Litter feedbacks, evolutionary change and exotic plant invasion

Understanding the mechanisms driving exotic plant invasions is important for designing successful invader control strategies. Previous studies have highlighted different invasion mechanisms, including alteration of nutrient cycles through plant-soil feedback and evolutionary change toward more competitive genotypes. We explored the possibility of these two mechanisms interactively affecting exotic plant invasion. Using data from recent experiments on the invasion of Phalaris arundinacea (Phalaris) in North American wetlands, we parameterized a classical resource competition model for nutrients and light, and expanded this model by including litter dynamics. We also examined the potential effects of evolutionary changes as observed in another set of recent experiments. Both litter feedbacks and observed evolutionary changes may increase Phalaris' performance in invasive habitats. These mechanisms may amplify each other and this synergy may accelerate Phalaris invasion in areas where it is already present, leading to a high-litter state with Phalaris outcompeting other species. In areas with low nutrient supply where Phalaris is not yet present, a combination of litter feedbacks and evolutionary change may induce a critical transition from a low-litter, native-species-dominated state toward a high-litter, Phalaris-dominated state. Our model results suggest that the importance of litter feedbacks and evolutionary change could be verified in future empirical studies through non-additive and nonlinear effects in litter addition experiments, and by increased litter:above-ground biomass ratios in biogeographical comparisons between plant communities with Phalaris in invasive and native habitats. The latter result may be caused by the accumulation of more recalcitrant litter from invasive genotypes. Synthesis.Our study illustrates how exotic plant invasions may be exacerbated by the interaction between litter feedbacks and evolutionary change. This interaction may induce a positive feedback in the invasion process, suggesting that even short-term events, such as a nutrient pulse, may lead to relatively large and rapid ecosystem shifts that could be long-lasting and difficult to reverse.