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 nearby islands may reduce bird colonizers of the focal island and diminish its biodiversity, resulting in a negative target effect. To test our hypothesis, we 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 42 islands in the largest archipelago of China, the Zhoushan Archipelago. Furthermore, we compared the predictive power of the distance to the large island under a set of relative area thresholds and the relative area of nearby islands on species richness under a set of distance thresholds to explore the role of nearby islands as a source and/or target island. 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 increased with increasing distance to the nearest larger island, indicating the negative target effect. Furthermore, the negative target effect depended on the area of nearby islands relative to the area of the focal island. Our finding of the negative target effect suggests islands located between the mainland and the focal island can be not only sources or stepping stones, but also colonization targets. This result demonstrates the importance of considering multiple geographical attributes of islands in island biogeographic studies, especially the characteristics related to source and/or target effects.
岛屿生物地理学平衡理论 (Equilibrium Theory of Island Biogeography) 提出的迁入-灭绝动态平衡 (colonization-extinction dynamic) 认为面积更大、隔离度更低的岛屿生物多样性将会更高。大陆通常被认为是岛屿生物的主要源, 因此存在着负的物种数-大陆距离关系。然而主岛 (focal island) 邻近的其他岛屿同样可能是潜在的物种源 (colonization source)。本研究进一步假设, 邻近岛屿既可以是源, 也可以是拓殖目标 (colonization target)。因此, 邻近岛屿可能吸引主岛的潜在拓殖种群, 降低目标岛屿的迁入率, 进而减少其生物多样性, 我们将其称之为负目标效应 (negative target effect)。 为了验证这一假设, 我们在中国最大的群岛 (浙江舟山群岛), 选取了面积和隔离度尽可能多样的42个岛屿开展鸟类物种调查, 并评估了各个岛屿地理属性 (包括岛屿面积、大陆距离和邻近岛屿相关的隔离度指标) 对鸟类的物种丰富度、功能多样性和系统发育多样性的影响。此外, 我们还比较了不同相对面积阈值下最近大岛距离和不同距离阈值下周边岛屿相对面积对物种丰富度的预测能力, 以探讨邻近岛屿作为源或目标时对岛屿物种丰富度的影响。 研究发现, 岛屿面积对物种丰富度、系统发育多样性和功能多样性有正影响, 而大陆距离只对物种丰富度有负影响。岛屿鸟类丰富度随着与最近大岛距离的增加而增加, 说明邻近岛屿表现出了负目标效应, 而非源效应。我们发现负目标效应受到邻近岛屿相对面积的影响。 负目标效应的发现表明邻近岛屿不仅仅可以作为物种源或是踏脚石 (stepping stone), 也可能是拓殖目标。这说明在岛屿生物地理学研究中考虑多重岛屿地理属性具有重要意义, 特别是那些有关源效应或目标效应的岛屿属性。.
Keywords: Zhoushan Archipelago; bird; functional diversity; island biogeography; phylogenetic diversity; target effect.
© 2022 The Authors. Journal of Animal Ecology © 2022 British Ecological Society.