Given that nearly all populations demonstrate phenotypic variation, it is critical to understand how this intraspecific diversity impacts species interactions. While we know that competitors induce phenotypic plasticity in many traits, how these plastic changes influence the likelihood of species coexistence is rarely tested. In fact, most previous experimental studies on this topic have confounded the impact of plasticity with the effects of the treatment that induces it. Here we manipulate plasticity using three temporary induction treatments in a field experiment and evaluate the consequences for the outcome of competition between all pairwise combinations of five species of annual plants. Focal seedlings were initially induced by their own species, their competitor, or neither, and after several weeks these inducers were removed. We nested this manipulation within a response surface competition experiment allowing us to evaluate how early life induction influenced the competitive performance of the different taxa. We tested the impact of plasticity on species coexistence by parameterizing a competition model with the field derived interaction data and quantified changes to the niche and fitness differences between the competitors.
Results/Conclusions
Our results suggest that the strength of competition changes with phenotypic plasticity, with the direction of the results varying by the competitive pair. The temporary induction treatments altered the phenotypes of the focal plants, including for example, leaf morphology. Results from the field parameterized competition models show that phenotypic plasticity changed key demographic parameters including growth rate when rare and both intraspecific and interspecific competition coefficients, though the extent of these changes depended on the identity of the inducers and the focal species. Counter to common expectations, induction by an interspecific competitor sometimes increased the strength of competition, weakening the ability of the species to coexist. Our results provide a first test of a new generation of more robust methods to understand how plasticity impacts the coexistence of competing species in a modern theoretical framework. Such information helps to better integrate how diversity within species impacts species diversity within communities.