Urban bumblebees are smaller and more phenotypically diverse than their rural counterparts

With urbanization identified as being one of the key drivers of change in global land use, and the rapid expansion of urban areas world-wide, it is relevant to evaluate how novel ecological conditions in cities shape species functional traits, which are essential for how species interact with their environments and with each other. Despite the many comparative studies on organisms living in urban and non-urban areas, our knowledge on species responses to urban environments remains limited. For one, much of the ecological research has assumed that the environment changes in a linear fashion from the city core to the city edges, whereas in reality the environments within the cities are highly heterogeneous. Furthermore, studies on species responses to these highly variable ecosystems are often based on interspecific mean trait values, which ignore the potential for high levels of intraspecific variation among individuals in key functional traits. The current study investigated intraspecific functional trait differences for four functional traits associated with body size, mobility and resource selection among rural and urban populations of two common bumblebee species, Bombus pascuorum and Bombus lapidarius, in urban centres and adjacent rural areas in Switzerland. We document shifts in functional traits towards smaller individuals and higher multidimensional trait variation in urban populations compared to rural conspecifics of both species. This shows that urban individuals for both species are on average smaller sized but populations are distinctively different from rural population by increasing their trait richness and diversifying their trait combinations. In addition, we found bimodality in tongue length within urban B. pascuorum populations. Our results suggest that urban and rural populations possibly experience differential selection pressures resulting in trait differences across and among populations. We argue that variations in the respective foraging landscapes in cities leads to smaller sized but phenotypically more diverse populations, and drive functional trait divergence.