The influence of intensity of interspecific competition on
foraging strategy for the three bees
Our study indicated that the intensity of interspecific competition
(estimated by niche overlap) was not correlated with bee abundance
across the sampling dates. The increasing bee abundance did not
accelerate the intensity of interspecific competition while the
mitigation of intensity of interspecific competition was not caused by a
decrease of bee abundance. Moreover, we found that the foraging strategy
was significantly correlated with competition intensity for all the
three bees (Figs. 3 and 4). We conclude that for these bumble bees, the
primary factor influencing the temporal change of foraging strategy
might be the abundance of interspecific instead of intraspecific
individuals.
The mainly pollen-seeking bee, B. festivus , foraged more on
nectarless flowers than those with nectar during our investigations.
However, its preference for reward type (pollen vs. nectar) did not
remain stable. Our results indicated that the intensity of interspecific
competition significantly influenced the foraging strategy of the bee.
The bee mainly visited nectarless flowers to collect pollen grains on
the days with low competition intensity. As competition intensity
increased, the bees tended to forage for nectar. Although pollen
provides important and necessary nutrimental requirement for bumble bee
(Roulston & Cane, 2000; Goulson & Darvill, 2004; Woodard, 2017;
Woodard et al., 2019), nectar may provide more energy than pollen (Baker
& Baker, 1983). The increase of intensity of interspecific competition
may decrease the availability of floral rewards, which may also reduce
the net energy intake for a single foraging trip. In this circumstance,
a shift to foraging for nectar may benefit colony development.
Although we did not detect niche differentiation in proboscis length and
flower use for the two mainly nectar-seeking bumble bees (B.
lepidus and B. friseanus ), because they foraged on a wide range
of flowering plants, the application of the index of composite flower
depth uncovered that such niche differentiation might occur. The fact
that the ratios of worker proboscis length for the species pairs are
1.18 (festivus :friseanus ) and 1.25
(friseanus :lepidus ), close to the ratio of 1.2 predicted
as a minimum for coexisting species (Inouye, 1978), suggests that
proboscis length might play some role in the resource partitioning. When
the intensity of interspecific competition was low, the flowers chosen
by B. friseanus presented a larger composite flower depth than
those visited by B. lepidus , the bee with relatively shorter
proboscis length (Fig. 4). The individuals of B. lepidus had a
much longer proboscis length than the composite flower depth of the
plants they visited. Although long-tubed flowers may prevent a
short-proboscid pollinator from removing nectar (unless they resort to
nectar robbing), long-proboscid pollinators may retain the option of
visiting short-tubed flowers for nectar (Nilsson, 1988; Johnson et al.,
2017; Vlašánková et al., 2017). Our study suggests that the match of
proboscis length and flower depth may not be important in the niche
differentiation in proboscis length and flower use at our study site.
However, the bees’ foraging preferences were significantly influenced by
intensity of interspecific competition. As the competition intensity
increased, B. friseanus , the bee with relatively longer
proboscis, shifted to visit plants with shorter flower depth, and the
bees with shorter proboscis (B. lepidus ) tended to visit plants
with longer flower depth. When the competition intensity reached its
highest point, the ratio of the composite flower depth to proboscis
length was in the range of 0.5 to 0.6 for both the two bees. However,
the plant assemblages visited by the two bees were still highly variable
except for the most dominant plant, Polygonum macrophyllum (Table
S3). When the competitor bee species reduced the availability of floral
resources, enhancement of foraging efficiency in terms of net energy
intake would be important for the coexisting bees. The match of
proboscis length and flower depth might highly enhance the foraging
efficiency, e.g. reducing the handling time on a flower (Inouye, 1980;
Ranta & Lundberg, 1980). The temporal dynamic in plant preference for
the two mainly nectar-seeking bees in response to competition intensity
contributed to our understanding of niche differentiation in proboscis
length and flower use.
In conclusion, our study indicated that the community context of
foraging strategy for bumble bees not only responded to intensity of
interspecific competition, but also depended on preference for reward
type (pollen vs. nectar). Moreover, the change of foraging strategy in
response to competition intensity depended on the species. For the
mainly pollen-seeking bee, B. festivus , when the intensity of
competition with the other two bees (B. friseanus and B.
lepidus ) increased, it was forced to adjust its foraging preference
between plants with and without nectar but not to change to target
plants with different flower depths. At relatively high abundance, the
latter two species did not respond to intensity of competition withB. festivus at all. The two bees mainly sought nectar, and an
increase in competition intensity impelled them to shift their flower
preferences to enhance the foraging efficiency. Our findings suggest
that in a community, intensity of interspecific competition, relative
bee abundance, and reward preference (pollen vs. nectar) might
collectively influence the niche differentiation in flower use for a
bumble bee species. The coexistence of multiple bumble bee species in a
community should be achieved by adjusting foraging strategy in response
to intensity of interspecific competition for floral resources.
Acknowledgements We thank Min Lv, De-Xin Liu, Yong-Deng He,
Si-Zhen Liu and Yi-Fan Ma for field assistance and Yan-Hui Zhao and Huan
Liang for helping in insect identification. This work was supported by
the National Natural Science Foundation of China (Nos. 31970253 and
31770255 to CFY, 31800194 to ZMY).