Artificial light at night and an invasive snail synergistically enhance the invasion of a nonnative macrophyte.

Herbivory shapes plant invasion outcomes, yet its role in aquatic plant invasions under artificial light at night (ALAN) remains poorly understood. We conducted three experiments using an invasive macrophyte Myriophyllum aquaticum, a native macrophyte community comprising Vallisneria natans, Hydrilla verticillata, Myriophyllum spicatum, as well as an invasive snail Pomacea canaliculata, and a native snail Cipangopaludina chinensis, in our study area, to test the combined effects of ALAN and herbivory on nonnative macrophyte invasions. In the absence of grazing, ALAN increased M. aquaticum height and total biomass, but had no effect on native species. In 4-day feeding assays, P. canaliculata consumed all native species but consistently avoided M. aquaticum under both light treatments. In community mesocosms, the invasive snail reduced native macrophyte biomass by 48.0% under No-ALAN and by 87.2% under ALAN without affecting M. aquaticum, thereby increasing proportional biomass of M. aquaticum within the community, with a stronger effect under ALAN. Therefore, ALAN may indirectly facilitate nonnative macrophyte invasions by amplifying their relative biomass within native communities, particularly in the presence of invasive herbivores, and may promote invasional meltdown through selective herbivory.