Galls in the city: Effects of urbanization on the interaction of a highly specialized fly (Neolasioptera sp) and a wildflower (Turnera subulata) in tropical environments

The artificial structures, increased pollution levels and hotter temperatures found in urban environments disrupt trophic interactions by impoverishing biotic communities and favouring organisms with high mobility, thermal and pollution tolerance. Yet, we still know little about the impacts of urbanization on the trophic interaction of organism in tropical urban ecosystems. In this study, we evaluated biotic and abiotic factors that potentially affect the association between stem galls and their host plant in a Brazilian tropical city. The abundance of galls varied with landscape characteristics and key predictors for their occurrence were plant height, herbivory, and impervious surface. The strong positive association between occurrence of galls and herbivory is noteworthy and suggestive that galls mediate the occurrence of other herbivorous arthropods. The landscape-level shifts toward highly urbanised areas reduce occurrence of galls, while increases the occurrence of the host plant. Although physical barriers could be a key driver for decrease in galls occurrence, we found plants harbouring galls even in highly urbanized sites suggesting gall-makers might cross physical barriers. The increased built-up cover disrupts this trophic interaction, yet it is challenging to pinpoint the key drivers. Galling insects are extreme resource specialists and host plant parts cut due to management practices (mowing) could reduce habitat availability and may have contributed to their decline in highly urbanised sites. As the world became more urbanised the understanding of the drivers that shape trophic interaction in urban environment becomes increasingly important.