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.