Complex population dynamics and control of the invasive biennial Alliaria petiolata (garlic mustard)

Controlling species invasions is a leading problem for applied ecology. While controlling populations expanding linearly or exponentially is straightforward, intervention in systems with complex dynamics can have complicated, and sometimes counterintuitive, consequences. Most invasive plant populations are stage-structured and density-dependent--a recipe for complex dynamics--and yet few population models have been created to explore the effects of control efforts on such species. We examined the demography of the invasive biennial plant Alliaria petiolata (garlic mustard) on the front of its spread into a natural area and found evidence of strong density dependence in vital rates of first-year rosette and second-year adult stage classes. We parameterized a density-dependent, stage-structured projection model using field-collected data. This model produces two-point cycles with alternating years in which adults vs. rosettes are more prevalent. Such population dynamics match observations in natural populations, suggesting that these complicated population dynamics may result from deterministic rules. We used this model to evaluate simulated management strategies, including herbicide treatment of rosettes and clipping or pulling of adult plants. Management of A. petiolata by inducing mortality of either rosettes or adults will not be effective at reducing population density unless the induced mortality is very high (>95% for rosettes and >85% for adults) and repeated every year. Indeed, induced mortality of rosettes can be counterproductive, causing increases in the stationary distribution of A. petiolata density. This species is typical of many invasive plants (stage-structured, short-lived, high fertility) and exhibits common forms of density dependence. Thus, the management implications of our study should apply broadly to other species with similar life histories. We suggest that management should focus on managing adults rather than rosettes, and on creating efficient control in targeted areas of the population, rather than spreading less efficient efforts widely.