Spatial and climatic variables independently drive elevational gradients in ant species richness in the Eastern Himalaya

Elevational gradients are considered important for understanding causes behind gradients in species richness due to the large variation in climate and habitat within a small spatial extent. Geometric constraints are thought to interact with environmental variables and influence elevational patterns in species richness. However, the geographic setting of most mountain ranges, particularly continuity with low elevation areas may reduce the effect of geometric constraints at lower elevations. In the present study, we test the effects of climatic gradients and continuity with the low elevation plains of the eastern Himalayan mountain range on patterns of species richness. We studied species richness of ants (Hymenoptera: Formicidae) on an elevational gradient between 600m and 2400m in the Eastern Himalaya–part of Himalaya biodiversity hotspot. Ants were sampled in nine elevational bands of 200m with four transects in each band using pitfall and Winkler traps. We used regression models to identify the most important environmental variables that predict species richness and used constrained null models to test the effects of contiguity between the mountain range and plains. We find a monotonic decline in species richness of ants with elevation. Temperature was a more important predictor of species richness than habitat complexity. Geometric constraints model weighted by temperature with a soft lower boundary and hard upper boundary best explained the species richness pattern. This suggests that a combination of climate and geometric constraints drive the elevational species richness patterns of ants.

由于在较小空间尺度内气候与栖息地存在显著异质性，海拔梯度（elevational gradients）被视作解析物种丰富度（species richness）梯度成因的关键研究对象。几何约束（geometric constraints）假说可与环境变量相互作用，进而调控物种丰富度的海拔分布格局。然而多数山脉的地理区位特征，尤其是其与低海拔区域的连续性，可能会削弱低海拔区段的几何约束效应。本研究以东喜马拉雅山脉与低海拔平原的连续性及气候梯度为研究变量，探究二者对物种丰富度分布格局的影响。本研究以隶属喜马拉雅生物多样性热点（Himalaya biodiversity hotspot）的东喜马拉雅地区为研究样区，针对600m至2400m的海拔梯度开展蚂蚁（膜翅目（Hymenoptera）：蚁科（Formicidae））物种丰富度调查。研究采用地面陷阱诱集法（pitfall trap）与温克勒诱集器（Winkler trap），在9个200m间隔的海拔带内各设置4条样线完成样本采集。本研究通过回归模型筛选出对物种丰富度预测能力最优的环境变量，并借助约束零模型检验山脉与平原的邻接效应。研究结果表明，蚂蚁物种丰富度随海拔升高呈单调递减趋势。相较于栖息地复杂度，气温是预测物种丰富度更为关键的环境因子。以气温为权重、兼具软下界与硬上界的几何约束模型最能解释本次观测到的物种丰富度分布格局。这意味着气候与几何约束的共同作用塑造了蚂蚁物种丰富度的海拔分布格局。