PS 74-14 - Niche overlap and diet breadth: Can bumble bee (Bombus spp.) foraging preferences reveal species vulnerabilities?

Friday, August 12, 2011
Exhibit Hall 3, Austin Convention Center
Rosemary L. Malfi and T'ai H. Roulston, Environmental Sciences, University of Virginia, Charlottesville, VA

Within the last two decades, notable shifts have taken place within the bumble bee (Bombus spp.) communities of North America: some species have experienced substantial, rapid declines, while others appear to be stable or even experiencing range expansion. Studies conducted in the UK and Europe implicate the loss of natural habitat and associated floral resources in overall bumble bee declines and species losses. Research also indicates that loss of host plants disproportionately impacts species with narrower diet breadths, and that these species are more likely to be long-tongued (presumably due to the reduction of long-tubed floral hosts). In contrast, species with more generalized diets have been shown to be more abundant. In this study we examined resource utilization by the bumble bee community at Blandy Experimental Farm in Boyce, VA in order evaluate (1) whether the diet of a potentially declining long-tongued species (B. fervidus) overlaps strongly with a more competitive, expanding species (B. impatiens), (2) whether the declining species specializes on host plants, and (3) if species abundance is associated with diet breadth. We surveyed and quantified all mass flowering species in two large meadows on a weekly basis from June 1 to August 15, 2010. Comprehensive collections of bees were then carried out on these flowering plants for a minimum of one person-hour per week (with collections on all plants being standardized by time each week).  For each bee species, Simpson’s index was used to assess diet breadth.  


Six bumble bee species were observed at Blandy Experimental Farm in total. The potentially declining and expanding species (B. fervidus and B. impatiens) possessed the narrowest and widest diet breadths respectively. In addition to possessing specialist tendencies, B. fervidushad little niche overlap with other species in the community (i.e. it visited plants other species were not visiting). Although B. impatiens had the widest diet breadth and was also the most abundant species (50% total collections), diet breadth, overall, was not associated with abundance (Spearman’s rho = 0.37, p = 0.46). However, we found that species abundance was positively correlated with abundance of most preferred host plant (Spearman’s rho = 0.97, p = 0.0012). The results of this study indicate that resource limitation may put B. fervidus at risk of decline when preferred host plants are reduced or lost, whereas B. impatiens may be resistant to extirpation due to its “super-generalist” tendencies.

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