The relative role of rivers, environmental heterogeneity and species traits in driving compositional changes in Southeastern Amazonian bird assemblages

Amazonian rivers have been proposed to act as geographic barriers to species dispersal, either driving allopatric speciation or defining current distribution limits. The strength of the barrier varies according to the species ecological characteristics and the river physical properties. Environmental heterogeneity may also drive compositional changes, but have hardly been assessed in Amazonia. Aiming to understand the contributions of riverine barriers and environmental heterogeneity in shaping compositional changes in Amazonian forest bird assemblages, we focus on the Tapajós River. We investigate how spatial variation in species composition is related to physical barriers (Tapajós and Jamanxim rivers), ecological characteristics of the species (distinct guilds) and environmental heterogeneity (canopy reflectance, soils and elevation). We sampled birds through point counts and mist nets on both banks of both the Tapajós and Jamanxim rivers. To test for relationships between bird composition and environmental data, we used Mantel and partial Mantel tests, NMDS and ANOVA + Tukey HSD. The Mantel tests showed that the clearest compositional changes occurred across the Tapajós River, which seems to act unequally as a significant barrier to the bird guilds. The Jamanxim River was not associated with differences in bird communities. Our results reinforce that the Tapajós River is a biogeographical boundary for birds, but environmental heterogeneity determines compositional variation within interfluves. These results contrast with diversity patterns described for other vertebrates, suggesting that upland forest birds singularly respond to large rivers as barriers in Amazonia, leading to erroneous extrapolations for interpreting biogeographic results for other Amazonian organisms.

有研究指出，亚马孙河流域的河流可作为物种扩散的地理屏障，既可能推动异地物种形成（allopatric speciation），也可界定物种当前的分布边界。此类屏障的强度随物种的生态特征及河流的物理属性而异。环境异质性同样可能驱动群落组成发生变化，但相关研究在亚马孙地区尚属匮乏。为探究河流屏障与环境异质性对亚马孙森林鸟类群落组成变化的调控作用，本研究以塔帕若斯河（Tapajós River）为研究对象。我们分析了鸟类群落组成的空间变异与三类因素的关联：物理屏障（塔帕若斯河与雅曼辛河（Jamanxim River））、物种生态特征（不同功能群）以及环境异质性（冠层反射率、土壤与海拔）。研究团队在塔帕若斯河与雅曼辛河的两岸，通过点计数法与雾网法开展鸟类采样。为检验鸟类群落组成与环境数据间的关联，我们采用了曼特尔（Mantel）检验、偏曼特尔检验、非度量多维标度法（NMDS，Non-metric Multidimensional Scaling）以及方差分析（ANOVA，Analysis of Variance）+ 图基诚实显著差异检验（Tukey HSD，Tukey's Honestly Significant Difference）。曼特尔检验结果显示，群落组成的最显著变化发生在塔帕若斯河两岸，该河对不同鸟类功能群的屏障作用并不均等。雅曼辛河并未与鸟类群落的差异存在显著关联。本研究结果进一步证实，塔帕若斯河是鸟类的生物地理边界，但环境异质性决定了河间地块内部的群落组成变异。本研究结果与其他脊椎动物的多样性分布模式存在显著差异，这表明亚马孙地区的高地森林鸟类对大型河流作为屏障的响应具有特异性，若以此类推解读其他亚马孙生物的生物地理研究结果，可能会得出错误结论。