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 SpreadINVASIVE HYBRID CORDGRASS (SPARTINA ALTERNIFLORA X FOLIOSA)RECRUITMENTDYNAMICS IN OPEN MUDFLATS OF SAN FRANCISCO BAYC.M. SLOOP 1,2 ,D.R.AYRES 1 AND D.R. STRONG 11 Section <strong>of</strong> Evolution and Ecology, University <strong>of</strong> California, Davis, One Shields Avenue, Davis, CA 956162 Laguna de Santa Rosa Foundation, 900 Sanford Road, Santa Rosa, CA 95401; christina@lagunafoundation.orgHybrid Spartina are currently expanding <strong>the</strong>ir range in <strong>the</strong> San Francisco Bay (SFB) at a rateexceeding exponential growth. A subset <strong>of</strong> transgressive hybrid Spartina plants that positivelyexceed <strong>the</strong> fitness trait values <strong>of</strong> <strong>the</strong>ir parents are competitively and reproductively superior to bothparents and o<strong>the</strong>r hybrids and likely drive <strong>the</strong> invasion. In order to colonize <strong>the</strong> vast open SFBmudflats and found new populations hybrid cordgrass plants have to evolve self-compatibility andexhibit rapid vegetative and lateral growth. The mudflat tidal cycle covers or exposes plants for upto six hours, so new seedlings have to be robust and fast growing to survive and establish. A smallnumber <strong>of</strong> hybrid and native Spartina have colonized <strong>the</strong> open mudflats along <strong>the</strong> eastern shore <strong>of</strong>SFB. To discern mudflat seedling recruitment dynamics we investigated (1) <strong>the</strong> numbers andlocations <strong>of</strong> recruiting seedlings at three SFB sites in 2003 and 2004 via GPS/GIS, and (2) <strong>the</strong>genetic relationship <strong>of</strong> established adult plants and parentage <strong>of</strong> seedlings using microsatellitemarkers. Our results identify all sampled seedlings as hybrids, and show a dramatic increase inseedling recruitment numbers in 2004. Molecular investigations reveal surrounding, inter-relatedadult plants as <strong>the</strong> most likely sires for most seedlings, and also show evidence <strong>of</strong> increased selffertilizationin isolated plants. We found seedling recruitment to be spatially heterogeneous alongshorelines, with local pockets <strong>of</strong> recruitment at invaded sites and highly local aggregations <strong>of</strong>seedlings in proximity to <strong>the</strong>ir seed (and pollen) parents. These results give support to transgressivehybrid plants as <strong>the</strong> drivers <strong>of</strong> <strong>the</strong> invasion.Keywords: Hybrid Spartina, seedling recruitment, self-compatibilityINTRODUCTIONIn light <strong>of</strong> <strong>the</strong> accelerating spread <strong>of</strong> Spartina hybrids inSFB (Ayres et al. 2004) we propose that highly invasiveindividuals at <strong>the</strong> forefront <strong>of</strong> <strong>the</strong> invasion possess extremephenotypes for traits important in hybrid cordgrass survivaland spread. We posit <strong>the</strong>se traits are i) self-compatibility,which enables single individuals to found new populations;ii) height, which confers <strong>the</strong> ability to grow low on <strong>the</strong> openmud <strong>of</strong> <strong>the</strong> intertidal plane; iii) lateral spread, which anchorsplants into <strong>the</strong> shifting substrate and allows colonization <strong>of</strong>occupied or unoccupied neighboring patches; iv) tolerance tohigh (40 parts per trillion (ppt)) salinity which allows plantsto growth higher on <strong>the</strong> intertidal plane in <strong>the</strong> range <strong>of</strong>Salicornia virginica, and v) <strong>the</strong> timing and abundance <strong>of</strong>flowering, which is important in siring ability on earlyflowering native S. foliosa and in seed production (Ayres etal. 2008). These traits cause <strong>the</strong> selective superiority <strong>of</strong> thoseindividuals that possess all or most <strong>of</strong> <strong>the</strong>m with regard too<strong>the</strong>r hybrids and both parent species, and we propose that<strong>the</strong>se hybrid individuals will drive <strong>the</strong> invasion <strong>of</strong> SFB openmudflats. We have found that Spartina hybrids show highlyvariable self-compatibility, while both hybrid parent species(S. alterniflora and S. foliosa) are marginally self-compatible(H. Davis and D. Ayres, unpublished data) (Fig. 1). Hybridindividuals thus greatly differ in <strong>the</strong>ir ability to successfullyset self-fertilized seed, and certain hybrid plants aretransgressive, exceeding both parent species and o<strong>the</strong>rhybrids in <strong>the</strong>ir ability to self-fertilize (Fig. 1).RESULTS AND DISCUSSIONWe performed GPS surveys in 2003 and 2004 at threesites along <strong>the</strong> eastern shores <strong>of</strong> SFB (Elsie Roemer Marsh,Robert’s Landing, Hayward Shoreline) and found a dramaticincrease in seedling establishment from 2003-2004 (Fig. 2).At Hayward shoreline all genetically surveyed seedlingswere hybrids. Overlapping GPS points and <strong>the</strong> relativelylarger size <strong>of</strong> some individuals at Hayward suggest someseedling survival from 2003 to 2004 (Fig. 2). Microsatelliteparentage analysis (Sloop et al. 2005; Blum et al. 2004;Gerber et al. 2003) <strong>of</strong> a sub-sample <strong>of</strong> GPS-surveyedseedlings and surrounding plants suggests that seedlingsoriginate most likely from local seed sources, ra<strong>the</strong>r thanfrom seeds swept in by <strong>the</strong> tides (Sloop et al. 2009). Ourparentage data show a large likelihood <strong>of</strong> establishingseedlings as <strong>the</strong> progeny <strong>of</strong> <strong>the</strong> surrounding clones. In 200338% <strong>of</strong> all establishing seedlings were most likely selffertilizedat Hayward Shoreline. Microsatellite analysis alsorevealed that at Robert’s Landing <strong>the</strong> majority <strong>of</strong> 2003 seedscollected from isolated hybrid plants were self-fertilized.-95-