Patterns of phylogenetic relatedness within communities have been widely used to infer the importance of different ecological and evolutionary processes during community assembly and invasions. Novel exotic plants are expected to have less niche overlap with resident plant species and/or have novel functional traits. Therefore, the population dynamics of exotic plant species are expected to be less influenced by biotic interactions. While many studies have examined patterns of phylogeny in invasions, few have experimentally tested these ideas, and those that do are often restricted to greenhouses or laboratories. Here we provide a comprehensive test of the relationship between phylogeny, functional traits, the strength of biotic interactions, and population dynamics for fourteen exotic plant species occurring at a single site in central USA. We experimentally manipulate competition in a field setting and parameterize population projection models to quantify the effect size of competition on plant fitness. Additionally, we obtained a phylogeny and collected functional trait information for 116 species at our field site. We subsequently constructed models relating phylogenetic novelty, functional novelty, and varying degrees of both types of information to the effect size of competition. Additionally, we investigate patterns of invasiveness and compare these results to those of our experiment to investigate whether examinations of patterns alone can reliably recover the relationships revealed through experimentation.
Results/Conclusions
We find a strong, negative relationship between phylogenetic novelty and the effect size of competition and that presence of this relationship depends on the spatial grain used to calculate the degree of novelty. Furthermore, we find that incorporating information on functional traits can substantially improve our ability to explain the effect size of competition. However, investigations of patterns did not reveal any relationship in any direction. These results indicate that more exotic novel species compete less strongly with resident species in the community and that competition is indeed an important mechanism that explains how exotic plants become invasive after establishing. Our research shows experimental evidence for a commonly involved mechanism to explain the success of novel exotic plant species.