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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