Disentangling spatial and island shape effects on bryophyte distribution in the Zhoushan Archipelago, China

Understanding the effects of island shape and dispersal limitation on species assemblages is important for the conservation of bryophytes in fragmented landscapes. In order to explore the comparative contributions of dispersal limitation vs environmental filtering, island shape vs other island attributes to bryophyte species assemblages in fragmented landscapes, we investigated bryophyte flora and environmental variables (island shape, area, elevation, and isolation degree) of 71 islands in the Zhoushan Archipelago in the East China Sea. We used redundancy analysis, canonical correspondence analysis and generalized linear mixed models to explore comparative contributions of dispersal limitation vs environmental filtering, shape vs other explanatory variables to bryophyte species composition and species richness. Dispersal limitation independently accounted for 15% of the total variation in species composition, which was higher than environmental filtering (6.6%). When island shape was the only constraining explanatory variable, it explained 22.5% of the total species richness variation. Variance partitioning showed that island shape per se explained 3.8% of the total variation in species richness and 18.7% of the variation was confounded by island shape and other variables. Island shape combined with area and elevation was a good predictor of bryophyte species richness, independently explaining 40% of the total SR variation. Among twenty tested bryophyte categories, eighteen categories had positive relationships of their species richness with island shape irregularity, their species richness linearly increasing with island shape irregularity to varying extents. Island shape irregularity exerted a stronger effect on the species richness of acrocarpous mosses than that of pleurocarpous mosses. Therefore, dispersal limitation exerted significant effects on bryophyte species assemblages, and island shape imposed a significant and taxon-specific effect on bryophyte species richness. Our findings implied that a forest reserve with a high irregular shape is favourable for bryophyte conservation.