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.