Multidimensional nestedness patterns and underlying processes of bird assemblages in the Zhoushan Archipelago, China

Nestedness is one of the important patterns in island biogeography, community ecology and conservation biology. However, most previous nestedness studies focus on the taxonomic dimension while neglecting the functional and phylogenetic processes in generating nestedness. Moreover, few studies have examined the seasonal change of the nestedness and underlying processes. Here, we examined the seasonal nestedness of bird assemblages in taxonomic, functional, and phylogenetic dimensions, and determined the underlying processes of nestedness patterns in the Zhoushan Archipelago, China. We surveyed the occurrence, abundance, and habitats of birds on 40 islands. We calculated taxonomic, functional, and phylogenetic nestedness using WNODF and treeNODF. We determined the processes underlying nestedness by relating nestedness ranks to island characteristics and species traits. The WNODF analyses showed that bird assemblages in winter and summer were both significantly nested. The habitat-by-site matrix was also significantly nested. The nestedness of bird assemblages was significantly correlated with island area, habitat diversity, habitat specificity, minimum area requirement, habitat specificity and hand-wing index (HWI) of birds in both seasons. While the distance to the nearest mainland (DTM) exerted the influence on nestedness in summer, the distance to the nearest larger island (DTNL) affected nestedness only in winter. However, the nestedness of bird assemblages was not caused by passive sampling or human disturbance. The results of treeNODF analyses illustrated that bird assemblages were functionally and phylogenetically nested in summer and winter, but the exact mechanisms were somewhat different in these two seasons. Overall, our results supported the habitat nestedness hypothesis, selective extinction hypothesis, and selective colonization hypothesis in both seasons. From a conservation viewpoint, we should protect islands with large area and diverse habitats, islands close to the mainland, and species with large area requirement and high habitat specificity to prevent local extinction.