Methods
Community surveys of aquatic plants
The field surveys were conducted in eastern, central, southern, and southwestern China. 236 sites where exotic aquatic plants had invaded were sampled from 2015 to 2018 (Fig. 1). To obtain peak species richness and standing biomass, surveys in the northern and southern areas of the Tropic of Cancer were conducted from July to September and from May to November, respectively. At each site, we first recorded the position (latitude and longitude), habitat type (lake, river, rapid, canal, wetland, pool, and reservoir), habitat size (Ahabitat , the continuous water area supporting the community), and species of plants. The origin of each species (native or exotic) was verified according to the Aquatic Plant of the World (Cook et al., 1974) and the List of Invasive Plants in China (http://www.iplant.cn/ias/protlist). Then, we used a systematic sampling technique to survey plant communities. Transects were laid out in species-rich and dense vegetation. The selection of the number of transects and quadrats, as well as the surveyed area, was based on the principle of sampling to accurately reflect the community structure. At sites with narrow vegetation (1 m < width < 5 m, mostly in small rivers and canals, a few in lakes and large rivers), one transect was placed, and 6 quadrats (1 m ×1 m, Fang et al. 2009) were sampled at 10 m intervals along the transect (Appendix S1: Fig. S1A). When the width of vegetation was more than 5 m, 2~4 transects (depending on the size of the community) were placed equidistantly perpendicular to the bank. These transects started from the banks and traversed the shallow rivers and pools (generally water depth < 4 m) if vegetation covered the whole habitat (Appendix S1: Fig. S1B); otherwise, they ended at the edge of vegetation or the limit water depth and distance (generally < 500 m) of manual sampling (Appendix S1: Fig. S1C). The interval between two transects was chosen at 5 m, 10 m, 15 m, 20 m, 30 m, 40 m, 50 m, 70 m, or 100 m according to the size of the community. 2~8 quadrats (1 m ×1 m, depending on the size of the community) were sampled equidistantly along each transect, and the interval between two quadrats was chosen at 5 m, 10 m, 15 m, 20 m, 30 m, or 50 m according to the size, species richness and structure of community (Appendix S1: Table. S1).