Modeling the spread and control of Spartina alterniflora in a Pacific estuary: The consequences of an Allee effect in a plant invasion

Field studies show a striking pattern in the fecundity of an invasive population of the cordgrass, Spartina alterniflora; seed production in low density plants at the edge of the invasion is dramatically lower than the fecundity of high density plants. This is an example of an Allee effect, defined generally as a positive relationship between any component of fitness and density of conspecifics.; In this dissertation, I explore the population consequences of this Allee effect on the spread rate of this invasion. I built a spatially-explicit stochastic simulation model and demonstrated that the Allee effect has slowed the invasion. The model predicts an average annual rate of areal spread with the Allee effect of ∼20% and a much higher spread rate, ∼30%, without the Allee effect. A sensitivity analysis shows that the invasion rate was sensitive to selfing rates with Allee dynamics, but not without.; I built a spatially-implicit, deterministic model and used it to find optimal eradication strategies. The goal was to see whether it is more efficient to first remove low-density plants that produce few propagules but spread rapidly by rhizomes or to remove high-density plants that spread slowly but produce most of the new recruits. I found that the optimal strategy was dependent on annual budget levels. At low budgets, it is necessary to remove the low-density areas first to achieve eradication but if a high budget is available then the optimal strategy is to prioritize high-density areas. Without an Allee effect the optimal strategy always prioritizes the removal of the fast-growing low-density areas.; In the final chapter, I review the literature of Allee effects in invasive species. Numerous studies taken together reveal an extensive theory on the consequences of Allee effects in biological invasions. Allee effects can cause longer lag times, slower spread and decreased establishment likelihood. Expected spatial ranges, distributions and patterns of species may be altered when an Allee effect is present. The presence an Allee effect and its successful or unsuccessful detection may affect control decisions, costs of control and change the estimation of the risk posed by potentially invasive species.