Proceedings of the Third International Conference on Invasive ...

Chapter 2: Spartina Distribution and Spread<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive Spartinabudget is lower than is necessary to eradicate <strong>the</strong> invasion, <strong>the</strong>optimization algorithm is unable to find a strategy thatremoves all <strong>the</strong> Spartina but instead finds <strong>the</strong> strategy thatminimizes <strong>the</strong> area occupied at <strong>the</strong> end <strong>of</strong> <strong>the</strong> control period.We calculated <strong>the</strong> minimum annual budget in terms <strong>of</strong>area removed that is necessary to eradicate an invasion withinten years when starting control after 40 to 100 years <strong>of</strong>unchecked invasion. The results are shown in Fig. 2. Thenecessary annual budget depends on <strong>the</strong> starting time and size<strong>of</strong> <strong>the</strong> invasion but roughly speaking it is necessary to removeannually at least <strong>the</strong> equivalent <strong>of</strong> about 15 to 20% <strong>of</strong> <strong>the</strong>starting area <strong>of</strong> <strong>the</strong> invasion.We found that <strong>the</strong> optimal allocation <strong>of</strong> resources(removal <strong>of</strong> clones versus meadows) depends on <strong>the</strong> annualbudget level (Fig. 3). For a 40 year old invasion that occupies~17,000 m 2 , if <strong>the</strong> budget is low (in this case removing at most3600m 2 per year; an area equivalent to about 22% <strong>of</strong> <strong>the</strong> initialinvasion), <strong>the</strong> optimal strategy is to remove clones firstallowing <strong>the</strong> meadows to initially expand and <strong>the</strong>n removemeadows once <strong>the</strong> clones are largely removed. If <strong>the</strong> budget ishigh (removing at most 5000 m 2 per year; an area equivalent to30% <strong>of</strong> <strong>the</strong> initial invasion), <strong>the</strong> optimal strategy is to remove<strong>the</strong> meadows first, allowing <strong>the</strong> clones to expand and startremoving clones in a later year. We also found that <strong>the</strong> clonefirststrategy is optimal when minimizing cost only andignoring <strong>the</strong> risk posed by escaping seeds and that <strong>the</strong> clonefirststrategy is <strong>the</strong> only possible way to eradicate an invasionwhen <strong>the</strong> budget is low. The meadow-first strategy can only beused when <strong>the</strong> budget is high and it is optimal because itsubstantially reduces <strong>the</strong> risk <strong>of</strong> escaping seed although it doesnot substantially reduce <strong>the</strong> cost. We also found that <strong>the</strong> clonefirststrategy is optimal when <strong>the</strong>re is no Allee effect since; inthis case, <strong>the</strong> clones produce as much seed as <strong>the</strong> meadows butgrow faster vegetatively (Taylor and Hastings 2004).CONCLUSIONSWe conclude that <strong>the</strong> Allee effect caused by pollenlimitation has dramatically slowed down <strong>the</strong> spread <strong>of</strong> thisplant and affects <strong>the</strong> optimal eradication strategy. The optimalcontrol strategy illustrates <strong>the</strong> importance <strong>of</strong> vegetative spreadin this plant. To control an invasion with limited or uncertainresources, <strong>the</strong> best and only viable strategy is to prioritize <strong>the</strong>removal <strong>of</strong> <strong>the</strong> fast growing, low density clones even though<strong>the</strong>y produce very few <strong>of</strong> <strong>the</strong> new seedlings. With higherbudgets and taking into account <strong>the</strong> risk <strong>of</strong> seed escaping tocolonize o<strong>the</strong>r sites, <strong>the</strong> optimal strategy is to prioritizeremoval <strong>of</strong> <strong>the</strong> slower-growing, high density meadows.ACKNOWLEDGMENTSWe would like to thank Don Strong, Debra Ayres,Miranda Wecker and John Lambrinos for helpful discussions.Field work was assisted by Washington State Department <strong>of</strong>Natural Resources and Washington State Fish and WildlifeService. This work was funded by <strong>the</strong> National ScienceFoundation Biocomplexity Grant #DEB0083583 (P.I. AlanHastings)REFERENCESCiville, J.C. 2005. Spatial and temporal analyses <strong>of</strong> an estuarineinvasion: Spartina alterniflora in Willapa Bay, WA. DoctoralDissertation. University <strong>of</strong> California at Davis, Davis, CA.Davis, H.G., C.M. Taylor, J.C. Civille, and D. R. Strong. 2004a. 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