Understanding the factors that affect community composition is essential for community ecology. The genetic similarity rule (GSR) identifies 3 variables (host genetic composition, phytochemistry, and the environment) that could affect community composition. Few studies have determined the relative influence of these variables on community composition. Using path analysis, we found that arthropod community similarity was better explained by geographic (56%) and environmental (32%) distance than genetic distance in clonal aspen (Populus tremuloides). Comparing our data with data from similar studies of poplars (P. fremontii and P. fremontii × P. angustifolia hybrids), we found that hybrid poplar stands had higher levels of genetic and arthropod diversity than did clonal aspen stands. We found a significant relationship between genetic and arthropod diversity only in hybrid stands and across Populus systems. Our findings agree with the GSR expectations that the importance of the genetic composition of the host in structuring communities depends in part on the amount of genetic variation present in the study system. This is relevant for management and restoration strategies of geographically restricted species and of disjunct populations of otherwise widespread species, as these tend to have lower effective population sizes and reduced levels of genetic diversity.