Data from: Direct and indirect effects of shrub encroachment on alpine grasslands mediated by plant-flower visitor interactions

Mutualistic interactions structure ecological communities and they are
strongly influenced by the combined effect of different drivers of global
change. Land-use changes and global warming have elicited rapid shrub
encroachment in alpine grasslands in recent decades, which may have
detrimental outcomes for native alpine forbs. In spite of the importance
of this process, we lack knowledge about how shrub encroachment modifies
community-wide patterns of plant–pollinator mutualistic interactions.
Based on the functional biodiversity hypothesis (FBH), which predicts
higher pollinator biodiversity in species-rich plant communities, we asked
whether the increase in nutritional resources available for pollinators
due to shrub expansion modifies pollinator niche breadth and species
richness, and whether these changes affect plant–plant interactions. For
this purpose, we compared quantitative plant–flower visitor interaction
network assemblages at replicated plots in two habitat types in dry
cryophilic grasslands of Sierra de Guadarrama (Spain): (i) encroached
pastures (EP) and (ii) pastures dominated by forb species where shrub
species are absent (PA). As predicted by FBH, flower visitor richness
increased in EP, but their niche breadth did not vary. Furthermore, shrubs
had more interactions with flower visitors and received more visits per
plant than forbs in EP in agreement with their significantly higher
linkage and strength. Overall, results revealed that moderate levels of
shrub encroachment affected the flower visitation patterns of forb species
in alpine grasslands as flower visitor diversity increased and plant–plant
competition for shared flower visitors became greater. These findings
highlight the need to use an integrative approach to study the cascading
effects of global change drivers on species interactions and their impact
on the structure and functioning of threatened ecological communities.

互利共生互作是生态群落结构的核心塑造力量，同时会受到全球变化各类驱动因子的联合效应的显著影响。近数十年来，土地利用变化与全球变暖导致高山草原出现了快速的灌木入侵现象，这可能对本土高山草本显花植物产生不利影响。尽管该过程的生态意义重大，但目前我们仍不清楚灌木入侵如何改变植物-传粉者互利共生互作的群落尺度格局。
基于功能生物多样性假说（Functional Biodiversity Hypothesis, FBH）——该假说预测物种丰富的植物群落中传粉者生物多样性更高——我们提出了两个研究问题：其一，灌木扩张为传粉者提供的营养资源增加，是否会改变传粉者的生态位宽度与物种丰富度；其二，此类变化是否会影响植物间的互作关系。为此，我们在西班牙瓜达拉马山（Sierra de Guadarrama）的干燥嗜冷草原中，选取两类生境的重复样地，对定量型植物-访花者互作网络群落进行了比较：（i）已发生灌木入侵的牧场（encroached pastures, EP）；（ii）以草本显花植物为优势种、无灌木入侵的牧场（pastures dominated by forb species where shrub species are absent, PA）。
正如功能生物多样性假说所预测，入侵牧场中的访花者物种丰富度显著提升，但其生态位宽度并未发生变化。此外，与入侵牧场中的草本显花植物相比，灌木与访花者的互作数量更多，单株植物获得的访花频次更高，这与其显著更高的互作连接度与互作强度相符。
整体而言，研究结果表明：适度的灌木入侵会改变高山草原中草本显花植物的访花模式——访花者多样性提升的同时，植物间对共享访花者的种间竞争也愈发激烈。
本研究结果凸显了采用整合性研究方法的必要性，以探究全球变化驱动因子对物种互作的级联效应，及其对受威胁生态群落结构与功能的影响。