Seed production and reproductive cycles determine the success of an annual invader in a stochastic simulation model

Predicting the behavior of invasive plant species is fundamental to better understand the ecology of invasions. We chose Sisymbrium loeselii L. (small tumbleweed mustard) as a target plant species because it is wide spread across the northern hemisphere in native (Europe) and non-native ranges (North America and parts of Asia). In non-native ranges, Sisymbrium causes damage in disturbed habitats, such as grasslands, roadsides, and along railway lines. We conducted a greenhouse experiment and a field study in order to collect data on its functional traits and population dynamics. We devised a stochastic simulation model for the population dynamics of Sisymbrium in competition with a test species to investigate mechanisms which are likely to result in invasion. A specified number of each of the two plant species are placed randomly in the habitat region as the initial conditions and the model simulates both species' reproduction over time, accounting for the number of seeds produced by each plant, the radius of seed dispersal from the parent, the germination rate, the number of reproduction cycles per year, and competition when seeds of different species occupy the same location. Competition is quantified using the experimentally determined relative interaction intensity (RII) value, an index that indicates the likelihood of the seed of one species winning in competition with a seed of another species in proximity. Model results demonstrate that the number of seeds produced per plant, number of reproductive cycles per year, and seed dispersal radius are important mechanisms for invasion, while competition between seeds in proximity has a smaller impact. We conclude that the invasion success of Sisymbrium can be attributed to the relatively large number of seeds and seed dispersal radius, with its competitive advantage between individual seeds playing a lesser role. The implications of these results for potential management of this invader are discussed.