Bumble bee species exhibit divergent responses to urbanisation in a Southern California landscape

Abstract

1. Bees are ecologically important pollinators that are threatened by disease, habitat degradation, and habitat loss. Bee species that share ecological traits (e.g. body size, diet breadth, and sociality) may often respond similarly to environmental disturbance; however, few studies have examined the extent to which closely related, ecologically similar species exhibit divergent responses to the same forms of disturbance.2. In the present study, the responses of bumble bees (Apidae: genus Bombus) to urbanization are examined by combining field surveys with the quantification of local‐ and landscape‐level variables using Geographic Information Systems (GIS). During the spring and summer of 2012 and 2013, 57, bumble bees were surveyed in 1‐ha plots in San Diego County, California that spanned a gradient of urbanisation. Species‐level, occurrence (presence/absence) data and logistic regressions were then used to identify determinants of Bombus occurrence across this landscape.3. Approximately 1000 Bombus individuals belonging to three species were observed: B. californicus Smith, B. melanopygus Cresson, and B. vosnesenskii Radoszkowski. The occurrence of all Bombus species was negatively related to impermeable surface cover within study plots, but species responded uniquely to other plot‐scale variables and exhibited distinctive patterns of scale‐dependency with respect to impermeable surface cover surrounding plots.4. Urbanisation negatively affected all three focal bumble bee species in this study. Species‐specific responses to plot‐scale and landscape‐scale variables presumably reflect interspecific trait differences (e.g. body size, tongue length, and foraging behaviour). Unique responses to urbanisation caution against pooling species into functional groups based merely on taxonomic relationships or perceived ecological similarities.