Jointly modeling niche width and phylogenetic distance to explain species co-occurrence

Competitive exclusion is most likely when there are large differences in competitive ability and the strength of competitive interactions between species is high, but predicting competitive outcomes is not straightforward. Assuming a trade-off between competitive ability and ecological generalism, we would predict larger competitive differences between species with different niche widths. Community phylogenetic theory predicts that competition will be stronger among more closely related species, assuming that phylogenetic distance reflects ecological similarity. We would therefore expect the probability of competitive exclusion to be highest among closely related species with different niche widths. Here, we assess how well differences in niche width and phylogenetic distance correlate with co-occurrences among 34 species of Cyperaceae (sedges) in the eastern Canadian subarctic. The Cyperaceae is a species-rich family, with many species sharing similar niches and environmental tolerances, making it a model clade for evaluating the importance of niche width differences and phylogenetic distances on co-occurrence. Consistent with both hypotheses, we found that higher co-occurrence scores correlated with species pairs that were distantly or only intermediately related and that had similar niche widths. Furthermore, we show that this correlation is stronger when considering only more recently diverged species pairs and that there is a triangular relationship between phylogenetic distance and species co-occurrence, suggesting that distantly related species might have both strong and weak competitive interactions. Using co-occurrence as a proxy for competitive outcomes, our results support both a negative correlation between phylogenetic distance and strength of competitive interactions, and a trade-off between niche width and competitive ability.