Species-Area Relationship: A Mesocosm Experiment with Annual Plants

1. The increase in species richness with increased island area is one of the most widely documented patterns in ecology and biogeography but its mechanisms are still under debate. Disentangling these mechanisms in natural systems is challenging due to various kinds of confounding effects. 2. Here we use a novel mesocosm experiment focusing on annual plants to disentangle the effects of two mechanisms that may lead to higher richness on large islands: increasing population sizes and increasing opportunities for within-island dispersal. Theoretical studies show that both mechanisms may contribute to higher richness on large islands, but no previous study has attempted to separate their effects. We also test an alternative, 'null' hypothesis, according to which the only mechanism underlying the increase in richness with increased area is a pure sampling effect (the 'passive sampling hypothesis'). 3. As expected, increasing area had a strong positive effect on species richness. However, the main mechanism underlying the difference in richness between small and large islands was passive sampling. Moreover, our results indicate that dispersal had a negative rather than positive effect on island richness. 4. Synthesis: Our results corroborate previous observational studies that failed to reject the passive sampling hypothesis. However, in contrast to previous studies, our findings are based on experimental manipulations of island area, under controlled uniform conditions, at small spatial scales that facilitate within-island dispersal, and in three independent systems that differed in resource availability and disturbance. These findings emphasize the need for a careful examination of sampling effects in future research of the species-area relationship.1. 物种丰富度随岛屿面积增大而升高，是生态学与生物地理学领域被广泛验证的经典格局之一，但其背后的作用机制仍存在争议。由于存在多种混杂效应，在自然系统中厘清这些机制极具挑战性。 2. 本研究依托一项针对一年生植物的新型中宇宙实验（mesocosm experiment），旨在拆解两种可能推高大岛屿物种丰富度的核心机制：种群规模扩大与岛内扩散机会增多。理论研究表明，这两种机制均可能推动大岛屿的物种丰富度提升，但此前尚无研究尝试分离二者的独立作用。我们同时检验了一种替代性的零假设：即面积驱动的物种丰富度提升仅源于纯粹的取样效应，也就是被动取样假说（passive sampling hypothesis）。 3. 正如预期，岛屿面积增大对物种丰富度存在显著正向影响。然而，大小岛屿间丰富度差异的核心驱动机制实为被动取样。此外，本研究结果显示，扩散对岛屿物种丰富度的影响为负向，而非正向。 4. 综合分析：本研究结果佐证了此前未推翻被动取样假说的观测类研究结论。但与以往研究不同的是，我们的发现基于受控均匀条件下的岛屿面积操控实验，实验空间尺度较小且便于岛内扩散，且依托三个资源可获得性与干扰程度各异的独立实验系统。本研究结果强调，在未来物种-面积关系（species-area relationship）相关研究中，需审慎审视取样效应的作用。