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).