Simultaneous effects of phylogenetic niche conservatism and competition on avian community structure

We currently have only a partial understanding of how phylogenetic relationships relate to patterns of community structure, in part because, for most groups of organisms, we do not know the extent to which ecological similarity results from common ancestry. Associations between phylogenetic relatedness and local community structure are particularly interesting for groups in which many species that span a gradient of phylogenetic divergence occur in potential sympatry. We explored the relationship between evolutionary relatedness and current species co-occurrence among the North American wood-warblers (Aves: Parulidae), a group of songbirds known both for its species diversity and for exhibiting high levels of sympatry at breeding sites. Species co-occurrences were derived from North American Breeding Bird Survey transects comprising 160,000 census points distributed across North America. The nested point-within-transect structure of this survey provides an unusual opportunity to remove larger-scale geographical effects on local community composition and thereby consider patterns of co-occurrence only among regionally sympatric pairs of species. We indexed evolutionary relatedness among all pairs of taxa by genetic distances based on long mitochondrial DNA protein-coding sequences. Most regionally sympatric taxon pairs rarely co-occur at local sites, and the most closely related never exhibit high local co-occurrences, as predicted if past or present competitive effects are strongest for these recently separated lineages. Quantile regression shows that, for a subset of taxa, local co-occurrence does increase with time since common ancestry, and that this apparent relaxation of competitive exclusion is strongest for distantly related species that have differentiated in fundamental ecological and behavioral traits, such as terrestrial vs. arboreal foraging. Comparisons against a null model of species co-occurrence further demonstrate that these patterns occur against a background of phylogenetic niche conservatism: across all phylogenetic distances, sympatric species co-occurred at higher rates than expected by chance, a pattern that might stem from a tendency by these species to show conservatism in their selection of similar general habitat types. Considered in concert, these analyses suggest the simultaneous mediation of local community structure by the ecological similarity of closely related species and by trait divergence among a subset of more distant lineages.