Bumble bee nest abundance, foraging distance, and host-plant reproduction: implications for management and conservation

Authors: GEIB, JENNIFER - Appalachian State University; Strange, James; GALEN, CANDACE - University Of Missouri

Submitted to: Ecological Applications
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/12/2014
Publication Date: 4/15/2015
Citation: Geib, J., Strange, J.P., Galen, C. 2015. Bumble bee nest abundance, foraging distance, and host-plant reproduction: implications for management and conservation. Ecological Applications. 25(3):768-778.

Interpretive Summary: Bumble bees are important pollinators of wildflowers and are indicator species of environmental health in alpine environments. To assess the relationship of bumble bees to plants in alpine meadows of Colorado, we assessed genetic structure of four bumble bee species. Additionally, we examined the seed set of two clover species in relation to pollinator population size in these meadows. We found that the different species of bumble bees have different population sizes and this varies both with elevation and within species. Finally, seed set of Parry’s Clover was found to be correlated with the population size of its primary pollinator, the High Country Bumble Bee, whereas seed set in the other clover studied was not correlated to the population size of any of the bumble bee species. Conservation and management implications of these findings are discussed in the manuscript.

Technical Abstract: Recent reports of global declines in pollinator species imply an urgent need to assess native pollinator population sizes and density dependent benefits for linked plants. Here, we estimated effective population sizes (Ne) of four native bumblebee species, Bombus balteatus, B. flavifrons, B. bifarius, and B. sylvicola, in alpine habitats of the Colorado Rocky Mountains using highly variable microsatellites to assess sibships among sampled workers. We also estimated species’ foraging ranges from nestmate distributions and evaluated habitat features that may restrict movement. We then examined relationships between Bombus nest frequencies and reproduction of two Bombus host plants, Trifolium dasyphyllum and T. parryi. In 2007-8, Ne varied among Bombus species and with elevation, ranging from 18.1 to 77.8 colonies per 0.01 km2. The long-tongued species B. balteatus was most common, especially high above treeline, but the subalpine species B. bifarius was unexpectedly abundant for this elevation range. B. balteatus nestmates were distributed among sites, while B. flavifrons nestmates were restricted. Neither intervening snowbeds nor elevation restricted nestmate distributions; however, isolation was associated with inter-peak valleys and dense willow stands. Bombus foraging ranges were smaller than expected based on lowland species, ranging from 25-110 m. Fruit set of a specialized host plant, Trifolium parryi, correlated with Ne of its primary pollinator, B. balteatus. However, for generalist T. dasyphyllum, fruit set showed little variance with Bombus Ne. Relationships of plant fecundity to abundances of foragers and colonies were similar; however, for foragers, only total Bombus density predicted seed production. Results suggest that forager abundance is a useful proxy for pollinator services. If that assumption is correct, observed declines in Bombus forager density after the extreme dry winter of 2011-12 may foreshadow local extinction for the bumblebee populations we studied.