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

Abstract

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.

Shedding light on trophic interactions: A field experiment on the effect of human population between latitudes on herbivory and predation patterns

“…In the higher latitude, herbivory pressure was smaller than in the lower latitude. Regardless of the latitude, an increase in HPD was not associated with different levels of herbivory across any type of herbivory which disagree with other studies in herbivore abundance (Dale & Frank, 2017;Meineke et al, 2013;Parsons & Frank, 2019) or leaf defoliation (Cuevas-Reyes et al, 2013;Rivkin & de Andrade, 2023). We explain our absence of effects of HPD on herbivory with the counter effects of biotic and abiotic factors occurring in urban areas.…”

“…Besides, other authors have found a decrease in herbivory with the increase in human population either with a decrease in chewer herbivory (Meineke et al, 2019), chewer, leaf miner and galler herbivory (Valdés-Correcher et al, 2022) or for chewer herbivory but not in other guilds (Moreira et al, 2019;Nuckols & Connor, 1995). Contrarily, other studies found an increase in herbivory abundance (Dale & Frank, 2014;Parsons & Frank, 2019) or intensity in chewer herbivory in urban areas (Christie & Hochuli, 2005;Cuevas-Reyes et al, 2013;Rivkin & de Andrade, 2023).…”

The effects of human population density on trophic interactions are contingent upon latitude

“…Despite widespread support for the ‘increasing disturbance’ hypothesis, some studies have also reported a positive effect of urbanization on herbivory in different parts of the world, including Brazil (Cuevas‐Reyes et al., 2013; Rivkin & de Andrade, 2023), Australia (Christie & Hochuli, 2005) and the USA (Cregg & Dix, 2001; Dale & Frank, 2017). Likewise, predation by birds and arthropods has been shown to increase across disturbance gradients in the Philippines (Posa et al., 2007) and in Europe (Kozlov et al., 2017).…”