Local adaptation and phenotypic plasticity both occurred in Wedelia trilobata invasion across a tropical island

The role of the local adaptation and phenotypic plasticity of invasive species in their invasion of new environments has historically been a debatable issue, particularly at small spatial scales (e.g., different habitats within an island). We selected seven field sites across Hainan Island, Hainan Province, China, to investigate the role of local adaptation and/or phenotypic plasticity in the successful invasion of Wedelia trilobata by a field survey, molecular marker analysis, and common garden experiment. In the field survey, the clonal growth characteristics of W. trilobata showed significant differences among the seven sites, suggesting that the species was able to adapt to different environments. The mean phenotypic plasticity index of W. trilobata was higher than that of other invasive plant species (0.61 vs 0.48). The analysis of the inter-simple sequence repeat molecular markers of 420 individuals from the seven sites revealed a Shannon's index that was similar to those of other invasive plants (0.29 vs 0.25). The nested analysis of the molecular variance in the genetic diversity of the population showed significant differences among the sites. In the common garden experiment, the growth characteristics of plants grown from the seven sites were significantly affected by light and density treatments but not by soil moisture. However, the responses of plants grown from different sites to light treatment varied. Plants from sunny sites had greater clonal traits than those from shady sites, indicating that local adaptation occurred in plant populations grown at some sites. Overall, our results implied that both phenotypic plasticity and local adaptation contributed to the successful invasion of W. trilobata across Hainan Island.