Correlates of Bumble Bee Occurrence in an Urbanized Southern California Landscape

Bees are ecologically important pollinators that are threatened by disease, urbanization, and habitat loss. Bee species that share broad ecological traits (e.g., body size, diet breadth) may often respond similarly to environmental disturbance; however, few studies have examined why closely related, ecologically similar bee species exhibit divergent responses to the same forms of disturbance. Here, we examine the responses of three bumble bee species (Apidae: genus Bombus) to urbanization by combining field surveys and Geographic Information Systems (GIS) analyses of local- and landscape-level variables. We surveyed 57, one-hectare plots selected across a gradient of urbanization from April to June in 2012 and 2013. We used field surveys and aerial and satellite imagery to quantify local and landscape-level variables thought to affect Bombus occurrence in an urbanized landscape. We observed approximately 1,000 bumble bee individuals belonging to three species during the two year observation period. Negative effects of urbanization on Bombus occurrence were evident in terms of reduced species richness for plots in urban areas compared to plots in fragments or reserves. Although occurrence values for each Bombus species were all strongly and negatively related to impermeable surface cover within plots, each species responded uniquely to different plot- scale variables and exhibited distinctive patterns of scale-dependency with respect to impermeable surface cover surrounding plots. Species-specific responses to plot- scale and landscape-scale variables partly reflects trait variation among the three species. Given the divergent responses exhibited by each Bombus in this study, our results caution against pooling species into functional groups based merely on taxonomic relationships or perceived ecological similaritie