| The freshwater ecosystems in China are facing many ecological crises,such as water eutrophication and biodiversity decline.Among the threatening factors,the invasion of aquatic organisms has not attracted enough attention.Many alien aquatic species were introduced by aquaculture and aquarium trade.Some alien aquatic plants were even widely used in freshwater ecological restoration project,such as Eichhornia crassipes,which made this vicious invasive species spread quickly and brought new threat into unhealthy waterbodies.The alien species of aquatic plants in China have several features:various sources,large numbers and complex flora components.It is necessary to conduct a comprehensive investigation and correct identification of alien species.Although lots of studies on alien aquatic plants have been conducted,some important issues are still unknown,e.g.,the species number of alien aquatic plants,their distribution patterns,and their invasive statuses.Most studies explored their invasion on the level of physiology,individual and population,not ecosystem,which will make it difficult for us to make accurate and effective early warning and control of aquatic plant invasion.Based on six-year field investigations in wide ranges,this study reviewed the floristic composition,cataloged the catalogs,and verified the origin and introduction of alien species of aquatic plants in China.Furthermore,this study explored the distribution pattern and diffusion mechanism of alien species,summarized the harm of alien species to the health of freshwater ecosystems,found the key ecological factors affecting the invasibility of native communities,and analyzed the reason causing invasive species widespread in the water body.The results are as follows:1.There are 478 alien species of aquatic vascular plants in China,belonging to 62 families and 158 genera.Among them,there are 452 species of angiosperms(258 species of monocotyledons plants,194 species of dicotyledonous plants),3 species of gymnosperms,23 species of ferns.When divided according to life-form,emergent,submerged,floating leaved and free-drifting plants species are 275(57.53%),149(31.17%),37(7.74%),and 17(3.56%),respectively.When divided according to family,Araceae,Alismataceae and Nymphaeaceae have most species and their numbers are 15 genera(9.49%)and 63 species(13.18%),6 genera(3.8%)and 44 species(9.21%)and 7 genera(4.43%)and 36 species(7.53%),respectively.When divided according to genus,Cryptocoryne,Echinodorus and Nymphaea have most species and their numbers are 36 species(7.53%),29 species(6.07%)and 19 species(3.97%),respectively.When divided according the place of origin,Asia,South America and North America are the top three and have 166 species,143 species and 126 species,respectively.Among 478 alien species,456 species were introduced for ornamental gardens.A total of 42 species(8.79%)were found to establish population in the field,and 12 species(2.51%)have damaged native freshwater ecosystems.Aquatic plant ornamental needs and human introduction for ecological restoration,as well as the complex geographical environment and climatic conditions make China become the country with the most alien species of aquatic plant.The introduction of alien species has changed the flora component and community structure of aquatic plants in China.The alien species that have been established in the field and those in aquariums and parks will potentially cause great harm to freshwater ecosystems in China.2.Alien species of aquatic plants have successfully invaded into 25 out of 34 provincial administrative regions in China,and they distribute mainly in Eastern Plains area and Yunnan-Guizhou Plateau,covering about one third range of China.Eichhornia crassipes,Alternanthera philoxeroides and Pistia stratiotes are the most widespread alien species.The frequency and coverage of invasive species varied greatly in different areas.In the seven areas of China,Southwest China,Central and East China,and South China were the top three areas suffering invasion.The number(occurrence frequency)of invasive species in the three areas was 8(81.48%),7(76.22%)and 4(69.57%),and the proportion of invasive species distribution to the total area of survey communities was 41%,26.8%and 33.15%,respectively.In North China,Northwest China and Northeast China,only one invasive species was found,and its occurrence frequency is 3.23%,4.38%and 0.53%,occupying 1.2%,1.62%and 0.27%of the total area of survey communities,respectively.In the Qinghai-Tibet plateau,2 invasive species were found with 1.68%occurrence frequency and occupying 0.05%of the total area of survey communities.Due to population size,economic development,traffic and trade developed,aquaculture and aquatic ecology,Southwest China,Central and East China,and South China are facing a greater risk of invasive species of aquatic plants.3.There are significant differences in the number of alien species of aquatic plants at different latitudes.The number increased first and then decreased with the increase of latitude and North latitude 25 degrees had the most number of alien species.Three distribution patterns were presented in alien species of aquatic plants in China:single point distribution,scattered and island distribution,and continuous patchy distribution.Although the invasive species E.crassipes and A.philoxeroides have a high degree of overlap in their distribution ranges,species competition and the difference of adaptability to the environment causes their population size to be complementary at different latitudes.4.There was no significant correlation between the abundance of native species and that of alien species.However,with the increase of biomass and coverage of native species,the abundance of alien species decreased significantly.the abundance of alien species increased significantly with the increasing of total phosphorus,total nitrogen and chemical oxygen demand of water.The latitude and habitat size of the community had no effect on the abundance of alien species in the community.The above results suggested that the reduction of biodiversity and coverage of native species and water eutrophication can increase the invasiveness of the community.5.The relative coverage of invasive species decreased with the increase of the abundance,relative coverage and biomass of native species,habitat size,and latitude,increased with the increase of the total nitrogen,total phosphorus and chemical oxygen demand of water,and was not affected by the altitude and the proportion of leaf herbivory.The same trends were presented in the biomass of invasive species.The above results suggested that the decrease of biodiversity and coverage of native species,water eutrophication,habitat fragmentation and global warming will aggravate the invasion of alien species of aquatic plants.The relative coverage,biomass,abundance of native species and total nitrogen,total phosphorus,chemical oxygen demand of water were the main factors determining the relative coverage of invasive species..The above results suggested that ecological restoration with improving vegetation coverage and biodiversity of native species,elimination of eutrophication and reduction of organic matter are the fundamental ways to control alien species of aquatic plants and eliminate their hazards.6.Based on the classification principle used in the book Invasive Plants Directory in China,the alien species of aquatic plants in China were divided into three types six subtypes at two levels:harmful alien species including malignant invasive species and local serious invasive species,temperate alien species including minor harmful alien species and undetermined alien species,and beneficial alien species including ornamental exotic species and edible exotic species.The species were classified according to the list of 100 malignant invasive species in the world published by the International Union for Conservation of Nature and Natural Resources,the published list of invasive and alien species in natural ecosystems of China,and the Handbook of Global Freshwater Invasive Species. |
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