Increased herbivory but not cyanogenesis is associated with urbanization in a tropical wildflower

Urbanization is associated with numerous changes to the biotic and abiotic environments, many of which lead to a loss of biodiversity. Some studies have documented increased herbivory rates in cities, which has been hypothesized to lead to the evolution of novel defence traits in plants. Yet evidence supporting this hypothesis is contradictory and entirely absent from South American cities. To address this research gap, we evaluated herbivory rates in the native urban wildflower, Turnera subulata (Turneraceae), along an urbanization gradient in Joao Pessoa, Brazil. We predicted that higher rates of herbivory in cities would lead to the expression of cyanogenesis, a chemical defence found in a closely related Turnera species. We estimated herbivory rates and screened for cyanogenesis in 32 populations along the urbanization gradient, quantified by the Human Footprint Index and the amount of impervious surface surrounding each site. We found herbivory rates increased in T. subulata populations with increasing urbanization, but we did not find evidence of cyanogenesis in any of the populations. Our results suggest that although herbivores respond positively to urbanization, the fitness effects of leaf herbivory may be insufficient to select for the evolution of cyanogenesis in some plants. Our results provide valuable insight into the effects of urbanization on plant-herbivore interactions in the tropics.