Universidade Federal da Paraiba Joao Pessoa, Brazil
Abstract: Biotic homogenisation and disassembled communities are inexorable consequences of urbanization. Urbanization also impacts trophic interactions via changes in community structure caused by increased impervious cover, pollution levels, warmer temperatures, and other abiotic alterations that filter in organisms with high mobility, thermal and pollution tolerance. Yet, we still know little about the impacts of urbanization on the trophic interaction of organism persisting urban ecosystems. Since environmental conditions vary throughout the urban landscape, we aimed to evaluate how different degrees of urbanization at different landscape scale and how thermal gradients impact trophic interactions. In this study, we evaluated biotic factors (trichome density, plant height, reproductive investment) and abiotic factors (temperature and proportion of sealed surface at local and landscape scale) that could affect parasitism by galls and the occurrence of hyperparasites in a tropical city. Increased urbanization favoured the host plant, Turnera subulata. Contrary to our predictions, increased urbanization reduced the occurrence of galls; the probability of a plant being infected by galls was significantly lower in urban areas and negatively associated with impervious cover. Moreover, increased temperature did not influence the likelihood of galls in plants. Some of the factors that predispose a host plant to be attacked by galls suggest a preference of galling insects for vigorous plants. An unexpected factor significantly increasing the likelihood of a host plant having of galls was the higher trichomes density, likely associated with a greater capacity of the host plant to protect the galls against possible predators. Numerous studies suggest that increased number of galls impair plant reproduction, but our data show that number of galls was not associated with change in energy allocated to reproduction in T. subulata. Although the built-up environment is an important physical barrier filtering out tiny arthropods, we found galls and parasitoids even in sites highly urbanized, suggesting they might cross physical barriers. Nevertheless, our results show that increased built-up cover disrupt trophic interactions, but this relation is complex and other factors, e.g. management practice (mowing) and extreme surface temperatures might play a role in explaining occurrence of galls in tropical cities. As the world became more urbanised understanding the drivers shaping trophic interaction in urban environment becomes increasingly important.
