A fundamental objective in ecology is to understand how plant species are organized into communities. One component of tackling this objective is a focus on competition. Currently there are two opposing views of how competitive interactions organize plant communities; commonly referred to as “assembly rules.” One view holds that plant competitive interactions are hierarchical or transitive in nature, and the relative composition of communities can be predicted by measuring the average competitive effect and response of constituent species. The other view holds that plant species rarely interact in simple hierarchies because in nature all individuals interact simultaneously in multiple species complexes and thus communities are organized by a suite of complex direct and indirect interactions that are non-transitive. While the role of competition in community assembly rules has generated much theoretical debate, experimental design and inaccurate tests of theory have largely hindered advances towards resolving the two views. Using a novel experimental design we created common garden communities comprised of five native northwest intermountain prairie species in an iterative omission design to test the role of direct and indirect interactions in the structuring of plant communities.
Results/Conclusions
Our results suggest that Linum lewisii exerts strong competitive effects on Festuca idahoensis when grown in simple pairwise competition plots (p=0.02); however, L. lewisii does not exert significant competitive effects on F. idahoensis when grown with other native species in community plots (p=0.181). In contrast, Pseudoreognaria spicatum exerts moderate competitive effects on F. idahoensis in pairwise competition (p=0.083) but very strong competitive effects within a multi-species community context (p=0.004). These shifts in species-specific competitive dominance indicate that simple direct competitive effects are poor predictors of more complex, natural and community derived suites of direct and indirect competitive effects. Our results suggest that plant communities are organized by networks of non-transitive interactions and that the indirect effects of plant-plant competition can maintain species coexistence and thus biodiversity within plant communities.