When I read this report, I immediately thought to myself, "well, that's probably because they have more non-native invasive species in the urban areas", but the researchers concluded that non-native invasives aren't the source of the diversity. I found this study very interesting, because I've studied butterfly diversity along an urban gradient and also had somewhat surprising results. We'll have to continue to study urban environments to more fully understand impacts on species diversity!
Di Mauro, D., T. Dietz, and L. Rockwood. 2007. Determining the Effect of Urbanization on Generalist Butterfly Species Diversity in Butterfly Gardens. Urban Ecosystems.
Kelly RP et al. "Genetic signatures of ecological diversity along an urbanization gradient" PeerJ. 2016.