Distribution patterns of alien invasive plants and their influences on native plants of Hainan Island

Aims Our objective was to explore the distribution patterns of 20 noxious invasive plants in Hainan Island notified by the National Ministry of Agriculture, and their influences on native plants. We also focused on investigating the traits of six most widely spread invasive plants and plant distributions in seven types of terrestrial ecosystems. Three hypotheses were deployed in this study: (1) the ecological effects of plant invasion were related to the characteristics of invaded ecosystems and successional phases of invaded communities; (2) the invasive plants rather affected certain functional groups than the whole communities; (3) and most invasive plant species tended to colonize frequently disturbed ecosystems, while they were usually excluded by intact forest ecosystems.Methods The plot survey was conducted to investigate the distributions of the 20 target invasive plants. The appearance frequency of the six most widely spread invasive plants (i.e., Chromolaena odorata, Praxelis clematidea, Mimosa pudica, Wedelia trilobata, Conyza sumatrensis, and Lantana camara) was compared among seven terrestrial ecosystems (i.e., secondary forest, farmland, grassland, abandoned land, plantation, village, and forest edge) to evaluate the invasibility of different ecosystems. The plots invaded by target species were classified, and species richness of native species in these invaded plots was compared using one-way ANOVAS. Relationships between the total coverage of invasive plants and the species richness of native plants were assessed using a linear regression model. Important findings (1) A total of 11 noxious invasive plants were found in Hainan Island, accounting for 55% of the target specie. (2) The target invasive species were mostly distributed in the northeastern and southwestern regions of Hainan Island, less in the southeastern region, and least in the central region. (3) An order of the intensity regarding plant invasiveness was expressed as Praxelis clematidea > Chromolaena odorata > Mimosa pudica > Wedelia trilobata > Lantana camara > Conyza sumatrensis. The appearance frequency of Conyza sumatrensis and Lantana camara showed no significant differences among the surveyed ecosystems, while Praxelis clematidea, Mimosa pudica and Wedelia trilobata had obviously higher appearance frequency in plantations and abandoned lands, in farmlands, and in plantations, respectively. (4) Farmlands, plantations and abandoned lands were the most frequently invaded ecosystems; but forest edges and natural secondary forests appeared to be less vulnerable to plant invasion. (5) The appearance frequency of invasive plants was negatively correlated with plant coverage of the community invaded by the target species. The total coverage of invasive plants and the richness of native plant species showed a positive correlation in grasslands when the coverage was within a certain extent, and a negative correlation both in forest edges and abandoned lands, and no obvious correlation in plantations farmlands and village sites.