The Negative Target Effect Of Bird Diversity Distribution In The Zhoushan Archipelago,China

The Equilibrium Theory of Island Biogeography postulates that larger and closer islands support higher biodiversity through the dynamic balance of colonization and extinction processes.The negative diversity-isolation(i.e.,the distance to the mainland)relationship is derived based on the assumption that the mainland is the only source pool for island biotas.However,nearby islands could also act as species sources for focal islands via a source effect.In this study,we move a further step and hypothesize that the nearby large islands may reduce bird colonizers of the focal island and diminish the its biodiversity,resulting in a negative target effect.To test our hypothesis,we selected42 islands in the largest archipelago of China,the Zhoushan Archipelago encompassing as much variations in area and isolation as possible and assessed the effects of island area and isolation(metrics considering both the mainland and nearby islands)on taxonomic(i.e.,species richness),functional,and phylogenetic diversity of terrestrial breeding birds on these islands.The islands were encompassing as much variations in area and isolation as possible.Furthermore,we compared the predictive power of the distance to the large island with relative different area thresholds and the relative area of nearby islands on species richness with different distance thresholds to explore the role of nearby islands as a source and/or target island.Further parameter-testing of the distance to the large island and the relative area of nearby islands indicated the negative target effect of the nearby large islands depended on their relative area to the focal island.A total of 150 species(excluding seabirds)were recorded,belonging to 15 orders and 51 families,accounting for 43.5%of birds on the mainland in the same period.We found that island area had a positive effect on species richness,phylogenetic diversity,and functional diversity,while the distance to the mainland had a negative effect only on species richness.Species richness on the focal island increases with the increasing distance to the nearest larger island,indicating the negative target effect.Our finding of the negative target effect suggested dispersal events among islands should be much more complex than traditional assumptions of straight dispersal trajectories.This result demonstrates the importance of considering multiple geographical attributes of islands in island biogeographic studies,especially the characters related to source and/or target effects.