Artificial light at night (ALAN) is an increasingly pervasive driver of global change that disrupts natural diel rhythms and alters species interactions. However, its effects on plant-pollinator networks across coupled day-night systems remain poorly understood, particularly in environmentally sensitive alpine ecosystems. Here, we conducted a field manipulation experiment in an alpine meadow on the Tibetan Plateau, comparing illuminated and unlit conditions, and constructed temporally explicit pollination networks by integrating diurnal and nocturnal interactions. Despite no detectable changes in plant community composition, ALAN significantly restructured pollinator-mediated interactions. At night, visitation by Lepidoptera (primarily moths), the dominant pollinators, declined sharply (-57.5%), coinciding with an overall reduction in visitation frequency (-35.4%) and a substantial decrease in network nestedness (-64.7%). In contrast, Diptera visitation increased markedly (+179.6%), partially offsetting the reduction in total visitation associated with the decline in Lepidoptera visitation. ALAN also increased nocturnal network modularity by 12.8%, indicating greater compartmentalization of nocturnal plant-pollinator interactions. Notably, these effects extended beyond the nocturnal period. During the day, total visitation remained unchanged, but strong shifts in pollinator composition emerged: Hymenoptera, Lepidoptera, and Coleoptera declined (-20.0%, -51.8%, and -61.7%, respectively), whereas Diptera increased (+46.3%). These changes were accompanied by a significant increase in network connectance (+16.1%), whereas nestedness and modularity remained unaffected by ALAN. The decline in nocturnal nestedness was primarily driven by reductions in Lepidoptera, whereas the increase in diurnal connectance was associated with increased Diptera visitation and reduced Hymenoptera visitation. Together, our results demonstrate that ALAN reorganizes plant-pollinator networks through differential responses among pollinator groups, with some groups declining while others increase in visitation and cross-diel cascading effects, thereby altering the organization of species interactions across both nocturnal and diurnal pollination systems. Our findings highlight the importance of integrating diel dynamics into ecological network research and underscore the need to mitigate light pollution in vulnerable alpine ecosystems.
Circadian Biology