Bumble bee (Bombus) distribution and diversity in Vermont, USA: a century of change

Bumble bees (Bombus) play key roles as pollinators in temperate ecosystems. Some North American species have declined due to factors that include habitat loss, parasites, pesticides, and climate change. In many regions conservation is hampered by lack of quantitative data on historical abundance and distribution, making status assessments difficult. From 2012 to 2014, with help from 53 citizen scientists, we conducted surveys to determine the status of bumble bees throughout Vermont, USA. For historical comparison, we identified and digitized bumble bee specimens from 13 public and private collections. Our dataset contained 12,319 records, which we separated into historic (1915-1999; n=1669) and modern (2000-2014; n=10,650) periods, with our survey contributing 94% of modern data. Of 17 species, four were not detected and four showed significant declines. Rarefaction indicated that both modern and historic datasets slightly underestimated known species richness, diversity, and abundance, but confirmed a strong decline for all three parameters. Declining species broadly accorded with those reported elsewhere in eastern North America, and included those in subgenera Bombus, Fervidobombus, and Psithyrus. Four species in the subgenus Pyrobombus (B. bimaculatus, B. impatiens, B. ternarius, and B. vagans) greatly increased in relative abundance in the modern period. Landscape factors such as road density, elevation, and land use strongly predicted distribution of some species. Species diversity was correlated positively with grasslands, and negatively with deciduous and mixed forest cover, while abundance was correlated positively with evergreen forest cover, yet negatively with deciduous forest.