A persistent challenge in ecology is to tease apart the in-
fluence of multiple processes acting simultaneously and interacting
in complex ways to shape the structure of species assemblages. We
implement a heuristic approach that relies on explicitly defining species
pools and permits assessment of the relative influence of the main
processes thought to shape assemblage structure: environmental filtering,
dispersal limitations, and biotic interactions. We illustrate our
approach using data on the assemblage composition and geographic
distribution of hummingbirds, a comprehensive phylogeny and morphological
traits. The implementation of several process-based species
pool definitions in null models suggests that temperature—but not precipitation
or dispersal limitation—acts as the main regional filter of assemblage
structure. Incorporating this environmental filter directly into
the definition of assemblage-specific species pools revealed an otherwise
hidden pattern of phylogenetic evenness, indicating that biotic interactions
might further influence hummingbird assemblage structure.
Such hidden patterns of assemblage structure call for a reexamination
of a multitude of phylogenetic- and trait-based studies that did not explicitly
consider potentially important processes in their definition of
the species pool. Our heuristic approach provides a transparent way
to explore patterns and refine interpretations of the underlying causes
of assemblage structure.