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).