Scaling up effects of measures mitigating pollinator loss from local- to landscape-level population responses

1. Declining pollinator populations have caused concern about consequences for food production, and have initiated an increasing number of initiatives that aim to mitigate pollinator loss through enhancement of floral resources. Studies evaluating effects of mitigation measures generally demonstrate positive responses of pollinators to floral resource enhancement. However, it remains unclear whether this represents landscape-level population effects or results from a spatial redistribution of individuals from otherwise unaffected populations. 2. Here we present a method for estimating landscape-level population effects using data from commonly used standardized pollinator transect surveys. The approach links local density responses of pollinators in both mitigation sites and surrounding landscape elements to the area these habitats occupy in mitigation landscapes as well as control landscapes to obtain landscape-level population estimates. 3. We demonstrate the method using data from a two-year study examining the effects of experimental wildflower enhancements on wild bumble bees and solitary bees in Dutch agricultural landscapes. The results show that conclusions based on local responses may differ significantly from those based on landscape-level responses. 4. Wildflower enhancements significantly enhanced landscape-level abundance of both bumble bees and solitary bees. Bumble bees showed a pronounced positive local density response in mitigation sites and the surrounding landscape that was in line with significant landscape-level increases in abundance. However, solitary bees showed no local response to mitigation sites, and the landscape-level increases in abundance only became apparent when the area of bee habitat was taken into account. 5. Incorporating the area of both newly created and pre-existing pollinator habitats into effect estimates accounts for density-dependent processes such as dilution, spill-over and local concentration of individuals. It therefore results in more reliable estimates of the response to mitigation measures of pollinators, as well as other mobile arthropod groups that are often being surveyed using transect surveys.

1. 传粉昆虫（pollinator）种群数量下降引发了人们对粮食生产相关后果的担忧，同时催生了越来越多旨在通过增强花资源供给以缓解传粉昆虫种群衰减的行动。现有评估种群缓解措施效果的研究普遍表明，传粉昆虫对花资源供给增强呈现正向响应。但目前仍不清楚这类响应究竟反映了景观尺度的种群效应，还是源于未受干扰种群个体的空间再分布。
2. 本研究提出一种基于常用标准化传粉昆虫样线调查（pollinator transect survey）数据的景观尺度种群效应估算方法。该方法将修复样地及周边景观单元内传粉昆虫的局域密度响应，与修复景观和对照景观中各类生境的占比面积相结合，以此得到景观尺度的种群估算结果。
3. 我们利用荷兰农业景观中一项为期两年的实验性野花增植对野生熊蜂（wild bumble bee）与独居蜂（solitary bee）影响的研究数据，对该方法进行了验证。结果显示，基于局域响应得出的结论，与基于景观尺度响应得出的结论可能存在显著差异。
4. 野花增植显著提升了熊蜂与独居蜂的景观尺度种群丰度。熊蜂在修复样地及周边景观中呈现出显著的正向局域密度响应，这与景观尺度上丰度的显著提升相一致。但独居蜂在修复样地中未表现出局域响应，其景观尺度的丰度提升仅在纳入蜂类生境面积后才显现出来。
5. 将新增及原有传粉昆虫生境的面积纳入效应估算，可涵盖个体的稀释效应、溢出效应与局域聚集等密度依赖过程。因此，该方法能够更可靠地估算传粉昆虫，以及其他常通过样线调查开展研究的移动性节肢动物类群对缓解措施的响应。