The role of phenotypic plasticity in biotic invasions: Reaction norms, niche breadth, and competition

Invasive species are exotic species that negatively impact the environment, economy, or human health. The number of species invasions has drastically increased in the past two centuries, most likely due to increased global trade and travel. Overall, very few exotic species become invasive, however. Thus, considerable research has focused on identifying which life history traits underlie invasive potential. Unfortunately, these studies have not yet provided an adequate way to predict the fate of an exotic species. Recently, phenotypic plasticity, the ability of a genotype to produce different phenotypes in response to environmental variation, has been proposed as an important attribute of an invasive species. Using six exotic annual Bromus species, three classified as invasive and three non-invasive, I investigated the role of phenotypic plasticity in biotic invasions both quantitatively and qualitatively using reaction norms. In addition, I investigated the role of plasticity in the current expansion of the invasive Bromus tectorum into the understory of the Ponderosa Pine forests in Colorado. I found no differences in plasticity between invasive and non-invasive species when grown alone or with a competitor. However, I did find that reaction norms differed between invasive and non-invasive Bromus when both soil moisture and competition are taken into consideration. Also, when grown individually, there were differences in the relationship between fitness variation, vegetative trait variation, and mean fitness. Specifically, invasive species had more variation in fitness, less variation in vegetative traits, and greater mean fitness relative to non-invasive species. Bromus tectorum individuals growing in the understory of a Ponderosa Pine canopy had greater plasticity in vegetative traits, but lower plasticity in photosynthetic rate than those growing in sites with full sunlight. Finally, I found that intraspecific competition was stronger than interspecific competition in invasive Bromus than in non-invasive Bromus. In conclusion, I found no support for the hypothesis that invasive species are more plastic than non-invasive species. Although reaction norm differences suggest that plasticity is important for invasive species, I found several species-specific effects regardless of invasive status. Thus, the dichotomous classification system (invasive or non-invasive) needs improvement to better describe invasive potential. In addition, I discovered important differences in the relationship between plasticity and competition. To understand how these relationships affect invasiveness, a conceptual framework needs to be developed. With an improved classification system and a strong conceptual framework, our ability to understand and ultimately predict which exotic species are more likely to become invasive will be improved