Inferring assembly processes from empirical community diversity patterns has always been a major goal in Ecology. Many empirical studies rely on the "filtering framework", which characterizes community assembly as a sequence of abiotic and biotic filters. The success of the ecological filtering framework lies in its theoretical foundation, linking environmental filtering to niche theory, and competitive interactions to coexistence theory. Empirical studies have provided evidence of environmental filtering in a wide range of environments. However, while competitive interactions are omnipresent, few applications of the filtering framework found significant evidence of competition in real-life settings. Consequently, the framework has been criticised for being overly simplistic. We argue that this unbalanced picture is likely due to specific conceptual challenges. First, many traits are commonly used in empirical work without a clear distinction between traits that capture species responses' to the environment vs. traits that capture the competitive interactions between species, and without consideration of how these two sets of traits may co-vary. Second, it neglects that environmental filter and competition can produce the same traits patterns. Third, the spatial scale at which the community is observed strongly impacts the resulting patterns. Here, we explore these three conceptual challenges and test how trait patterns vary depending on different assembly processes, traits and scales vary. Using a theoretical simulation model, we demonstrate that the trait patterns resulting from environmental filtering and competition respond differently to variations in traits' correlation structure and observation scales. We then identify the actual conditions in which it is possible to distinguish signals of distinct assembly processes from patterns, given the correlation and relevance of traits and the inherent constraints of the observational scale.
