Individual bumble bees are locavores in a Rocky Mountain meadow

Background/Question/Methods

Pollinators must make decisions about where to forage in landscapes where floral resources are patchy at many spatial scales. These foraging decisions are expected to be economical, for example, pollinators may prefer to forage in large and less isolated flower patches. These resulting foraging decisions can influence plant pollination and reproductive success. Despite this importance, we still have a poor understanding of how pollinators respond to patches which vary in flower abundance and isolation at broader spatial scales in natural habitats. In particular, it is uncertain how individual pollinators use a patchy resource over time, which will determine how pollen moves in a plant population. We marked and resighted known individual bumble bees (Bombus spp.) and recorded the abundance of bumble bees visiting 12 flowering Delphinium barbeyi patches that varied in size (from 9 to 440 m²) and isolation (from 1 to 46 m distance to nearest patch) in a subalpine meadow in the Colorado Rocky Mountains.

Results/Conclusions

At the patch-level, bumble bee visits per flower increased with flower patch size, and for small flower patches, bee visits per flower declined with increasing patch isolation. At an individual bee level, half of the individuals we saw many times foraged only in one of 12 patches in the meadow, with individual patch-use having a median of one and ranging from one to four patches. A randomization test showed that 26 of 27 individuals were foraging in patches in the meadow non-randomly. For those individuals who foraged in more than one patch, those additional patches tended to be nearby. Bombus nevadensis, B. flavifrons and B. appositus were the most common bumble bees seen, yet we saw very few marked B. nevadensis again, suggesting that this larger bee is less site faithful and/or may have a larger foraging range than the other two common, but smaller, bumble bees. Overall, we show that when floral resources are abundant, many individual bumble bees commonly forage very locally in a meadow, and that they concentrate their foraging in large and close patches, which may have implications for the pollination of D. barbeyi.