Source pool diversity and proximity shape the compositional uniqueness of insular mammal assemblages worldwide

Islands have been the test bed of several theories in community ecology, biogeography, and evolutionary biology. Progress within these disciplines has given a more comprehensive and mechanistic understanding of the processes governing variation in species richness among islands. However, it remains unclear whether these same processes also explain variation in species and phylogenetic composition among islands. Integrating theory from ecology and biogeography, we infer the roles of dispersal, selection, and stochasticity on the composition of insular assemblages within archipelagos. We further assess the influence of source pool diversity and connectivity on the compositional uniqueness of insular assemblages. Island systems worldwide. We compiled data on species composition of non-volant mammals on ∼200 islands in nine archipelagos distributed worldwide from the literature. We used variation partitioning to quantify the relative influence of the environment (selection) and geographic distance (dispersal) relative to a null model (stochasticity, randomness) on taxonomic and phylogenetic compositional turnover within archipelagos. We then used a linear mixed model to gain further insight into the underlying mechanisms shaping variation in assemblage composition among islands at a global scale. Specifically, we assessed the influence of source pool diversity, isolation from the source pool, and island characteristics on compositional uniqueness. Our results suggest that within-archipelago variation in the composition of insular mammal assemblages is associated with stochastic or unmeasured processes rather than abiotic selection or dispersal limitation. The diversity and proximity of the source pool, as well as some island characteristics, explained variation in phylogenetic, but not taxonomic, compositional uniqueness globally. Within archipelagos, the largely unexplained variation in compositional turnover points to the overwhelming influence of extinction mediated by ecological drift or other stochastic processes, which obscures or overrides the signature of selection and/or dispersal. Globally, isolated islands associated with highly diverse source pools exhibit high phylogenetic uniqueness whereas well-connected islands associated with small source pools show the opposite trend. Phylogenetically unique assemblages also tend to occur on islands with a small elevational span and low annual temperature variation. Taken together, our results suggest that source pool diversity, along with the potential for colonization from those pools, plays an important role in shaping the composition of insular mammal assemblages worldwide.岛屿一直是群落生态学、生物地理学以及进化生物学领域诸多理论的验证实验平台。上述学科的研究进展，已使我们对调控岛屿间物种丰富度差异的生态过程形成了更为全面且机制性的认知。然而，目前仍不清楚这些相同的生态过程，是否同样可以解释岛屿间物种组成与系统发育组成的差异。本研究整合生态学与生物地理学理论，推断扩散、选择以及随机过程在群岛内部岛屿生物群落组成中所扮演的角色，并进一步评估了源群落库多样性与连通性对岛屿生物群落组成独特性的影响。本研究以全球岛屿系统为研究对象：从已发表文献中搜集了全球9个群岛共约200个岛屿上的非飞行哺乳类物种组成数据。本研究采用变异分割分析，以零模型（随机过程、随机性）为参照，量化了环境因子（选择作用）与地理距离（扩散作用）对群岛内部物种组成与系统发育组成周转的相对影响；随后使用线性混合模型，进一步解析全球尺度下驱动岛屿间生物群落组成差异的潜在机制，具体评估了源群落库多样性、与源群落库的隔离程度以及岛屿自身特征对生物群落组成独特性的影响。研究结果显示，群岛内部岛屿哺乳类生物群落的组成差异，主要与随机过程或未被观测的过程相关，而非非生物选择或扩散限制。在全球尺度上，源群落库的多样性与邻近程度，以及部分岛屿特征，可以解释系统发育组成独特性的差异，但无法解释物种组成独特性的差异。在群岛内部，组成周转中大量未被解释的差异，表明生态漂变或其他随机过程介导的灭绝过程占据主导地位，这掩盖甚至抵消了选择作用和/或扩散作用的信号。全球尺度下，与高多样性源群落库相连的孤立岛屿，其系统发育独特性较高；而与低多样性源群落库连通性良好的岛屿，则呈现相反的趋势。系统发育独特性较高的生物群落，也多分布在海拔跨度较小、年温度变化较低的岛屿上。综合来看，本研究结果表明，源群落库多样性以及从这些库中进行定植的潜力，在塑造全球岛屿哺乳类生物群落组成的过程中发挥着重要作用。