Proceedings of the Third International Conference on Invasive ...

<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 2: Spartina Distribution and Spreadexpansion than elevated hybrid growth rate. Elevated hybridgrowth rates and seedling survival both result in a rapiddecline in <strong>the</strong> frequency <strong>of</strong> native genotypes: essentially as<strong>the</strong> area covered by hybrid cordgrass increases, <strong>the</strong>probability <strong>of</strong> hybrid pollen fertilizing <strong>the</strong> native alsoincreases. This ‘pollen-swamping' suggests that ultimatelyall new recruits to <strong>the</strong> Spartina population will be hybrid,resulting in extinction <strong>of</strong> <strong>the</strong> native through introgression.In both recruitment scenarios, <strong>the</strong> effect <strong>of</strong> elevatedrates <strong>of</strong> hybrid pollen production on <strong>the</strong> rate <strong>of</strong> invasion isnegligible, assuming <strong>the</strong> hybrids have no o<strong>the</strong>r fitnessadvantage. However, <strong>the</strong> frequency <strong>of</strong> native S. foliosagenotypes declines at a much faster rate than in <strong>the</strong> nullmodel (where all genotypes produce <strong>the</strong> same amount <strong>of</strong>viable pollen). If elevated hybrid pollen production iscoupled with an increased hybrid growth rate or seedlingsurvival, <strong>the</strong> resulting invasion proceeds faster than it wouldfor elevated hybrid growth rates or seedling survival alone.The effects <strong>of</strong> <strong>the</strong> differing life-history processes on <strong>the</strong> rate<strong>of</strong> invasion <strong>of</strong> hybrid Spartina genotypes are summarized inTable 1.DISCUSSIONHere we have shown that a model which allows forelevated fitness traits (growth, seedling survival, pollenproduction) in hybrid Spartina can explain its explosivepropagation through San Francisco Bay. As well aspredicting a huge increase in <strong>the</strong> total area covered bySpartina in <strong>the</strong> Bay, <strong>the</strong> model also shows a sharp decline in<strong>the</strong> frequency <strong>of</strong> pure S. foliosa genotypes over <strong>the</strong> nextcentury, as an increasingly higher percentage <strong>of</strong> seedlingshave hybrid origin. If this invasion continues unchecked, <strong>the</strong>native S. foliosa will become extinct in <strong>the</strong> Bay. O<strong>the</strong>rmodels <strong>of</strong> invasion and hybridization predict <strong>the</strong> ultimateextinction <strong>of</strong> <strong>the</strong> species (Huxel 1999; Wolf et al. 2001). Thework presented here extends previous modeling efforts bylinking <strong>the</strong> population dynamics and genetics <strong>of</strong> <strong>the</strong>invasion, making transparent <strong>the</strong> roles <strong>of</strong> geneticrecombination and selection acting on favourable phenotypictraits.Hybrid Spartina needs to be eradicated to ensure <strong>the</strong>persistence <strong>of</strong> <strong>the</strong> native S. foliosa in San Francisco Bay.Super-exponential population growth <strong>of</strong> hybrids may bedriven by different life-history processes depending on <strong>the</strong>level <strong>of</strong> local recruitment. In accordance with recentmodeling work on <strong>the</strong> control <strong>of</strong> S. alterniflora in WillapaBay, Washington (Taylor and Hastings 2004), this suggeststhat control efforts at sites with differing levels <strong>of</strong> seedlingrecruitment should be targeted at different stage classes <strong>of</strong>Spartina. At low recruitment sites, control should beTable 1: Effects <strong>of</strong> elevated life-history traits and parental compatibilityon <strong>the</strong> rate <strong>of</strong> invasion <strong>of</strong> hybrid Spartina.TraitAll genotypesequally fitElevated growth rate(G)Elevated seedlingsurvival (S)Elevated pollenproduction (M)Effect on growth rateExponential growth; increase infrequency <strong>of</strong> hybridsSuper-exponential; effect greatestwhen seedling recruitment lowSuper-exponential; effect greatestwhen seedling recruitment highExponential growth; rapid increasein hybrid frequency; superexponentialonly in conjunction wi<strong>the</strong>levated S or Mtargeted at <strong>the</strong> removal <strong>of</strong> fast-growing isolated clones. Atsites with potentially a high level <strong>of</strong> recruitment, such asmarsh restoration sites, control should focus on <strong>the</strong> removal<strong>of</strong> seedlings, or herbicide treatment <strong>of</strong> hybrid Spartinameadows during <strong>the</strong> pollination season.ACKNOWLEDGMENTSWe gratefully ackowledge funding from NSFBiocomplexity Grant AESRJ85. RJH is grateful to CazTaylor and John Lambrinos for helpful discussions.REFERENCESAyres, D.R., D.R. Strong, and P. Baye, 2003. Spartina foliosa(Poaceae) – a common species on <strong>the</strong> road to rarity? Madrono50:209-213.Ayres, D.R., D.L. Smith, K. Zaremba, S.Klohr, and D.R. Strong,2004. Spread <strong>of</strong> exotic cordgrasses and hybrids (Spartina sp.) in<strong>the</strong> tidal marshes <strong>of</strong> San Francisco Bay, California, USA. BiologicalInvasions 6: 221-231.Bulmer, M.G. 1980. The ma<strong>the</strong>matical <strong>the</strong>ory <strong>of</strong> quantitative genetics.Oxford University Press, UK.Hall, R.J., A. Hastings, and D.R. Ayres, 2006. Explaining <strong>the</strong> explosion:modelling hybrid invasions. <strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> RoyalSociety B 273:1385-1389.Huxel, G.R. 1999. Rapid displacement <strong>of</strong> native species by invasivespecies: effects <strong>of</strong> hybridization. Biological Conservation89:143-152.Taylor, C.M., and A. Hastings, 2004. Finding optimal controlstrategies for an invasive grass using a density-structured model.Journal <strong>of</strong> Applied Ecology 41:1049-1057.Turchin, P. 2003. Complex population dynamics. Princeton UniversityPress, USA.Williamson, M. 1996. Biological invasions. Chapman and Hall,London, UK.Wolf, D.E., N. Takebayashi, and L.H. Rieseberg, 2001. Predicting<strong>the</strong> risk <strong>of</strong> extinction through hybridization. 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