Results/Conclusions Our field surveys recorded 541 plant vascular species on 156 islands of sizes ranging from 0.02 and 1153.88 ha. The species-area relationship (SAR) for the 156 studied land-bridge islands is: S = cA^z, z = 0.17, R² = 0.56. Our analysis showed that isolation contributed much less to the variation in species diversity (R²=0.03). Based on the geographic isolation by rivers before dam construction, we divided all sampled islands into three groups and computed SARs for each. Z-values of SARs of these groups showed different sensitivities of species diversity to habitat loss. We also found that a group of islands had more species than a single island with the same total area. This difference, however, declined as the total area or the number of islands in the group increased. Overall, our study indicates that habitat loss plays a more important role in maintaining plant biodiversity than habitat fragmentation in the Thousand Island Lake region, and the factors at different scales affect the community organization and assembly rules.