Beta diversity and specialization in plant-pollinator networks along an elevational gradient

Aim: To assess whether the reduced nutritional resources available for pollinators due to plant community simplification along an elevational plant-diversity gradient changes pollinator niche breadth and richness. Additionally, we evaluated how body size and proboscis length of pollinators shifted along the gradient, and whether these changes were related to pollinator niche breadth.
Location: An elevational gradient (2,350-3,520 m a.s.l.) on the oceanic high-mountain strato-volcano of El Teide (Tenerife, Canary Islands).
Taxon: Flowering plant and pollinator species.
Methods: We compared quantitative plant–pollinator networks along the plant-diversity gradient. We calculated a set of niche-based topological metrics that capture the degree of specialization, niche breadth and niche overlap. Furthermore, we obtained β-diversity measures and the proportion of replacement and richness components.
Results: There was an overall decline in species richness of pollinators with increasing elevation. This decline was mainly driven by the loss of species along the elevational gradient, which conformed a nested subset pattern. The whole network showed less specialization, greater connectance and lower modularity towards the summit. At high elevations, pollinators were more generalized and less selective in their flower choice, showing a greater trophic niche breadth compared to pollinators at lower elevations. Mean body size of pollinators increased with elevation, and species body size and proboscis length were positively associated with the number of plant species visited.
Main conclusions. Overall, results indicated that the elevational gradient filters pollinator species, probably according to their thermal tolerance and ability to exploit a wide range of trophic resources. The finding that pollinators become more generalized and opportunistic at higher elevations is a novel result, which may have implications for new research into how ecological networks vary over environmental gradients. From an applied perspective, our results highlight the importance of considering the spatial variation of species assemblages when aiming to construct functionally reliable interaction networks along environmental gradients.

研究目的：旨在评估沿海拔植物多样性梯度的植物群落简化所导致的传粉者可利用营养资源减少，是否会改变传粉者的生态位宽度与物种丰富度。此外，本研究还探究了传粉者的体型大小和喙长沿该梯度的变化趋势，以及这些变化是否与传粉者生态位宽度存在关联。
研究地点：位于加那利群岛特内里费岛的海洋性高海拔层状火山泰德峰（El Teide），其海拔梯度范围为2350至3520米（a.s.l.）。
研究类群：显花植物与传粉者物种。
研究方法：本研究对比了沿植物多样性梯度的定量植物-传粉者网络（plant-pollinator networks）。我们计算了一系列可表征特化程度、生态位宽度与生态位重叠的基于生态位的拓扑指标（niche-based topological metrics）。此外，我们还获取了β多样性（β-diversity）测度值，以及物种替代组分与丰富度组分的占比。
研究结果：随着海拔升高，传粉者的物种丰富度整体呈下降趋势，该下降主要由沿海拔梯度的物种丧失驱动，且呈现嵌套子集格局。在趋近火山顶峰时，整个传粉网络的特化程度降低、连接度升高、模块化程度下降。高海拔区域的传粉者相较于低海拔区域的同类，其花部选择更为泛化、选择性更低，营养生态位宽度更大。传粉者的平均体型随海拔升高而增大，且物种体型与喙长均与访问的植物物种数量呈正相关。
主要结论：整体而言，研究结果表明海拔梯度对传粉者物种存在过滤作用，其过滤依据可能是传粉者的热耐受能力以及利用多样营养资源的能力。本研究发现传粉者在高海拔区域更为泛化且具机会主义特征，这是一项新颖的研究结果，可为探究生态网络沿环境梯度的变化规律提供新的研究方向。从应用视角来看，本研究结果强调，若要构建沿环境梯度的功能可靠的物种互作网络，需考虑物种集合的空间异质性。