Evidence of exploitative competition between honey bees and native bees in two California landscapes

Human-mediated species introductions provide real-time experiments in how communities respond to interspecific competition. For example, managed honey bees (Apis mellifera L.) have been widely introduced outside their native range and may compete with native bees for pollen and nectar. Indeed, multiple studies suggest that honey bees and native bees overlap in their use of floral resources. However, for resource overlap to negatively impact resource collection by native bees, resource availability must also decline, and few studies investigate impacts of honey bee competition on native bee floral visits and floral resource availability simultaneously. In this study, we investigate impacts of increasing honey bee abundance on native bee visitation patterns, pollen diets, and nectar and pollen resource availability in two Californian landscapes: wildflower plantings in the Central Valley and montane meadows in the Sierra. We collected data on bee visits to flowers, pollen and nectar availability, and pollen carried on bee bodies across multiple sites in the Sierra and Central Valley. We then constructed plant-pollinator visitation networks to assess how increasing honey bee abundance impacted perceived apparent competition (PAC), a measure of niche overlap, and pollinator specialization (d'). We also compared PAC values against null expectations to address whether observed changes in niche overlap were greater or less than what we would expect given the relative abundances of interacting partners. We find clear evidence of exploitative competition in both ecosystems based on the following results: (1) honey bee competition increased niche overlap between honey bees and native bees, (2) increased honey bee abundance led to decreased pollen and nectar availability in flowers, and (3) native bee communities responded to competition by shifting their floral visits, with some becoming more specialized and others becoming more generalized depending on the ecosystem and bee taxon considered. Although native bees can adapt to honey bee competition by shifting their floral visits, the coexistence of honey bees and native bees is tenuous and will depend on floral resource availability. Preserving and augmenting floral resources is therefore essential in mitigating negative impacts of honey bee competition.

人类活动介导的物种引种，为探究群落对种间竞争的响应机制提供了实时实验体系。以人工饲养的西方蜜蜂（Apis mellifera L.）为例，该物种已被广泛引种至其自然分布范围之外，可能会与本土蜜蜂竞争花粉与花蜜资源。多项研究表明，西方蜜蜂与本土蜜蜂在花部资源的利用上存在重叠。然而，若要使资源重叠对本土蜜蜂的资源采集产生负面影响，花部资源的可获得性也必须同步下降；目前鲜有研究同时探究西方蜜蜂的种间竞争对本土蜜蜂访花行为，以及花部资源可获得性的影响。本研究聚焦美国加利福尼亚州的两类生境：中央谷地的野生花卉种植区与内华达山脉的山地草甸，探究西方蜜蜂种群密度升高对本土蜜蜂访花模式、花粉食谱，以及花蜜与花粉资源可获得性的影响。研究人员在中央谷地与内华达山脉的多个样地中，采集了蜜蜂访花记录、花部花蜜与花粉可获得性数据，以及蜜蜂体表携带的花粉样本。随后，研究人员构建了植物-传粉者访花网络，以评估西方蜜蜂种群密度升高如何影响感知表观竞争（perceived apparent competition, PAC）——这一指标用于衡量生态位重叠程度——以及传粉者特化程度（d'）。此外，研究人员将PAC值与零假设预期值进行对比，以明确观测到的生态位重叠变化幅度，是否相较于基于互作物种相对丰度的预期值更高或更低。基于以下结果，本研究在两类生境中均找到了资源利用型竞争的明确证据：（1）西方蜜蜂的种间竞争提升了其与本土蜜蜂之间的生态位重叠；（2）西方蜜蜂种群密度升高导致花部花粉与花蜜资源可获得性下降；（3）本土蜜蜂群落通过调整访花策略响应种间竞争：根据生境类型与蜜蜂类群的不同，部分类群的特化程度升高，而部分类群的泛化程度提升。尽管本土蜜蜂可通过调整访花策略适应西方蜜蜂的种间竞争，但二者的共存状态仍较为脆弱，且高度依赖花部资源的可获得性。因此，保护并扩充花部资源，是缓解西方蜜蜂种间竞争负面影响的核心举措。