Habitat-scale relationships between nest abundance of bumblebees and reproductive success of their host plants

Background/Question/Methods

Recent reports on the status of pollinators in North America and documented declines of multiple pollinating species have highlighted the need for improved monitoring of pollinator populations and better understanding of consequences of pollinator population sizes for linked plants. I used highly variable microsatellites to assess sibships among sampled worker bees of four native North American Bombus species, B. balteatus, B. flavifrons, B. bifarius, and B. sylvicola, and estimated the number of nests of each species in alpine habitats in the central Rocky Mountains, Colorado. I estimated foraging ranges for each species from nestmate distributions and evaluated habitat features that may function as foraging or dispersal barriers. I also examined relationships between habitat-level nest abundance and reproductive rates of two bumblebee-dependent host plants, Trifolium dasyphyllum and T. parryi.

Results/Conclusions

Nest abundance varied among the four Bombus species, and between high and low alpine habitats. Effective population sizes of the four species ranged from 18.1 colonies to 77.8 colonies per .01 km² habitat, with the long-tongued species B. balteatus most abundant, especially high above treeline. B. balteatus nestmates were distributed among sites while B. flavifrons nestmates were primarily found within sites. Snowbeds and elevational gradients did not restrict nestmate distributions. However, isolation was associated with interpeak valleys and/or dense willow canopy. Foraging ranges of both subalpine and alpine Bombus species were generally smaller than those published for lowland species, with means ranging from 25-110 m. Fruit set of a specialized host plant, Trifolium parryi, increased with effective population size of its bumblebee partner, B. balteatus. However, the congeneric generalist species, T. dasyphyllum, showed little variance in fruit set with the number of Bombus nests. There was a nearly 1:1 correspondence of abundance of foragers of each species and their nest numbers within habitats. Relationships of plant reproductive rates with forager abundance were similar to that with nest abundance, except that total number of Bombus foragers was the only significant predictor for seed production of either clover. These results suggest that for natural resource managers, forager abundance is an appropriate measure for monitoring the status of pollinator services.