Understanding how the phylogenetic relatedness of species impacts community assembly, co-occurrence, and abundance is a burgeoning discipline at the interface of ecology and evolution. Phylogenetically related species are expected to co-occur if their ecological niches are evolutionarily conserved, because these species will share many traits that allow them to live in similar habitats. However, if co-occurring species are too similar in their ecological niches, competition for resources is expected to repel such species from co-occurring or result in divergence of their niches. Using two genera (Psychotria and Palicourea) in the clade Psychotrieae (Rubiaceae), we examined the impact of phylogenetic relatedness on the co-occurrence and variation in abundance among these species in the forests of Costa Rica, Central America. We used co-occurrence and abundance data collected on 240 transects nested in seven Psychotrieae assemblages across Costa Rica and a phylogenetic hypothesis of species relationships using DNA sequence data to examine the phylogenetic structure of Psychotrieae assemblages.
Results/Conclusions

We found that Psychotrieae assemblages are overall significantly phylogenetically overdispersed, indicating that co-occurring species are less related than expected by chance. Examining the seven assemblages individually, we found that the phylogenetic structure of assemblages differed significantly, with some assemblages overdispersed and others clustered (more related than expected by chance). On the whole, species found at higher elevations sites were more abundant. Within one heavily sampled assemblage, we found an inverse relationship between the phylogenetic relatedness of species on transects and the variation in abundance among species on transects, indicating that closely related species are more similar in their abundances. However, when species are found on phylogenetically clustered transects, their average abundances are lower than when they are found on phylogenetically overdispersed transects. When the relationship between phylogenetic relatedness and variation in abundance was examined across assemblages, they were positively correlated, in opposition to the trend found at the local scale.