Jessica Orozco, 2011

San Francisco State University

Diet and diversity: Bumblebee (Bombus spp.) foraging dynamics and community response to flowering plant diversity in Virginia grassland meadows

Bumblebees (Bombus spp.) are important pollinators, both ecologically and economically. Many bumblebee species have experienced significant declines in both North America and Europe. Habitat loss and fragmentation associated with agricultural intensification are purported causes of decline in several cases. Studies also suggest that reduction of host plant abundance and diversity associated with this type of land use change disproportionately impacts bumblebee species with specialized diets, and that these species tend to be long-tongued. Natural habitat loss may increase competition for available resources, making the study of resource utilization and partitioning in bumblebee communities both a relevant and important pursuit. In this study, we examined bumblebee community response to floral resource availability and diversity in grassland meadows throughout the northern Shenandoah Valley in Virginia, and whether diet breadth (i.e. degree of specialization) and tongue length are correlated among bee species in this region. Between June 5 and July 25 of 2011, we surveyed and quantified flowering species in seven grassland meadows in Virginia's Northern Shenandoah Valley for a total of two visits per site. Collections of bees were conducted following each floral survey, and identifications were made on-site. The proboscis (tongue) lengths of bumblebees were measured using digital calipers in the laboratory. Results show that floral diversity (Shannon Index) and abundance in meadows is positively associated with bumble bee species diversity (Shannon Index) ( F(1,5) = 8.82, p = 0.031, as well as bee density (no. bees per m2) (F(1,5) = 8.82, p = 0.031). We found no significant relationship between pairwise niche overlap and floral diversity and abundance. Our data also show that diet breadth and tongue length of measured bumblebee species were strongly negatively correlated (r=-0.87, N=6, p=0.02), revealing that species with longer tongues exhibit specialist tendencies. Although we found no significant association between tongue length and specific host plant families (F stat and p value), principal components analysis of the proportion of visits made by each bumblebee species to different plant families illustrates that the six bumble bee species observed across study sites do occupy distinct dietary niche space . These findings demonstrate the importance of resource availability and diversity in fostering diverse bumblebee communities.


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