Introduced honey bees increase host plant abundance but decrease native bumble bee species richness and abundance

Long-term variation in the population density of introduced honey bees
(Apis mellifera) has been shown to be associated with variations in floral
traits in alpine lotus (Saussurea nigrescens). However, it remains to be
determined whether a high density of honey bees affects the abundance of
nectariferous plants and the species richness and abundance of native
bumble bees. We predicted that a high density of introduced honey bees
lasting three decades would decrease the species richness and abundance of
native bumble bees but increase the abundance of honeybee host plant
species. Here, the field experiments were conducted to examine the
diversity of nectariferous plants and native bumble bees along the typical
gradients of honey bee density (high density of honey bee at close apiary
and low density of honey bee at distant of apiary). We investigated
nectariferous plant abundance, floral and seed traits, bumble bee species
richness and abundance at sites with either a high or low honey bee
density in an alpine meadow. Our results demonstrated that an increased
population of introduced honey bees was associated with increased host
plant abundance and flower/capitula number per plant but decreased nectar
volume per flower, seed mass, species richness and abundance of native
bumble bees. The bumble bee visitation rate was positively correlated with
nectar volume per flower at sites close to and far from apiaries. The
honey bee visitation rate was positively correlated with flower/capitula
number per plant at sites close to apiaries and nectar volume per flower
at sites far from apiaries. Seed mass was negatively correlated with
nectariferous plant abundance. Our findings showed that introduced honey
bees decreased the species richness and abundance of native bumble bees,
attributed to evolutionary decrease nectar resources among honey bee host
plant species, but increased the abundance of nectariferous plants,
attributed to the production of many small seeds by plants. This suggests
that long-term high-density beekeeping affects the biodiversity of honey
bee host plants and native bumble bees. Our results provide new insights
into the mechanisms of maintaining the biodiversity of nectariferous
plants and native bumble bees.