DOI: 10.25338/b80s8f
title: Evidence of exploitative competition between honey bees and native bees in two California landscapes

description: 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.
description: <strong>Field surveys of plant-pollinator
 interactions</strong>: We conducted this work in the California
 Central Valley at five replicated wildflower plantings neighboring almond
 orchards which we sampled in 2017 and 2018. We also sampled 15 montane
 meadows in the Central Sierra Nevada in 2019. In both ecosystems, we
 surveyed pollinators and their visits to flowering plants over multiple
 sample rounds. For network analyses, we excluded bees not identified to
 morphospecies (~3% of all specimens). Because we were exclusively
 interested in documenting bee competition, we also excluded non-bee floral
 visitors from network analyses.
 <em><strong>Assessing pollen diet composition In the
 lab</strong></em>, we counted and identified pollen grains on
 the bodies of collected bee specimens, focusing on scopal loads. We
 calculated pollen fidelity as the number of pollen grains from the plant
 species from which the specimen was caught divided by the total number of
 pollen grains in the swabbed sample. We calculated pollen diversity using
 the Shannon-Weiner diversity index.  
 <em><strong>Quantifying pollen resource depletion In
 both systems</strong>,</em> at the end of each sampling day,
 we measured pollen and nectar availability in the field using one to three
 flowers on 10–20 “open-pollinated” plants and 10-20 unvisited control
 plants which were bagged on site arrival (between 6h30 and 7h30) and prior
 to pollinator activity. We measured pollen availability as the proportion
 of dehisced anthers with pollen visible to the naked eye and measured
 nectar availability using 1µL capillary tubes. 
 <em><strong>Network
 metrics:</strong></em> For each site and sample round in each
 year and system, we generated unique plant x pollinator visitation
 networks. In total, we generated 40 networks across two years of sampling
 for our Central Valley sites and 48 networks for our Sierra sites. For
 each network, we used the bipartite package and R to calculate pollinator
 specialization (d’), complementary specialization (H2'), and
 perceived apparent competition (PAC).  Because PAC is sensitive to the
 relative abundance of competing species, we compared observed values of
 PAC against PAC values calculated using null networks. To perform this
 comparison, we generated 500 randomized null networks using the r2rtable
 method, which resamples interactions but keeps row and column sums
 constant and compared observed metrics against null network metrics using
 Z-scores. To complement calculations of PAC between honey bees and native
 bees, we also calculated the mean PAC value for all pairwise comparisons
 of native bees against other native bees. For more details about methods
 and statistical tests used to analyse these data, please see the
 associated publication.