Consequences of an Allee Effect in the Invasion of a Pacific Estuary by Spartina alterniflora

Results from both a spatially explicit stochastic simulation model and a spatially implicit deterministic model show that a weak Allee effect can slow the spread of an invasive plant, Spartina alterniflora, in a Pacific Coast estuary. The mean rate of spread with the Allee effect is ~19%; removing the Allee effect results in a mean rate of spread of ~31%. Sensitivity analysis both with and without the Allee effect reveal that seedling establishment, inflorescence density, and outcrossed seed production are key factors determining invasion rate. When there is an Allee effect, the invasion rate is sensitive to variation in self-pollinated seed production. By structuring the population according to density classes instead of the more usual age or stage classes, the deterministic model demonstrates a novel way of representing density dependence and an Allee effect in population biology. This approach could be applied to model the population dynamics of any species that spreads from multiple foci and in which these foci later coalesce.