Urbanization is quickly changing natural and agricultural landscapes, with consequences for the herbivorous arthropods dwelling in or near cities. Here, we review the evidence for the effects of urbanization on the ecology and evolution of plant-herbivore interactions. We first summarize how abiotic factors associated with urbanization affect the ecology and evolution of herbivorous arthropods. Next, we explore how urbanization affects plant-herbivore interactions, by considering how urban environments may disrupt top-down and bottom-up ecological processes that affect herbivory. Abiotic changes in the urban environment, such as the urban heat island effect, have caused shifts in phenology for some herbivorous arthropods. Other abiotic changes in urban areas, including water availability, pollution, and habitat fragmentation, have resulted in changes to physiology, behavior, and population abundance. Native species richness tends to decline in urban areas; however, changes in abundance appear to be species specific. These shifts in ecology suggest that urbanization could affect both adaptive and non-adaptive evolution of herbivorous arthropods and their host plants in urban environments. However, plant-herbivore interactions may be dramatically altered if either arthropods or plants are unable to tolerate urban environments. Thus, while some species can physiologically acclimate or genetically adapt to the abiotic urban environment, many species are expected to decline in abundance. We conclude with suggestions for future research to advance our understanding of how urbanization alters the ecology and evolution of plant-herbivore interactions.

Urban environments represent globally replicated, large-scale disturbances to the landscape, providing an ideal opportunity to study parallel evolution in natural populations on a large scale. In recent years, there has been a rapid increase in the number of studies investigating evolutionary responses of a diverse range of taxa across multiple cities. Although parallel evolutionary responses across independent urban environments will depend on the extent to which urban environments converge on similar biotic and abiotic environments, the extent to which cities are environmentally similar has not yet fully been integrated into studies of urban evolution. This chapter begins by asking: Do species display parallel evolutionary responses across independent urban environments? It then briefly reviews a subset of the environmental factors that have driven parallel responses to cities (heat islands, pollution, and habitat fragmentation) and discusses some of the potential causes of non-parallelism. Finally, it ends with practical considerations for the design of future studies aiming to examine parallel evolutionary responses to urbanization. Understanding the shared and unique features of urban environments and identifying parallel species responses to rapid and ongoing urban development will provide important insight into the ubiquity of parallel evolution in nature.

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Urbanization exerts many pressures on species, yet little is known about how these pressures impact species interactions. Studies of urban plant-pollinator systems provide mounting evidence that urbanization impairs pollinator movement in fragmented urban landscapes, yet the consequences for pollinator-mediated plant reproduction remains unclear. In non-urban areas, habitat corridors can facilitate the movement of organisms including pollinators, but whether these corridors facilitate plant-pollinator interactions in urban areas remains understudied. To examine how urban environments and green corridors influence plant-pollinator interactions, we measured reproductive success in the native plant common milkweed (Asclepias syriaca), and the community structure of its pollinators, for two years along two urban-rural transects in the Greater Toronto Area, Canada, one of which followed a green corridor. We found that urbanization decreased male fitness (i.e., pollen removal), increased fruit set (i.e., mean no. of follicles per inflorescence), and inconsistently affected female fitness (i.e., no. of follicles) in A. syriaca. Urbanization simultaneously decreased pollinator abundance but increased pollinator richness. Proximity to a green corridor inconsistently affected male fitness but increased reproductive effort (i.e., no. of inflorescences) in A. syriaca, while pollinator diversity and richness was lower in corridors. Notably, there were no consistent relationships between pollinator community structure and reproductive success in A. syriaca in both the presence, and absence, of a green corridor. These results demonstrate the complexity with which urbanization, green corridors, and pollinator communities can shape the reproductive investment and fitness of native plant populations.

Urbanization is altering landscapes globally at an unprecedented rate. While ecological differences between urban and rural environments often promote phenotypic divergence among populations, it is unclear to what degree these trait differences arise from genetic divergence as opposed to phenotypic plasticity. Furthermore, little is known about how specific landscape elements, such as green corridors, impact genetic divergence in urban environments. We tested the hypotheses that: 1) urbanization, and 2) proximity to an urban green corridor influence genetic divergence in Asclepias syriaca (common milkweed) populations for phenotypic traits. Using seeds from 52 populations along three urban-to-rural subtransects in the Greater Toronto Area, one of which followed a green corridor, we grew ~1000 plants in a common garden and observed >20 ecologically-important traits associated with plant defense/damage, reproduction, and growth over four years. We found significant heritable variation for eight traits within A. syriaca populations and weak phenotypic divergence among populations. However, neither urbanization nor an urban green corridor influenced genetic divergence in individual traits or multivariate phenotype. These findings contrast with the expanding literature demonstrating that urbanization promotes rapid evolutionary change and offer preliminary insights into the eco-evolutionary role of green corridors in urban environments.