Data from: Natural habitat loss and exotic plants reduce the functional diversity of flower visitors in a heterogeneous subtropical landscape

1. Functional diversity (FD) of pollinators can increase plant reproductive output and the stability of plant-pollinator communities. Yet, in times of worldwide pollinator declines, effects of global change on pollinator FD remain poorly understood. Loss of natural habitat and exotic plant invasions are two major drivers of global change that particularly threaten pollinator diversity. 2. In a subtropical South African landscape, we investigated changes in the FD of flower visitor assemblages on native and exotic plants along gradients of natural habitat loss and relative abundance of exotic plants. We used a dataset of 1434 flower visitor individuals sampled on 131 focal plants and calculated the FD in three flower visitor traits that are strongly related to plant-flower visitor interactions and pollination processes: proboscis length, proboscis diameter and body length. 3. Multivariate FD of flower visitors decreased with both increasing natural habitat loss and relative exotic abundance. Importantly, changes in FD went beyond those in flower visitor richness. Furthermore, flower visitor richness was not related to either natural habitat loss or relative exotic abundance. Loss in multivariate FD seemed to be mediated by complementary reductions of FD in proboscis length with natural habitat loss and of FD in body length with both global change drivers. Correspondingly, we recorded lower abundances of long-tongued flower visitors with natural habitat loss and reduced variance in body size with both drivers. In contrast, FD in proboscis diameter was unaffected by either driver. All effects of the two global change drivers were non-interactive. 4. Our results show that both natural habitat loss and exotic plants negatively affect flower visitor FD, which may imperil pollination of specialised plant species in degraded habitats. In contrast, flower visitor richness may not cover all facets of flower visitor FD that are relevant to pollination processes, and here future studies are needed. Distinct responses of visitor traits to the two drivers suggest limited options to infer relations of one trait to another. Finally, additive effects of natural habitat loss and exotic plant invasions highlight the need to consider multiple drivers of global change when investigating ecosystem processes at a community scale.

1. 传粉者的功能多样性（Functional diversity, FD）可提升植物繁殖产出与植物-传粉者群落的稳定性。然而在全球传粉者衰退的当下，全球变化对传粉者功能多样性的影响仍知之甚少。自然生境丧失与外来植物入侵是全球变化的两大主要驱动因子，对传粉者多样性构成尤为严重的威胁。
2. 本研究在南非亚热带景观中，围绕自然生境丧失梯度与外来植物相对丰度梯度，探究了本土与外来植物上的访花昆虫类群功能多样性的变化。我们依托131株目标植物上采集的1434只访花昆虫个体数据集，计算了三类与植物-访花者互作及传粉过程密切相关的访花者功能多样性指标：喙长、喙径与体长。
3. 访花昆虫的多变量功能多样性随自然生境丧失程度加剧与外来植物相对丰度升高均呈下降趋势。重要的是，功能多样性的变化范畴超出了访花者物种丰富度的变化范畴。此外，访花者物种丰富度与自然生境丧失或外来植物相对丰度均无关联。多变量功能多样性的下降似乎由两类互补性下降所介导：自然生境丧失引发喙长功能多样性下降，而两类全球变化驱动因子均导致体长功能多样性下降。相应地，我们观测到随着自然生境丧失，长舌访花者的丰度降低，且两类驱动因子均导致体型方差减小。与之相反，喙径的功能多样性未受任一驱动因子影响。两大全球变化驱动因子的效应均无交互作用。
4. 本研究结果表明，自然生境丧失与外来植物入侵均会对访花昆虫功能多样性产生负面影响，这或会威胁退化生境中特化植物物种的传粉过程。相较而言，访花者物种丰富度或许未能涵盖传粉过程相关的所有访花者功能多样性维度，未来亟需开展相关研究。访花者性状对两类驱动因子的差异化响应表明，推断单一性状与其他性状间关联的可行途径十分有限。最后，自然生境丧失与外来植物入侵的加性效应凸显出，在群落尺度下研究生态系统过程时，需纳入全球变化的多重驱动因子进行考量。