Simulating control strategies for a spatially structured weed invasion:: Spartina alterniflora (Loisel) in Pacific Coast estuaries

Patterns in the spatial arrangement of invasive plant populations can provide opportunity for strategic placement of control efforts. Smooth cordgrass (Spartina alterniflora) is rapidly invading the intertidal mudflats of Pacific Coast estuaries. Its pattern of spread is distinctive. Seedlings establish in open mud and then grow vegetatively to form expanding circular patches, which dot the mudflats and eventually coalesce into a contiguous monospecific meadow. The invasion typically begins in the upper tide zone and then moves down the tidal gradient. A spatially explicit model was used to simulate the spread of S. alterniflora and compare various strategies for control in a situation where only a fraction of the total infestation could be controlled each year. A strategy of killing outlying patches first and then attacking the dense meadows (moving up the tidal gradient) led to eradication with up to 44% less time and effort than a strategy of attacking the dense meadows first and outlying patches second (moving down the tidal gradient). In the control of contiguous meadows located adjacent to the shoreline, the best strategy was to approach one end of the infestation, moving across the meadow to the other end. Suppression of seeds was not an effective control strategy. In general, effective control strategies were those that first eliminate the plant in areas where current or future vegetative growth is greatest. Application of these results in control programs for S. alterniflora and similar invasive species could greatly reduce the costs of control work and improve the likelihood of local or complete eradication.