Artificial light at night (ALAN) has emerged as a major source of disturbance for organisms by altering their behavior, physiology, and fitness, with consequences for ecosystem functioning. Yet, we still know relatively little about how it affects herbivores and the ecosystem function they provide. By experimentally illuminating not only captive individuals from two slug species but also entire wild communities, we tested whether nocturnal lighting alters circadian activity patterns, fitness parameters, feeding, and moving behaviors and whether this affects herbivory damage. In one of the two illuminated slug species, we found that ALAN reduced its activity at night, juvenile growth, and survival rate. At the community level, feeding activity of slugs was reduced in artificially illuminated sites, and this was related to reduced herbivory. We conclude that ALAN affects the fitness of slugs and disrupts their activity, with consequences for herbivory. This will most likely have indirect consequences for other ecosystem processes.
