Progress on Mechanisms and Impacts of Wetland Plant Invasions: A Twenty-Year Retrospective Analysis and Priorities for the Next Twenty

Abstract Invasive plants are ubiquitous features of many wetland systems, resulting in impacts that are extremely costly in both economic and ecological terms. Approximately twenty years ago, these impacts and many of the mechanisms underlying invasion were crystallized in a pair of now-classic review papers on the topic. These two contributions have guided substantial research efforts over the past two decades. Here, using a state-of-the-art review, we present an overview of research progress from the past 20 years and identify research priorities for the next 20 years. We structure these insights around key themes that emerge from those earlier reviews and emphasize ways wetland invasions might be distinguished from plant invasions in uplands. We first highlight research progress and priorities around the impacts of wetland plant invasions. We then do the same for seven broad mechanisms that have been postulated to enhance invasion success in wetlands. Invasive plants clearly impact wetlands across all levels of ecological organization and up to landscape scales, but generalizable conclusions are still lacking concerning what drives variation in impact magnitudes. One of the key mechanisms underlying invasion success reflects site-level variation in resource availability and, although we know that increased resources often lead to more opportunities for invasion, the role of discrete resource pulses and the way resource availability may interact with propagule pressure variability are poorly understood. Second, although release from natural enemies is a long-held potential driver of invasion success, recent insights support speculation that these effects may be especially pronounced in wetlands. Third, although most invasive wetland plants are considered good competitors, their simultaneous role as stress tolerators raises questions about whether expectations regarding life history tradeoffs may be less applicable for wetland invaders than for the upland species around which these theories were designed. Fourth, despite a keen understanding that wetland plant invaders are opportunistic, establishing quickly in response to disturbance, the central importance of seed and seedling traits has not been adequately studied to integrate life history theory across ontogenetic stages. Fifth, although many invasive wetland plants reproduce both sexually and asexually, the extent to which this mixed strategy contributes to their success as invaders has not been rigorously assessed. Sixth, despite observations that wetland invaders often exhibit a high degree of phenotypic variability, causal connections between the success of wetland invaders and either high population genetic diversity or high phenotypic plasticity have not been clearly established. Lastly, despite long-standing interests in whether interspecific hybridization contributes to wetland invader success, recent studies have highlighted evolutionary processes leading to variation in polyploidy and genome size as alternatives requiring additional study. We conclude with a vision for prioritizing wetland plant invasions research, presenting insights from this review aimed at inspiring future studies on remaining key uncertainties regarding the relative impacts of wetland vs. upland invaders and the relative importance of phenotypic plasticity, population genetic diversity, enemy release, and anthropogenic disturbances for influencing the success of invasive plants in wetland vs. upland systems.