where a is the semi-major axis, or half the length or width
(whichever was longest) of a flower’s receptacle or capitulum, bis the semi-minor axis, or half the length or width (whichever was
shortest) of a flower’s receptacle or capitulum, and h is the
height of a flower or inflorescence (Fig 2c and Appendix Table A2).
To determine which measurement of floral dimensions was the best proxy
for FR amount, literature searches for daily nectar sugar mass (µg/day)
and pollen volume (in µl/flower) were conducted for all flowering
species encountered; these measurements have been previously used to
assess FR available to pollinators (Hicks et al. 2016, Baude et al.
2016). Literature sources that provided counts of pollen grains per
flower and volumes of individual pollen grains were used to calculate an
estimate of pollen volume per flower for species for which we could not
find measurements of total pollen volume. Nectar sugar mass was obtained
for 46 species and pollen volume for 33 species of the 96 encountered
(see Appendix Tables A3–A4 for full species lists). Pearson
correlations between nectar sugar mass or pollen volume and the length,
width, height, surface area, and volume measurements of each species
(all variables log-transformed to approximate normal distributions) were
used to determine which floral dimension could best estimate the amount
of FR.
For all bee genera other than Peponapis , the abundance of FR in
the landscape surrounding each sampling location was calculated by
determining the mean FR value per flower of each species and multiplying
this value by the count of each flower in a quadrat. In the genusPeponapis , pollen is collected exclusively from Cucurbitaspp. (Hurd et al. 1974). Therefore, in models of Peponapisvisits, the abundance of FR in the landscape surrounding each sampling
location was calculated from the mean FR value per squash
(Cucurbita spp.) or cucumber (Cucumis sativus ) flower,
since 99.7% of all visits observed were to squash and 0.3% were to
cucumber. While the other bee genera we considered include some
oligolectic (pollen-specialist) species in our study area, they are not
uniformly specialized on a single plant taxon, so all rewarding plant
taxa were included in calculations of FR for these bees. The mean
abundance of FR per 1 m2 was then calculated across
quadrats for each transect, and the median of the transect-level values
was calculated for each land type during each time period. This number
was then multiplied by the total area of each land type within 250 m,
500 m, and 750 m around a sampling location to obtain an estimate of the
total FR at a given spatial scale during a given time period.
Statistical
analysis
All statistical analyses were performed in R version 3.6.1 (R Core Team
2019). Analyses of bee visitation rate per transect were conducted on
bee genera that were present in at least four out of 27 sites during a
given time period. Spatial autocorrelation among sites in the number of
visits by each genus in a given time period was assessed using Moran’s I
(Paradis et al. 2004). Visits from Apis mellifera were found to
be spatially autocorrelated across all time periods (p = 0.02, Moran’s I
|observed – expected| = 0.14), likely due to the
presence of hives on certain farms, so were not analyzed.
Generalized linear mixed models were run with a zero-inflated negative
binomial distribution and log link function, using the glmmTMB package
(Brooks et al. 2017). Models treated the total number of bee visits
observed within a transect as the response variable and were run
separately for each genus; all models included a log offset to account
for varying lengths of observation time based on transect sizes, and the
crossed random effects of time period and site. For genera that were
present in at least four sites in only a subset of the time periods,
only models including data for those time periods were run. The model
for the null hypothesis was of the form