Environmental factors act as drivers of species coexistence or competition. Mesic environments favor the action of parasites and predators on gall communities, while the factors that determine the structure of gall communities in xeric environments remain unknown. We evaluated the structure of gall communities along an environmental gradient defined by intrinsic plant characteristics, soil fertility and aridity, and investigated the role of competition as a structuring force of gall communities in xeric environments. We created null models to compare observed and simulated patterns of co-occurrence of galls and used the C-score index to assess community aggregation or segregation. We used the NES C-score (standardized C-score) to compare patterns of co-occurrence with parameters of environmental quality. Xeric environments had poorer and more arid soils and more-sclerophyllous plants than mesic environments, which was reflected in the distribution patterns of gall communities. Values of the C-score index revealed a segregated distribution of gall morphospecies in xeric environments, but a random distribution in mesic environments. The low availability of resources for oviposition and the high density of gallers in xeric environments reinforce interspecific competition as an important structuring force for gall communities in these environments.

This study was developed at seven sites with the presence of a population of C. langsdorfii, each located in a different plant formation. The sites were chosen to include a gradient of environmental stress, ranging from dry environments, such as rupestrian grasslands and ironstone outcrops, to humid environments, such as Atlantic forest and riparian forest. The distance between sampling areas ranges from 90 to 500 km. We obtained meteorological data for each of the studied sites for the 24 months preceding the study — a period corresponding to vegetative investment by each plant.