler's bat also use stopover sites during migration with high site fidelity. In contrast to most birds, the stopover site was also used for mating and migratory costs in terms of survival seemed to be low. Transients' analyses highlighted strong individual variation in site use which makes particularly challenging the study and modelling of their populations as well as their conservation.