Co-invading macrophytes and herbivores interact to significantly alter community stability and invasibility: evidence from a freshwater mesocosm experiment

Biological invasions significantly alter community functional diversity, stability, and ecosystem multifunctionality. However, the impact of co-invasion by multiple macrophytes and herbivores on community stability and invasibility in aquatic ecosystems remains largely unknown. Here, native plants Hydrilla verticillata and Vallisneria natans, and invasive species Myriophyllum aquaticum , Elodea nuttallii , and Pomacea canaliculata were used to investigate the effects of co-invasion on community stability and invasibility. Our results showed that (1) co-invasive interactions between the two invasive macrophytes are antagonistic; (2) exotic herbivores significantly alter the invasiveness of invasive species, but the magnitude depends on the invasive species and its community composition; and (3) co-invasive plants and animals affect community stability and invasibility by altering functional trait diversity. Our study confirms that the invasion success of co-occurring, invasive plants depends on interactions among the invasive and native macrophytes, and their effects on community stability and invasibility can be regulated by the grazing preferences of invasive herbivores. Our study determined that in freshwater ecosystems, co-occurring invaders including macrophytes and herbivores can result in unanticipated effects on community stability and invasibility due to trade-offs among niche complementarity effect and competition between macrophytes and the selecting effect of grazers. Our results underscore the need to consider complex interactions between co-invading macrophytes and herbivores across trophic levels when assessing the impact of co-occurring invasion on community stability. Furthermore, when managing invasive species, a key consideration is whether priority should be given to sites invaded by individual species or to removing dominant invaders to suppress the expansion of subordinate species. • Interactions between invasive macrophytes are antagonistic. • Invasive herbivores modulate the success of plant invasions. • Co-invaders alter ecosystem via niche complementarity, competition, grazer selection.