https://besjournals.onlinelibrary.wiley.com/doi/10.1111/1365-2656.13973

Abstract


   1. 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.
   2. 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.
   3. 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.
   4. 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.
   5. 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. In two California ecosystems, honey bee
   competition decreases pollen and nectar resource availability in flowers
   and alters native bee diets with potential implications for bee
   conservation and wildlands management.

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