Landscape context affects both capture probability and abundance of solitary bees in cities

It is intuitive to hypothesize that urban landscapes are broadly hostile to insects, but responses of wild bees to urbanization are inconsistent and often positive. Many people have interpreted this to mean that bees can persist in small habitat fragments in urban landscapes (hereafter the "Patch of Dreams" hypothesis). Another possible mechanism for this surprising pattern is that bees are more attracted to sampling sites or traps in resource-poor landscapes, and the apparent abundance of bees in cities is due to higher capture probability, not higher abundance (the "Only Show in Town" hypothesis). We used a mark-recapture study of the bicolored sweat bee (Agapostemon virescens) to separate capture probability from true abundance. We estimated the contributions of each factor (capture probability vs. abundance) to visitation rates of experimental flower patches in flower-rich gardens and paired non-garden sites along a landscape-scale impervious surface gradient (~35%-95% impervious surface). Bee abundance declined along the gradient in both site types (garden vs. non-garden), and this decline was more apparent for true abundance than for the number of visits to flower patches. Capture probability was higher in non-garden than garden sites and, along the landscape gradient, capture probability increased with impervious surface in non-garden but decreased with impervious surface in garden sites. These results support the Only Show in Town hypothesis at local scales, and at landscape scales when local resources are sparse. This finding emphasizes that the number of bees seen visiting flowers is not a consistent metric of population size across varying resource environments. Understanding the relationship between the sampling process and true abundance at landscape scales is an important priority for future research. In addition to these results, all of our experimental patches were used by A. virescens. This finding emphasizes the importance of urban gardens for supporting native bees, since even isolated flower patches were used by A. virescens in highly fragmented landscapes.