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Road to ruin: Herbivory in a common tropical weed (Turnera subulata) along a rural-urban gradient

Urbanization is associated with numerous changes to the biotic and abiotic environment, many of which degrade biodiversity. For example, cities often have increased pollution levels that harm wildlife. Nevertheless, the elevated concentration of some gases can fertilize urban plants, with concomitant positive effects on herbivore populations that could select defence phenotypes in plants. However, actual evidence supporting increased herbivory in urban environments remains scarce and contradictory. Giving the gaps in our knowledge, particularly in tropical cities, we evaluate herbivory on Turnera subulata, a common weed, along a rural-urban gradient in Joao Pessoa, Brazil. We expected higher rates of herbivory in urban areas and we predicted these populations to evolve cyanogenesis. We quantified herbivory and screened for cyanogenesis in 32 populations (617 plants) along the urbanization. Our results show that urban populations had significantly higher herbivory rates than rural populations. The likelihood of herbivory was positively correlated to increased amount of impervious surface, although there was no difference in amount of leaf loss to herbivory. Despite elevated herbivory, we found no evidence for the evolution of cyanongenesis in any of the populations. Habitat loss, predator release, and nutrient enrichment likely act in concert to benefit of leaf-eating arthropods, influencing the herbivory patterns observed in our study. HIGHLIGHTS Higher arthropod herbivory in cities can drive the evolution of urban plant defence We found increased herbivory in urban habitat, mostly by lace bugs We found a significant positive effect of impervious surface on herbivory Extensive impervious surfaces and stressful urban environment facilitate herbivory, but do not lead to the expression of HCN, a antiherbivore defense chemical