Estimating genus-specific effects of non-native honey bees and urbanization on wild bee communities: A case study in Maryland, United States.

Non-native species have the potential to detrimentally affect native species through resource competition, disease transmission, and other forms of antagonism. The western honey bee (Apis mellifera) is one such species that has been widely introduced beyond its native range for hundreds of years. There are strong concerns in the United States, and other countries, about the strain that high-density, managed honey bee populations could pose to already imperiled wild bee communities. While there is some experimental evidence of honey bees competing with wild bees for resources, few studies have connected landscape-scale honey bee apiary density with down-stream consequences for wild bee communities. Here, using a dataset from Maryland, US and joint species distribution models, we provide the largest scale, most phylogenetically resolved assessment of non-native honey bee density effects on wild bee abundance to date. As beekeeping in Maryland primarily consists of urban beekeeping, we also assessed the relative impact of developed land on wild bee communities. Six of the 33 wild bee genera we assessed showed a high probability (> 90 %) of a negative association with apiary density and/or developed land. These bees were primarily late-season, specialist genera (several long-horned genera represented) or small, ground nesting, season-long foragers (including several sweat bee genera). Conversely, developed land was associated with an increase in relative abundance for some genera including invasive Anthidium and other urban garden-associated genera. We discuss several avenues to ameliorate potentially detrimental effects of beekeeping and urbanization on the most imperiled wild bee groups. We additionally offer methodological insights based on sampling efficiency of different methods (hand netting, pan trapping, vane trapping), highlighting large variation in effect sizes across genera. The magnitude of sampling effect was very high, relative to the observed ecological effects, demonstrating the importance of integrated sampling, particularly for multi-species or community level assessments.

Trait variation along an urban-rural gradient in Asian dayflower: the contribution of phenotypic plasticity and genetic divergence.

Environmental changes associated with urbanisation can exert pressure that facilitates both adaptations and plastic responses in plants. Field surveys or common garden experiments (CGE) alone cannot differentiate between the observed phenotypic traits resulting from plastic responses versus evolutionary adaptations to urban environments. We conducted a field survey of habitat environmental factors and four vegetative traits in 12 Commelina communis populations along an urban-rural gradient in the Osaka-Kobe megacity area. We collected seedlings from six of the 12 populations, transplanted them into a greenhouse, and measured six vegetative traits. We investigated correlations between pairs of measured traits in both the field survey and CGE. Plant height and leaf area increased significantly in increasingly developed land areas (DLA) in the field survey, whereas no such variations were found in the CGE, suggesting that the observed phenotypic variation was due to a plastic response to urban eutrophication. Leaf number and specific leaf area (SLA) significantly decreased with increasing DLA in the CGE, suggesting the adaptation of these traits to urban environments. Positive correlations between plant height and leaf area were only observed in urban populations in both the field survey and CGE, indicating the evolution of this trait correlation in urban environments. It has been suggested that urban environments promote both plastic response and genetic divergence of a set of traits in native plants. Our findings suggest that low leaf number, SLA, and positive plant height-leaf area correlations have evolved. In addition, larger plant size is achieved via phenotypic plasticity in urban environments.