Finding optimal control strategies for invasive species:: a density-structured model for Spartina alterniflora

1. There are very few general guidelines available to improve the process of eradicating invasive species. We addressed the basic question of whether it is more efficient to prioritize the removal of outliers or core populations of an invasive grass, Spartina alterniflora. 2. We used a structured model in which the population of the invasive species is partitioned into different classes based on local conspecific density in order to investigate density-based eradication strategies. 3. The Spartina population at a Pacific coast estuary in Washington state, USA, is subject to an Allee effect: at low density plants produce fewer propagules than in high-density coalesced meadows. Plants at low density, however, are able to spread faster vegetatively. We used a genetic algorithm to find the optimal division of resources between removal of high-density and low-density plants. We explored the consequences of the Allee effect, contrasting financial budgets available for control operations and the inclusion of seedling control in addition to the control of mature plants. 4. We found that the optimal strategy was dependent on the annual budget available for control. At low and medium budgets, it was necessary to remove the low-density plants first to achieve eradication, but if more money was available then the optimal strategy was to prioritize high-density areas. 5. Without an Allee effect the optimal strategy would always be to prioritize the removal of fast-growing, low-density sub-populations. Seedling control did not change the optimal strategy but did, in some cases, reduce the cost of eradication. 6. Synthesis and applications. Given the uncertainty in future budgets allocated for control of invasive Spartina, we recommend a strategy that prioritizes the removal of low-density subpopulations of S. alterniflora over high-density subpopulations. The Allee effect in this system is not sufficiently strong to outweigh the importance of the rapid vegetative spread of the plants at low density.