Urbanization is one the main threats to biodiversity and its effects are projected to increase in magnitude, hence stressing the importance to better understand how urbanization may affect vital ecosystem services, such as pollination. Studies on the impact of urbanization have largely focused on only one aspect of the pollination process (e.g. pollinator visitation, pollen deposition), potentially misrepresenting the full complexity of urbanization effects on pollination services. Evaluating impacts across multiple components of pollination can also help pinpoint the underlying mechanisms driving urbanization effects. This study evaluates how urbanization affects multiple aspects of the pollination process in common milkweed populations, including pollinator community composition and visitation rate, pollen removal and deposition on stigmas. Urbanization altered floral visitor composition, with small bees having a larger presence in urbanized areas. Insect visitation rate and pollen removal were also higher in more urbanized areas, perhaps suggesting a positive impact of urbanization as predicted by the intermediate disturbance hypothesis. However, pollen deposition on stigmas, which is a key component of fertilization success, was not different between natural and urbanized sites. Our findings highlight the complexity evaluating urbanization effects on pollination, as these likely depend on the specific aspect of pollination evaluated and on the of the intensity of disturbance. Our study thus stresses the importance of evaluating multiple components of the pollination process in order to fully understand overall effects and the mechanisms underlying urbanization effects on this vital ecosystem service

Studies that aim to understand the processes that generate and organize plant diversity in nature have a long history in Ecology. Among these, pollinator-mediated plant-plant interactions that occur by altering pollinator floral preferences have been at the forefront in this field. Current evidence however indicates that plants can interact directly via heterospecific pollen (HP) transfer, that these interactions are ubiquitous, and can have strong fitness effects with implications for floral evolution, speciation and community assembly. Hence, interest in understanding their role in the diversification and organization of plant communities is rapidly rising. The existence of geographic mosaics of species interactions and their role in shaping patterns of diversity is also well recognized. However, after 40 years of research, the importance of geographic mosaics in HP intensity and effects remain poorly known, thus ignoring its potential in shaping patterns of diversity at local and global scales. Here, I develop a conceptual framework and summarize existing evidence for the ecological and evolutionary consequences of geographic mosaics in HP transfer interactions and outline future directions in this field.