Pollination deficits increase with urbanization in Chicago

Urban growth is occurring rapidly, and the land use changes associated with urbanization may have consequences for pollinators and the plants that rely on them. Despite its importance in the face of pollinator declines and expanding urban ecosystems, our understanding of the effects of urbanization on pollination mutualisms is scant. There is both evidence that urban areas support diverse pollinator communities and evidence that they degrade them. The influence of urbanization on the pollination of urban plants is even less understood. Urban agriculture relies on plant-pollinator interactions for crop production, providing a relevant framework to study pollination in an urban context. We therefore grew 240 plants across six sites at varying levels of urbanization in Chicago, Illinois, to investigate how urbanization relates to pollination of a generalized pollination system in Cucurbita pepo (squash) and a more specialized pollination system in the buzz-pollinated Solanum lycopersicum (tomato). We used a pollen limitation experiment to test whether the reproduction of plants at urban farms is pollen-limited and whether the magnitude of pollen limitation varies with the extent of urbanization (quantified as the percent of surrounding impervious surface around each site). We also examined how pollinator visitation rates vary with urbanization. In S. lycopersicum but not C. pepo, the pollen addition treatment had a consistent and significant positive effect on reproductive success, indicating that plants of S. lycopersicum are pollen-limited in our study area. The magnitude of this pollen limitation (the difference in fruit set between paired control and pollen-supplemented plants) increased with greater impervious surface, illustrating that S. lycopersicum plants at more urban sites are more pollen-limited. The limited evidence for pollen limitation in the more generalized C. pepo suggests that plants with more specialized pollination systems are subject to greater pollen limitation in urban environments. Together, our results demonstrate that urban plants are likely experiencing deficits in pollination services, but in ways that vary with both the type of pollination system and the level of urbanization in the surrounding area.