Do herbivorous fish amplify the competitive dominance of floating-leaved plants over submerged plants?

Growing interest in the relationship between herbivorous fish and aquatic plants has revealed multiple pathways by which fish can influence aquatic plant biomass and coverage. However, the impact of herbivorous fish on species competition and community assembly in aquatic plants remains poorly understood. Here, based on a mesocosm experiment with four aquatic plant species (two floating-leaved and two submerged) and one herbivorous fish, we quantified competition coefficients among plant species and fish effects using a modified Ricker model. Floating-leaved plants exhibited stronger intraspecific than interspecific competition, indicating competitive superiority over submerged species. Crucially, selective grazing by herbivorous fish on submerged plants indirectly facilitated the dominance of floating-leaved species, exacerbating competitive asymmetry. These results demonstrate that herbivory indirectly affects aquatic plant community structure, not simply by removing biomass but also via higher-order interactions. Our research extends herbivore-plant interaction theory to freshwater ecosystems and offers guidance for aquatic plant restoration and biological management.