Artificial light at night as an environmental pollutant: An integrative approach across taxa, biological functions, and scientific disciplines.

With each day, the world is becoming number of people move into urban areas, and consequently these increase in area to consume rural and natural land (Seto, Guneralp, & Hutyra, 2012). Among the numerous changes that accompany urban sprawl, artificial light at night (ALAN) is one of the most immediate and evident (Falchi et al., 2016; Longcore & Rich, 2004). From a variety of anthropogenic sources, artificial light is introduced at times (the night) and in places where it does not naturally occur (Falchi et al., 2016). The characteristics of the introduced light sources are also very different from natural light. Artificial light is often of an intensity higher than naturally occurring light at night, for instance, because of moon or starlight (Gaston, Bennie, Davies, & Hopkins, 2013). Furthermore, the spectral properties of artificial light are often enriched of a specific wavelength, in particular, the blue portion of the spectrum (Gaston et al., 2013; Navara & Nelson, 2007). ALAN is increasing at a steady pace globally (6% per annum) (Falchi et al., 2016) but with tremendous spatial variation (from negative trends to positive changes of up to 20% per annum; Bennie, Davies, Duffy, Inger, & Gaston, 2014; Bennie, Duffy, Davies, Correa-Cano, & Gaston, 2015). Although the proportion of the Earth’s surface covered by urban land is below 5% (Seto et al., 2012), between 10% and 20% of the global land experiences some degree of ALAN (Kyba et al., 2015). This is mainly due to the sky glow effect (Kyba et al., 2015) but further because artificial light is also used outside of urban areas, for instance on roads connecting different cities or in remote industrial installations (Falchi et al., 2016). A key question for biologists is whether the alteration of natural lightscapes by ALAN has any consequence for the organisms that inhabit light polluted areas, including humans (Dominoni, Borniger, & Nelson, 2016; Gaston et al., 2013; Navara & Nelson, 2007). Species have evolved over millions of years in habitats where daily, lunar, and seasonal cycles are dominant sources of environmental variation are driven by changes in light regimes (Foster & Kreitzmann, 2004; Helm et al., 2013). Organisms have thus developed specific molecular, physiological, and behavioral adaptations to such rhythms of life (Foster & Kreitzmann, 2004; Helm et al., 2013). How are these organismal adaptations coping with a light polluted world? How do responses at the individual level scale up to influence, populations, communities, and ultimately ecosystems? Importantly, can we identify lighting practices that minimize the environmental impacts of ALAN? In recent years the scientific interest in such questions, and more generally in the biological impacts of ALAN, has bloomed. This has led to an explosion of research papers that have investigated a plethora of effects of ALAN on individual organisms, species, and communities. This special issue was conceived to illustrate the breadth of research questions that the study of light pollution has focused on in recent years. Moreover, we aimed at highlighting recent developments and challenges in this field. We focus on three of these. First, the need to investigate the ecological effects of ALAN in a diverse array of species representing the extraordinary diversity of life, from microbes to plants and from invertebrates to all vertebrate classes. Second, the need for studies at different levels of biological organization, from molecules to physiology, behavior, populations, and communities. Third, the need to establish intensity and spectral-dependent effects of ALAN, with the ultimate goal to produce relatively simple guidelines that would inform policy-makers and produce tangible impacts on the way that lighting systems are designed, produced, and ultimately installed. We believe that to meet these challenges an integrative and multidisciplinary approach is needed.