Revitalizing our understanding of species distributions and assembly in community ecology requires greater use of functional (physiological) approaches based on quantifiable factors such as energetics. Here, we explore niche partitioning between bumble and honey bees by comparing a measure of within-patch foraging efficiency, the ratio of flower visitation rate (proportional to energy gain) to body mass (energy cost). This explained a remarkable 74% of the variation in the proportions of bumble to honey bees across 22 plant species and was confirmed using detailed energy calculations. Bumble bees visited flowers at a greater rate (realizing greater energy benefits) than honey bees, but were heavier (incurring greater energy costs) and predominated only on plant species where their benefit : cost ratio was higher than for honey bees. Importantly, the competition between honey bees and bumble bees had no consistent winner, thus highlighting the importance of plant diversity to the coexistence of competing bees. By contrast, tongue : corolla-tube-length ratio explained only 7% of the variation (nonsignificant). Our results confirm the importance of energetics in understanding community ecology and bee foraging niche and highlight the energetic tightrope navigated by foraging bees, since approximately half the nectar energy gained was expended in its collection.

1. Bramble (Rubus fruticosus L. agg.) is a common summer-flowering plant native to the United Kingdom. Multiple complementary approaches were used to evaluate its ecological value to the honeybee (Apis mellifera), bumblebees (Bombus spp.) and other flower-visiting insects in Sussex, England. 2. Regional surveys of insect groups at seven sites across 2 years showed that foraging activity on bramble was dominated by honeybees (60.2%; n = 28 surveys) and bumblebees (17.4%), compared to non-Apis/Bombus bees (2.8%), hoverflies (Syrphidae, 7.9%), non-syrphid flies (0.6%), butterflies (6.4%), wasps (0.4%) and beetles (4.4%). Foraging insect community structure was highly similar spatially, but varied significantly between survey months (June and July). 3. In detailed local surveys at one rural and one urban location, there was a diverse range of insect taxa foraging on the bramble flowers, including species of conservation concern (Bombus humilis, Coenonympha pamphilus and Limenitis camilla). 4. Pollen trapping at 12 honeybee hives in four locations showed that an average of 31% of pollen pellets collected by honeybees from late May to early August were bramble, with a peak of 66-86% per location. 5. Bramble was present in 54 out of 60 200 × 200 m randomly selected grid squares surveyed over a large area across Sussex. Plants were recorded in multiple habitat types in both urban and rural areas. 6. Bramble is sometimes considered an undesirable plant or a "thug" that outcompetes other wild flowers; however, these findings confirm that it is highly valuable for flower-visiting insects. Wherever conflicts of interest and management strategies allow, bramble should be maintained and promoted for wildlife and insect conservation.