Evaluation of Common Methods for Sampling Invertebrate Pollinator Assemblages: Net Sampling Out-Perform Pan Traps

Methods for sampling ecological assemblages strive to be efficient, repeatable, and representative. Unknowingly, common methods may be limited in terms of revealing species function and so of less value for comparative studies. The global decline in pollination services has stimulated surveys of flower-visiting invertebrates, using pan traps and net sampling. We explore the relative merits of these two methods in terms of species discovery, quantifying abundance, function, and composition, and responses of species to changing floral resources. Using a spatially-nested design we sampled across a 5000 km2 area of arid grasslands, including 432 hours of net sampling and 1296 pan trap-days, between June 2010 and July 2011. Net sampling yielded 22% more species and 30% higher abundance than pan traps, and better reflected the spatio-temporal variation of floral resources. Species composition differed significantly between methods; from 436 total species, 25% were sampled by both methods, 50% only by nets, and the remaining 25% only by pans. Apart from being less comprehensive, if pan traps do not sample flower-visitors, the link to pollination is questionable. By contrast, net sampling functionally linked species to pollination through behavioural observations of flower-visitation interaction frequency. Netted specimens are also necessary for evidence of pollen transport. Benefits of net-based sampling outweighed minor differences in overall sampling effort. As pan traps and net sampling methods are not equivalent for sampling invertebrate-flower interactions, we recommend net sampling of invertebrate pollinator assemblages, especially if datasets are intended to document declines in pollination and guide measures to retain this important ecosystem service.生态集合体（ecological assemblages）采样方法始终以高效性、可重复性与代表性为核心目标。人们往往未意识到，常规采样方法在揭示物种功能方面存在局限，进而在比较研究中的应用价值有限。 受全球传粉服务（pollination services）衰退的影响，学界开展了大量访花无脊椎动物（flower-visiting invertebrates）调查，其中色盘诱捕器（pan traps）与网捕采样（net sampling）是两种常用方法。 本研究围绕两种方法在物种发现、多度量化、功能解析与群落组成表征，以及物种对蜜源资源（floral resources）变化的响应这几个维度，对比分析其相对优势。2010年6月至2011年7月期间，研究团队采用空间嵌套设计（spatially-nested design），在5000平方千米的干旱草原区域开展采样工作，累计完成432小时网捕采样与1296个色盘诱捕器诱捕日。 相较于色盘诱捕器，网捕采样获得的物种数多22%、个体多度（abundance）高30%，且能更精准地反映蜜源资源的时空动态（spatio-temporal variation）。两种方法获取的物种组成（species composition）存在显著差异：在总计436个物种中，25%可被两种方法同时采集到，50%仅通过网捕采样获得，剩余25%仅由色盘诱捕器捕获。 除采样全面性不足外，若色盘诱捕器无法采集到访花昆虫，则其与传粉过程的关联便难以成立。与之相反，网捕采样可通过观测访花交互频率的行为学数据，将物种与传粉功能建立直接关联。此外，网捕获得的标本也是验证花粉传递（pollen transport）现象的必要材料。尽管整体采样工作量仅存在小幅差异，但网捕采样的优势更为显著。 鉴于色盘诱捕器与网捕采样在访花无脊椎动物与植物互作的采样中并不等效，本研究建议优先采用网捕采样开展无脊椎动物传粉者类群（invertebrate pollinator assemblages）调查，尤其是当数据集用于记录传粉服务衰退情况并指导保护措施以保留这一重要生态系统服务（ecosystem service）功能时。