The Role of Observation Scales, Trait Correlations, and Competitive Regimes in Community Assembly Patterns

Inferring assembly processes from empirical community diversity patterns has long been a central goal in Ecology. Many studies rely on the "filtering framework," which characterizes community assembly as a sequence of abiotic and competitive interactions. The framework's success stems from its theoretical foundation, linking environmental filtering to niche theory and competitive interactions to coexistence theory. Empirical studies have provided substantial evidence for environmental filtering across diverse ecosystems. However, despite the ubiquity of competitive interactions, empirical applications of the filtering framework have rarely detected significant competition effects. Consequently, the framework has been criticized as overly simplistic. We argue that this imbalance arises from three conceptual challenges. First, empirical studies often use traits without clearly distinguishing between those that capture species' responses to the environment versus those that mediate competition, nor do they consider how these trait sets may co-vary. Second, the framework overlooks that environmental filtering and competition can produce similar trait patterns. Third, the spatial scale at which communities are observed strongly impacts the resulting trait patterns. Here, we explore these three challenges and test how trait patterns vary depending on different assembly processes, trait correlations, and spatial scales. Using a theoretical simulation model, we demonstrate that trait patterns resulting from environmental filtering and competition exhibit different sensitivities to variations in trait correlation structure and observation scales. We then identify the conditions under which distinct assembly processes can be inferred from observed trait patterns, given the correlation and relevance of traits and the inherent constraints of the observational scales.