Functional trait diversity of bumble bees along an urban-rural gradient

Cities are predicted to select for a subset of generalist, functionally similar species from the regional species pool that are well-adapted to the urban environment, resulting in a loss and homogenization of insect biodiversity. Although declines in insect populations have been observed with increasing urbanization, diverse communities of insects have been reported in cities. Understanding the contexts in which urban ecosystems serve as refuges for insect biodiversity is required to inform conservation initiatives targeted for pollinators such as bumble bees (Bombus spp.). The objectives of this study were to use a functional trait-based approach to: 1) determine whether functional homogenization of bumble bees occurs with increasing urbanization; and 2) evaluate the relative importance of landscape and local drivers of bumble bee functional diversity. Bumble bees were collected in 2019 and 2020 along an urban-rural gradient in Madison, WI. The regional pool of bumble bee species was determined for a 200 km area surrounding the city using records from Bumble Bee Watch and published literature. A database of 42 functional traits was compiled for each species in the regional pool. Functional traits were determined from the literature or via microscopic measurement of specimens. Functional homogenization will be investigated using measures of beta-diversity and null models. The relative importance of landscape and local drivers will be distinguished by evaluating trait-environment relationships. A focus on functional traits and diversity to characterize bumble bee communities has greater comparative applicability among cities globally, and therefore, will broadly contribute to urban pollinator conservation.