PhD student Michigan State University, Michigan, United States
Abstract: The relationships between species’ evolutionary histories and their coexistence and assembly into current communities has fascinated ecologists for many years. At least since Darwin, ecologists have wondered how species’ evolutionary histories might help determine their interactions and therefore shape patterns of community composition and diversity. Complicating these relationships, modern coexistence theory has shown how trait differences among species may contribute to both niche and fitness differences among species, therefore promoting coexistence, competitive exclusion, or both. Recent efforts have begun trying to tease apart phylogenetic effects on niche and fitness differences, but we still lack empirical tests to evaluate how phylogenetic relatedness and traits affect communities.
In this study, we conducted a competition experiment to assess how phylogenetic relatedness and traits mediate niche and fitness differences among 15 herbaceous plant species that co-occur in prairie and oak habitats in the Pacific Northwest. To parameterize population models and estimate strengths of intra- and inter-specific competition, we grew plants at varying densities of each pairwise competitor species and estimated pairwise competition coefficients based on individual plant fecundity. We further measured key functional traits including specific leaf area, plant height, and root dry weight in order to test whether phylogenetic distance and/or trait differences predict competitive outcomes. We hypothesized that coexistence between closely related species is less likely due to increased niche overlap caused by evolutionary conserved traits, but also that different traits associated with fitness advantages might help determine coexistence outcomes.
Our results support the idea that there may be complex relationships between phylogenetic relationships, trait differences, and the niche and fitness differences that determine coexistence. In particular, we found significant relationships between species interaction coefficients and phylogenetic and trait distances. Moreover, with increasing phylogenetic distance between species, niche differences tended to increase, which stabilizes coexistence, but fitness differences also strongly increased, which tends to destabilize coexistence. Finally, we found contrasting patterns depending on phylogenic scale of analysis, suggesting that the trait differences relevant to coexistence may only be relevant over recent evolutionary history, not deeper phylogenetic history. Together, these results offer new insights on the evolutionary signatures embedded in current-day community coexistence dynamics.
