Assistant Professor Universidade Federal da Paraiba Joao Pessoa, Paraiba, Brazil
Urban areas are often characterized by high densities of herbivorous arthropods and increased herbivory, while there is often a decline in pollinators with increase in built landscape. Higher herbivory rates may lead to natural selection for greater defence phenotypes in plants. However, evidence supporting increased herbivory leading to the evolution of plant defence in urban environments is contradictory, and absent from tropical regions of the world. I evaluated herbivory, and phenotypic changes, on Turnera subulata, along an urbanization gradient in Joao Pessoa, Brazil. My objectives were: (i) to determine if the levels of herbivory change with urbanization; (ii) to determine if increased herbivory selects for mechanical defence and flower size. I predict that higher rates of herbivory in urban areas would lead these populations to evolve mechanical defences (increased number of trichomes), and given that urbanization can erode the abundance of pollinators, I expect urban population to show larger floral displays to attract pollinators. I selected a total of eight sites in the following gradients of urbanization: (a) highly urbanized, (b) mildly urbanized (c) low urbanized, and (d) three rural areas outside the city. All the populations I sampled were located beside roads.
I found increased herbivory in urban habitats. The average rate of leaf damage was significantly higher in more urbanized areas (F (3,257) = 4.44; p= 0.005), but this was not reflected in the number of leaf tricomas. Contrarily, there was a slight trend for less trichomes with increased urbanization level (p = 0.06). Urban and rural plants also showed significant differences height, fruit production, number and size of flowers. Plants in urban sites showed larger flower size (F (3,192) = 7.63; p < 0.001) and produced more flowers. Interestingly, plants with different floral morph (pin versus thrum) were impacted differently by herbivores in urban sites. Long styled plants suffered higher herbivore damage (t = 3.16; df = 137; p = 0.002) in urban sites. The extensive impervious surfaces and stressful urban environment causes significant shifts in arthropods’ communities, that could drive phenotypic change in T. subulata.