Artificial light at night can worsen competitive outcomes between native and invasive grasses

Artificial light at night (ALAN) is an important global disturbance that affects community dynamics, but how it affects species interactions, such as competition, is not well understood and needs investigation. In plant communities, some exotic-invasive plant species seem to be facilitated by ALAN, which may alter competitive outcomes with native plant communities and promote invasion. Despite these intriguing results, whether ALAN affects competitive outcomes between native and invasive species is understudied. Here we tested how ALAN affects competitive outcomes between two native prairie grasses and two grasses that are exotic-invasive in North America. We planted each native species with a range of densities of each exotic species, thereby creating a competition gradient for the native species. We grew these competition treatments under ambient or ALAN light treatments to test if ALAN altered competitive outcomes between native and exotic grasses. We found that ALAN-stimulated growth was significantly greater for our exotic grasses than for the native grasses as predicted, and this enhanced their competitive impact on native species as seen in decreases in native carbon:nitrogen ratios. Our experiment provides support for the hypothesis that the stimulating effect of ALAN on exotic plant growth can increase the competitive ability of exotic grasses and thus the negative effect of these invasive species on native plants.