Functional and phylogenetic structure of forest bird assemblages along an Afrotropical elevational gradient

Aim: To evaluate community assembly processes, we comprehensively analysed
avian functional and phylogenetic diversity along the only complete
elevational forest gradient in west-central Africa. We aimed to reveal the
signals of environmental filtering and interspecific competition in
community assembly and determine whether increases in species richness
across elevations are associated with niche space packing or expansion.
Location: Mount Cameroon, west-central Africa. Taxon: Birds. Methods: We
analysed data on elevational distributions of birds collected within 10
years from the lowland forest up to the treeline. We calculated functional
richness, evenness, and nearest neighbour distance, as well as
phylogenetic mean pairwise dissimilarity and mean nearest taxon distance.
Community assembly processes were inferred from each metric, after
controlling for species richness. To estimate the importance of niche
packing versus niche expansion, we assessed whether species of
species-richer assemblages occurred within or outside the niche space of
species-poorer assemblages. Results: Stronger signals of environmental
filtering were observed at mid-elevations. Functional distances among
species increased at high elevations, supporting the importance of the
Fox’s guild assembly rule (Fox, 1987). Species richness increments were
mainly associated with niche packing. We found random basal phylogenetic
structure across elevations, whereas terminal phylogenetic clustering
appeared in lowland and highland areas. Main conclusions: Environmental
filtering showed a rather weak effect towards higher altitudes over the
highly productive and relatively short tropical forest elevation gradient,
where niche packing was the dominant process behind the species richness
increments. Functional distances among species increased at higher
elevations, suggesting lower species density in ecological space. At the
same time, mild environmental conditions and a high level of resources in
the lowlands may sustain functionally redundant species, downplaying the
role of competition. Phylogenetic clustering at low and high elevations
further suggests the existence of distinct lowland and highland bird
faunas.

目标：为评估群落组装过程（community assembly processes），我们全面分析了中非中西部唯一完整海拔森林梯度（elevational forest gradient）上的鸟类功能多样性（functional diversity）与系统发育多样性（phylogenetic diversity）。旨在揭示群落组装中环境过滤（environmental filtering）与种间竞争（interspecific competition）的信号，并确定海拔梯度上物种丰富度（species richness）的增加是否与生态位空间填充（niche space packing）或扩展（expansion）相关。
位置：喀麦隆山（Mount Cameroon），中非中西部。
分类群：鸟类。
方法：我们分析了10年间从低地森林（lowland forest）至林线（treeline）采集的鸟类海拔分布数据。计算了功能丰富度（functional richness）、均匀度（evenness）、最近邻距离（nearest neighbour distance），以及系统发育平均成对差异度（phylogenetic mean pairwise dissimilarity）和平均最近分类群距离（mean nearest taxon distance）。在控制物种丰富度后，通过各指标推断群落组装过程。为评估生态位填充与生态位扩展的相对重要性，我们分析了物种丰富度更高的群落中的物种是否出现在物种丰富度较低群落的生态位空间内或外。
结果：在中海拔区域观察到更强的环境过滤信号。高海拔区域物种间功能距离（functional distances）增加，支持Fox功能群组装规则（Fox’s guild assembly rule, Fox, 1987）的重要性。物种丰富度增加主要与生态位空间填充相关。我们发现海拔梯度上基部系统发育结构（basal phylogenetic structure）随机，而末端系统发育聚类（terminal phylogenetic clustering）出现在低地与高地区域。
主要结论：在生产力高且相对较短的热带森林海拔梯度上，环境过滤对更高海拔区域的影响较弱，而生态位填充是物种丰富度增加背后的主导过程。高海拔区域物种间功能距离增加，表明生态位空间中物种密度较低。同时，低地温和的环境条件与丰富的资源可能维持功能冗余物种（functionally redundant species），从而弱化竞争作用。低地与高地的系统发育聚类进一步表明存在独特的低地与高地鸟类区系（bird faunas）。