In the limited number of studies on biotic resistance in freshwater ecosystems, experimental studies have supported that native plants play a role in resisting exotic plant invasion, but field observations have often reported positive relationships between exotic and native plant richness. 

To determine the reasons for the lack of evidence supporting biotic resistance in field observations, we surveyed the species, biomass, and coverage of exotic and native aquatic plants in 2801 quadrats at 287 sites in various freshwater ecosystems of China and examined the impacts of scale and environmental factors on the relationships between native and exotic plant richness and performance.

At both the site and quadrat scales, we observed a positive correlation between the richness of exotic and native plants. However, at the quadrat scale, high native plant richness was associated with a reduction in the biomass of exotic emergent and floating plants as well as the biomass of exotic submerged plants in low-nutrient water bodies. An increase in native plant performance, a metric that integrates the biomass and coverage of native plants, was associated with a decrease in both the richness and biomass of exotic plants at both scales. Furthermore, with increasing native plant performance at the quadrat scale, the richness of exotic plants declined more strongly in low-nutrient water bodies than in high-nutrient water bodies, and the decline in the biomass of exotic emergent and floating plants was more severe in shallow water than in deep water.

Synthesis Our results indicate that native plant performance is more important than richness for resistance to invasion in freshwater ecosystems, and the strength of biotic resistance is related to environmental factors. This study highlights the importance of environmental variables and multiple native community features and invasibility metrics in field observational studies of invasion.