Data analysis
Based on the total bumble bee counts (for which all castes were pooled),
our analyses focused on the three most common species: Bombus
lepidus , B. friseanus , and B. festivus . The reason for
using data for all castes combined was to provide an overall measure of
dietary similarity among species. To quantify the effects of temporal
variability and availability of floral resources, the data were
collected on each of the sampling dates. On a sampling day, the
abundance of each bumble bee species was recorded by the accumulated
numbers of visits for the species from all transects and averaged to a
single transect level. To quantify use of the available resources, the
accumulated numbers of a flowering plant visited by a bumble bee species
were also noted from all the sampled transects on the day. For each of
the bumble bee species, the proportion of nectar collectors out of all
recorded visits was also calculated on each sampling day.
In this study, we aimed to find the pattern of the partitioning for
floral resources among the three bees, and the temporal dynamic of the
pattern, and moreover, the influence of availability of floral resources
on the shift in foraging strategy of the bees. For the partitioning of
floral resources among the three bees, we first compared the bee
abundance on each sampling day through the flowering season of the
community. Second, we analyzed the differences in plants visited by each
of the bumble bee species between species pairs for the three bees in
2018 and 2019. The resource use of each bumble bee species in a year
included the flower identity and the total number of visits; a paired
t-test was used to detect the difference in plant composition between
species pairs under IBM SPSS 22 (IBM Corp, 2013). We also calculated the
relative frequency of a bee on a flowering plant on a sampling day. The
top 15 plants that received most visits by the bees were investigated.
The relative visiting frequency refers to the proportion of the number
of visits for a plant out of the total number of visits on all plants
for a bee on a sampling day. To detect the influence of availability of
floral resources on foraging strategy of the bees, we investigated the
intensity of interspecific competition for floral resources on plant
preference.
The flower diversity and availability of floral resources may vary
temporally, as does the abundance of bees, thereby creating different
levels of intensity of interspecific competition throughout the
flowering season. The overlap of floral-resource usage between bumble
bee species has been used widely to evaluate the intensity of
interspecific competition among bees in a community (Goulson et al.,
2008b; Nishikawa & Shimamura, 2016). In this study, this measure of
niche overlap was also applied to quantify the competition intensity
among the three bumble bee species for floral resources across the
different sampling dates. We used the measure of intensity of
competition between species pairs was that proposed by Colwell and
Futuyma (1971). The niche overlap between bumble bee species i and h was
calculated by the formula:
1-1/2\(\sum_{k}{|P_{\text{ik}}}\)-\(P_{\text{hk}}\)|
where
\begin{equation}
P_{\text{ik}}=\frac{\text{Individuals\ of\ species\ i\ visting\ plant\ species\ k}}{\text{Total\ individuals\ of\ species\ i}}\nonumber \\
\end{equation}To detect the plant preference of a bumble bee in response to changes in
competition intensity, we analyzed the relationship between the values
of niche overlap and those of the proportion of nectar collectors out of
total visitors for each of the bumble bee species. In addition, the
relationship between abundance of each bumble bee and its niche overlap
with the other bees was quantified by the Pearson correlation under IBM
SPSS 22 (IBM Corp, 2013).
Niche differentiation in proboscis length and flower use is thought to
play an important role in partitioning of floral resources among bumble
bees (Heinrich, 1976; Inouye, 1980; Pyke, 1982). However, such a
correlation could be complicated because of the temporal and spatial
variations in identity and availability of floral resources (Pyke et
al., 2012; Miller-Struttmann & Galen, 2014). A bumble bee may visit a
wide range of plant species with varied floral depth (see also Ranta &
Lundberg, 1980). To show better the foraging preference for flowers with
different traits for a bumble bee, we developed an index of composite
flower depth. On each sampling day, all the flowers visited by one
bumble bee species collectively formed a composite flower depth, which
was calculated as follows:
Composite flower depth\(\mathrm{=}\mathrm{\ }x_{1}f_{1}+x_{2}f_{2}+\ldots+x_{k}f_{k}\)
Where, xk means the flower depth of species k;
fk represents the percentage of visits received by plant
species k out of the total visits made by bumble bee species on a
sampling day. The plants with nectarless flowers were excluded in
calculation of composite flower depth. To detect the influence of
intensity of interspecific competition for floral resources on foraging
strategy of bumble bees, we analyzed the relationship between the niche
overlap and composite flower depth for each of the bumble bee species by
using Pearson correlation under IBM SPSS 22 (IBM Corp, 2013).