Sensitivity of Metrics of Phylogenetic Structure to Scale, Source of Data and Species Pool of Hummingbird Assemblages along Elevational Gradients

Patterns of phylogenetic structure of assemblages are increasingly used to gain insight into the ecological and evolutionary processes involved in the assembly of co-occurring species. Metrics of phylogenetic structure can be sensitive to scaling issues and data availability. Here we empirically assess the sensitivity of four metrics of phylogenetic structure of assemblages to changes in (i) the source of data, (ii) the spatial grain at which assemblages are defined, and (iii) the definition of species pools using hummingbird (Trochilidae) assemblages along an elevational gradient in Colombia. We also discuss some of the implications in terms of the potential mechanisms driving these patterns. To explore how source of data influence phylogenetic structure we defined assemblages using three sources of data: field inventories, museum specimens, and range maps. Assemblages were defined at two spatial grains: coarse-grained (elevational bands of 800-m width) and fine-grained (1-km2 plots). We used three different species pools: all species contained in assemblages, all species within half-degree quadrats, and all species either above or below 2000 m elevation. Metrics considering phylogenetic relationships among all species within assemblages showed phylogenetic clustering at high elevations and phylogenetic evenness in the lowlands, whereas those metrics considering only the closest co-occurring relatives showed the opposite trend. This result suggests that using multiple metrics of phylogenetic structure should provide greater insight into the mechanisms shaping assemblage structure. The source and spatial grain of data had important influences on estimates of both richness and phylogenetic structure. Metrics considering the co-occurrence of close relatives were particularly sensitive to changes in the spatial grain. Assemblages based on range maps included more species and showed less phylogenetic structure than assemblages based on museum or field inventories. Coarse-grained assemblages included more distantly related species and thus showed a more even phylogenetic structure than fine-grained assemblages. Our results emphasize the importance of carefully selecting the scale, source of data and metric used in analysis of the phylogenetic structure of assemblages.

群落（assemblages）的系统发育结构（phylogenetic structure）格局正日益被用于解析共现物种群落构建过程中涉及的生态与进化机制。系统发育结构度量指标（metrics of phylogenetic structure）对尺度效应与数据可得性较为敏感。本研究以哥伦比亚沿海拔梯度分布的蜂鸟（蜂鸟科Trochilidae）群落为研究对象，实证评估了4种群落系统发育结构度量指标对三类变量变化的响应：(i) 数据来源，(ii) 群落界定的空间粒度，(iii) 物种种库（species pools）的定义方式。本研究同时探讨了驱动上述格局形成的潜在机制及其相关研究启示。为探究数据来源对群落系统发育结构的影响，本研究采用三类数据来源界定群落：野外调查数据（field inventories）、博物馆标本数据（museum specimens）以及分布区范围数据（range maps）。群落以两种空间粒度进行界定：粗粒度（宽度为800米的海拔带）与细粒度（1平方千米样地）。本研究采用三种不同的物种种库定义方式：仅包含群落内出现的物种、分布在半度样方内的所有物种，以及海拔2000米以上或以下的所有物种。仅考虑群落内所有物种间系统发育关系的度量指标显示，高海拔区域呈现系统发育聚集格局，低海拔区域则呈现系统发育均匀格局；而仅考虑共现近缘物种关系的度量指标则呈现相反的趋势。该结果表明，采用多种系统发育结构度量指标能够更深入地解析塑造群落结构的核心机制。数据来源与空间粒度对物种丰富度及系统发育结构的估算均具有显著影响。考虑近缘物种的共现关系的度量指标对空间粒度的变化尤为敏感。基于分布区范围数据界定的群落相较于基于博物馆标本或野外调查数据的群落，所包含的物种数量更多，但其系统发育结构特征却更不显著。粗粒度群落相较于细粒度群落，包含的物种间亲缘关系更远，因此其系统发育结构更趋于均匀。本研究结果强调，在开展群落系统发育结构分析时，需谨慎选择分析尺度、数据来源与度量指标。