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(community ecology)、生物地理学(biogeography)与进化生物学(evolutionary biology)诸多理论的天然验证实验平台。这些学科的研究进展，使得学界对调控岛屿间物种丰富度变异的过程形成了更为全面且机制性的认知。然而，目前尚不清楚这些过程是否同样能够解释岛屿间物种与系统发育组成的变异。 本研究整合生态学与生物地理学理论，推断扩散(dispersal)、选择(selection)与随机性(stochasticity)在群岛(archipelagos)内部岛屿生物群落(insular assemblages)组成中的作用，并进一步评估物种源库(source pool)多样性与连通性(connectivity)对岛屿生物群落组成独特性(compositional uniqueness)的影响。 本研究的研究对象为全球各类岛屿系统，从公开文献中搜集了全球9个群岛、合计约200个岛屿的陆生哺乳动物(non-volant mammals)物种组成数据。本研究采用变异分解法(variation partitioning)，以零模型(null model，代表随机性与随机过程)作为参照，量化了环境因子（代表选择作用）与地理距离（代表扩散作用）对群岛内部分类与系统发育组成更替(compositional turnover)的相对影响。随后，本研究采用线性混合模型(linear mixed model)，进一步解析全球尺度下调控岛屿间生物群落组成变异的潜在机制，具体评估了物种源库多样性、与源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，群岛内部陆生哺乳动物群落的组成变异，与随机性过程或未被观测的过程相关，而非非生物选择(abiotic selection)或扩散限制(dispersal limitation)。全球尺度下，物种源库的多样性与邻近程度，以及部分岛屿特征，能够解释系统发育组成独特性的变异，但无法解释分类组成独特性的变异。在群岛内部，组成更替的变异大多无法被现有框架解释，这表明生态漂变(ecological drift)或其他随机性过程介导的灭绝作用占据主导地位，掩盖甚至抵消了选择与/或扩散作用的信号。全球范围内，与高多样性物种源库相关联的孤立岛屿，其系统发育组成独特性较高；而与低多样性物种源库相关联的连通性良好的岛屿，则呈现相反的趋势。系统发育组成独特的生物群落，也多分布于海拔跨度(elevational span)较小、年温度变异较低的岛屿之上。 综合来看，本研究结果表明，物种源库多样性以及从这些源库开展物种定植(colonization)的潜力，在塑造全球陆生哺乳动物岛屿群落组成的过程中发挥着关键作用。