Beta diversity reflects the variation of species composition across different sites, within a region of interest or along environmental gradients. It is a key concept of the understanding of ecosystem function, the conservation of biodiversity and the design of natural reserves. There are two main current hypotheses about the forming and maintaining mechanisms of beta diversity: dispersal limitation and niche differentiation. The dispersal hypothesis addressed that the differences of species distribution are created through spatially limited dispersal. The niche hypothesis emphasizes the variations of environmental condition among sites, which can offer suitable habitats for different species to settle. These two processes differently affect the respective patterns of plant groups (e.g. including tree, shrub, herb and liana) and drive them playing different roles in the formation of beta diversity. However, most current studies primarily focus on improving the analytical approaches of beta diversity and calculating various indices of beta diversity in different sites. Thousand Island Lake (TIL) is a large man-made hydroelectric reservoir with over 1078 land-bridge islands in western Zhejiang province, China, which is an ideal fragmented landscape for researching beta diversity and its drivers.
We studied the pattern of beta diversity of vascular plants on 154 islands in the TIL and its major causes by using quadrat method, Jaccard dissimilarity, Spearman correlation analysis and Non-metric Multidimensional Scaling (NMDS) ordination analysis. The results showed that the impacts on beta diversity from different environmental characteristics varied with different plant groups. The beta diversity of liana and shrubs is significantly correlated with island area, which means their beta diversity mainly due to the area differences among islands. The more the area lost, the higher the beta diversity of these two plant groups. The beta diversity of trees, in contrast, was mainly influenced by the distance between islands. With the increase of the distance between island pairs, the beta diversity of trees in these islands also raised. The beta diversity of herb on these 154 islands showed no significant correlation with the shortest distance to the mainland or with the nearest-neighbor island. NMDS ordination analysis also showed that island area, shape index, perimeter to area ratio and the shortest distance to the mainland are four important environmental characteristics to determine the distribution pattern of beta diversity of vascular plants on the islands of TIL. We conclude that the beta diversity of vascular plants on the 154 land-bridge islands in the TIL is both governed by dispersal limitation and niche differentiation.