Partitioning of floral resources by the three dominant bees
Three components may contribute to form the niche of a bumble bee, namely, nesting site, foraging preferences on flower type, and emergence time (Schoener, 1974). We don’t have information about nest sites (likely all subterranean) for the species we studied, but have investigated the other two components. For the two mainly nectar-seeking bees with higher abundance, B. lepidus and B. friseanus , the peak abundance was obviously staggered. In addition, the colony cycle of B. lepidus was shorter than B. friseanus (Fig. 1). Goulson and Darvill (2004) suggested that to avoid strong competition for limited resources, some rare bee species might have short colony cycles in order to coexist with the common species by specializing on short-lived high-quality resources to rear larvae quickly. The staggered emergence time may help to contribute to the coexistence of the bees.
Although the study meadow provided a large variety of flowering plants, the plants chosen for foraging did not vary significantly among all the species pairs of bees in the two years. It has been suggested that plant preference of a bee may change spatially and temporally (Jha & Kremen, 2013; Miller-Struttmann & Galen, 2014). We did not find niche differentiation in proboscis length or flower use in this meadow. Additionally, bumble bees may have intrinsic preferences for reward type (pollen vs. nectar) (Roulston & Cane, 2000). Niche differentiation in proboscis length and flower use may not occur if bees only rarely, if ever, seek nectar. On the other hand, our results indicated that plant preference obviously varied across the flowering period of the meadow. For the two mainly nectar-seeking bees, niche differentiation in proboscis length and flower use was not detected by analyzing the collective data of a year. It might exist in a complicated way and depend on community spatial and temporal factors, which allow the bees to adjust their foraging strategy in response to changes in availability of floral resources.