Evaluating the dependence of urban pollinators on ornamental, non-native, and ‘weedy’ floral resources

Urban landscapes are often florally rich due to extensive management of cultivated plants around the city. High abundance and diversity of these floral resources has been linked to more abundant and diverse pollinator communities, but little is known about how urban pollinators select from the wide variety of available flowers. This study provides unique insight into urban plant-pollinator interactions by examining how many plant taxa, and which taxa, are visited by pollinators. Over a three-year period, we observed foraging of urban pollinators across 63 neighborhoods in Chicago, Illinois (USA). We observed 1815 plant-pollinator interactions between 24 pollinator morpho-types and 106 plant taxa. An additional 57 plant taxa were flowering but not visited by pollinators. On average, each pollinator morpho-type visited 19 plant taxa, and most pollinators showed generalist tendencies. We identified 42 plant taxa that were visited by more pollinator morpho-types than their floral abundance would predict; we call these ‘highly attractive’ plants. In general, perennial and native plants received more pollinator visits than their counterparts, and ornamental plants were visited by more species than unintentional ‘weeds’. However, the two most-visited plant taxa were non-native, perennial weeds. Our results suggest that many flowering plants in cities are not visited by pollinators. Furthermore, the plant-pollinator network could be reduced to only four plant taxa without losing any pollinator morpho-types. To enhance urban pollinator conservation, urban residents can select ornamental plants from our list of ‘highly attractive’ plant taxa, or can allow some of the highly attractive ‘weeds’ to persist in their gardens.