Variation in pollinator-mediated plant reproduction across an urbanization gradient.

Urbanization alters the landscape, degrades and fragments habitats, and can have a profound effect on species interactions. Plant-pollinator networks may be particularly sensitive to urbanization, because plants and their insect pollinators have been shown to respond to urbanization both positively and negatively. To better understand the relationship between urbanization, pollinator behavior, and season on pollinator-mediated plant reproduction, we created 30 experimental plant populations along an urbanization gradient in the Greater Toronto Area, Canada. To test how urbanization affects plant reproduction and between-patch pollen dispersal, we created a standard hermaphroditic plant patch at each site, and a male-sterile plant patch at a subset of sites. We measured plant reproduction in the early and late summer in each of 2 years. Plants in urban sites produced significantly fewer flowers than plants in the nonurban sites, whereas seed number per plant either increased or decreased with urbanization, depending on the season. Experimental populations in urban sites also exhibited reduced pollen dispersal between patches. Pollen dispersal between patches was greatest early in the summer and declined with increased impervious surface and proximity to the city. Together, our results are likely caused by variation in environmental conditions and pollinator services across the urban gradient, resulting in pollen limitation and pollen dispersal differences among sites. Our work adds to the small but growing body of literature on urban plant-pollinator interactions and suggests that responses to urbanization are context-dependent.