Environmental Filtering and Resident Diversity Govern Different Stages of Invasive Macrophyte Success in Serbian Freshwaters

Freshwater biodiversity is increasingly threatened by biological invasions, yet the ecological processes governing invasion establishment and subsequent dominance remain poorly understood. We investigated environmental, community, and functional determinants of invasion success for three invasive aquatic macrophytes (Cabomba caroliniana, Elodea canadensis, and Paspalum distichum) across 102 freshwater sites in Serbia. The three invaders occupied distinct environmental niches, with C. caroliniana confined to canals, E. canadensis occurring mainly in reservoirs, and P. distichum associated primarily with rivers. The two submerged species occupied highly distinctive positions within regional functional trait space, whereas the emergent amphibious P. distichum was considerably less functionally distinctive. Invaded communities exhibited significantly higher functional dispersion, while invasion dominance declined with increasing resident Shannon diversity, which explained 67.5% of the variation in invasive abundance. Together, these findings support a multi-stage framework in which environmental filtering determines invader identity, resident community functional structure influences invasion occurrence, and resident taxonomic diversity constrains invasion dominance. Distinguishing invasion occurrence from invasion dominance provides a more informative basis for freshwater biodiversity monitoring and invasive species management.