Quantitative Historical Change in Bumblebee (Bombus spp.) Assemblages of Red Clover Fields

BackgroundFlower visiting insects provide a vitally important pollination service for many crops and wild plants. Recent decline of pollinating insects due to anthropogenic modification of habitats and climate, in particular from 1950's onwards, is a major and widespread concern. However, few studies document the extent of declines in species diversity, and no studies have previously quantified local abundance declines. We here make a quantitative assessment of recent historical changes in bumblebee assemblages by comparing contemporary and historical survey data. Methodology/Principal FindingsWe take advantage of detailed, quantitative historical survey data from the 1930's on bumblebee (Bombus spp.) abundances and species composition in red clover (Trifolium pratense) fields, an important floral resource and an attractant of all bumblebee species. We used the historical survey data as a pre-industrialization baseline, and repeated the same sampling protocol at nearly the same localities at present, hence setting up a historical experiment. We detected historical changes in abundances (bees/m2) of both workers (the “pollinatory units”) and queens (effective population size), in addition to species composition. In particular, long-tongued bumblebee species showed consistent and dramatic declines in species richness and abundances throughout the flowering season of red clover, while short-tongued species were largely unaffected. Of 12 Bombus species observed in the 1930's, five species were not observed at present. The latter were all long-tongued, late-emerging species. Conclusions/SignificanceBecause bumblebees are important pollinators, historical changes in local bumblebee assemblages are expected to severely affect plant reproduction, in particular long-tubed species, which are pollinated by long-tongued bumblebees.

背景：访花昆虫（Flower visiting insects）可为多种农作物与野生植物提供至关重要的授粉服务（pollination service）。自20世纪50年代起，受人类活动对栖息地的改造以及气候变化影响，授粉昆虫种群持续衰退，这已成为一项广受关注的重大议题。然而，目前鲜有研究记录物种多样性下降的具体幅度，且此前尚无研究量化本地种群丰度（abundances）的衰退情况。本研究通过对比当代与历史调查数据，对熊蜂（Bombus spp.）群落的近期历史变化进行定量评估。

研究方法与主要结果：我们利用了20世纪30年代留存的详细定量历史调查数据，该数据记录了红三叶（Trifolium pratense）田块中的熊蜂丰度与物种组成——红三叶是一类重要的蜜源植物，可吸引所有熊蜂物种到访。我们将该历史调查数据作为工业化前的基线参考，并在几乎相同的样地中重复了完全一致的采样方案（sampling protocol），由此构建了历史对照实验。除物种组成外，我们还观测到工蜂（即"授粉功能单元"，pollinatory units）与蜂王（有效种群规模，effective population size）的单位面积蜂量（只/平方米）均发生了历史变化。具体而言，长舌型熊蜂物种在红三叶整个花期内的物种丰富度（species richness）与种群丰度均呈现出持续且显著的下降趋势，而短舌型熊蜂物种则基本未受影响。在20世纪30年代记录到的12种熊蜂中，有5种在本次当代调查中未被发现，这些缺失的物种均为长舌型、晚出现的类群。

结论与意义：由于熊蜂是重要的授粉昆虫，本地熊蜂群落的历史变化预计会对植物繁殖造成严重影响，尤其是依赖长舌型熊蜂授粉的长花距植物物种。