Generalizability of findings
Honey bees were absent in most meadows before we experimentally introduced hives and the native bee community was abundant and diverse. In systems where honey bees are a natural element of bee communities or when other pollinator populations are diminished (e.g., in disturbed or agricultural systems), the negative effects we observed might be lessened or even reversed. For example, where native pollinator populations have been reduced due to habitat fragmentation or other stressors, honey bees can “rescue” plant populations from reproductive failure (Dick 2001), and, after honey bees have become naturalized, removing them may disrupt pollination of plants they would otherwise visit (Nabors et al. 2018).
However, regardless of whether honey bees are native or naturalized, dramatic increases of any species could disrupt species interactions and ecological processes (Geslin et al. 2017), particularly when floral resources are limited. For example, in France, where honey bees are native, highly abundant managed honey bees can over-exploit limited floral resources, reducing pollen and nectar collection by wild bees (Henry & Rodet 2018). Indeed, although we studied only one plant species in a specific context, there are likely many systems for which introducing honey bees or other highly abundant generalist pollinators may indirectly reduce pollination by competitively displacing other pollinators. Several recent meta-analyses have revealed that honey bees are less effective than other bees (Földesi et al. 2021; Pageet al. 2021). Furthermore, honey bees have been implicated in the extirpation of native bee species (Portman et al. 2018) and frequently compete with other pollinators for limited pollen and nectar resources (Cane & Tepedino 2016; Hudewenz & Klein 2015; Thomson 2004; Thomson 2016). Hive density is negatively correlated with wild bee abundance and diversity in many ecosystems (Angelella et al.2021; Ropars et al. 2020; Valido et al. 2019) and honey bees are replacing wild bees as floral visitors in some areas (Herrera 2020). Plant pollination declines when ineffective pollinators are over-represented in plant visitor communities (Hansen et al.2002; Vaughton 1996). Thus, indirect negative effects of honey bee introductions may be common where wild pollinator communities already effectively pollinate native plants.