Our understanding of how biotic interactions influence animal community assembly is largely restricted to local systems due to the difficulty of obtaining ecologically meaningful assemblage data across large spatial extents. We used a unique dataset of thousands of spatio-phenologically high-resolution assemblages across three distinct European regions together with a functional diversity approach to understand community assembly of dragonflies and damselflies (Odonata), a group characterized by a pronounced competitive reproductive biology. We found that dragonfly, but not damselfly, assemblages were consistently overdispersed in the morphological traits driving inter-specific reproductive encounters, which supports the role of competition. As predicted by ecological theory, support for competition varied spatially and seasonally as a result of changes in temperature stress and niche packing. Our study provides uniquely consistent and general evidence for the role of competition in animal community assembly, and illustrates mechanistically how spatio-temporal diversity patterns arise from variation in assembly processes.