Landscape composition and forage availability as predictors of bumblebee abundance and species richness in agricultural and forested settings in western Massachusetts

Scully, Caroline
Department of Environmental Studies
Bumblebees are one of the most important native pollinators for wild native and cultivated plants in North America. Scientists blame the decline of native bee populations on introduction of pathogens to wild colonies and on habitat destruction, degradation, and fragmentation caused by intensive agriculture practices and human development. Populations of four species of bumblebees native to North America have severely declined in recent years. Current and historic populations of native bee species are poorly understood.

This study examined bumblebee abundance and species richness and surrounding landscape composition at 37 sites across a 560-square kilometer region of northwestern Massachusetts in Berkshire County. Sites were separated from each other by at least three kilometers and were within open areas including abandoned agricultural fields, pasture and hay fields, and large residential lots. I sampled bumblebees and forage from these sites during summer and fall in 2008.

The community composition for bumblebee species was close to what I had expected, though the relative abundances were surprising. Bumblebee abundance and species richness data were distinctly different between the summer and fall data sets. Expected positive correlations between forage availability and bumblebee abundance and species richness were found, but only in the summer. Observed associations included a positive correlation between fall bumblebee abundance and percent of all forest cover within circles with radii of up to 1,500 meters and negative relationships with agriculture and open natural areas within approximately the same analytical area as forest. There were no significant relationships between summer bumblebee abundance and the landscape variables of percent forest, agricultural, open natural, or developed land cover or corridors through forest. In contrast, fall species richness was negatively associated with all forest percent cover and positively associated with developed land percent cover around sites within circles with radii from 250 to 10,000 meters. No relationships were found for summer species richness and landscape variables. Corridor length was negatively correlated with bumblebee species richness at 500 meters in the summer. Relationships were detected for species richness with elevation, positive in the summer and negative in the fall. Seven female Bombus terricola individuals, a currently rare species, were found at six different sites.