Skyglow and especially direct streetlight pollution alter moth communities

Light pollution caused by artificial light at night (ALAN) affects biodiversity worldwide and is still increasing. Species vary in their response to ALAN, but the impact on communities remains poorly understood. Effects of indirect light pollution by skyglow and direct streetlight pollution on communities have usually been addressed separately, and in most studies so far, urbanisation levels are a confounding factor for ALAN effects per se. Here, we sampled garden moth communities according to a study design that integrated two levels of direct streetlight pollution and two regional skyglow levels to test for additive effects of both types of ALAN. We also tested whether ALAN conditions related to community changes in species' morphological, behavioural and life-history traits. Both skyglow and streetlights negatively affected moth abundance. Species richness and diversity decreased under direct streetlight, independent of skyglow levels. Both types of ALAN affected moth community composition and we identified several relationships with species traits in the local moth communities. Skyglow filtered for hibernating stage and larval habitation, while streetlight pollution filtered for voltinism, larval food specificity, habitat openness and hibernating stage. Our study provides evidence that both skyglow and, more strongly, streetlight pollution affect moth communities. A significant decline in moth abundance will have consequences for ecosystem functioning through reduced food availability for predators and diminished pollinator services. At the community level, ALAN operates as a filter for species and species traits, and based on our results, we argue that future studies should consider both skyglow and direct light pollution.