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<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaChapter 1: Spartina Biology250200Clonal Spread (m-squ)150100500-50Fig. 2. Clonal spread in area <strong>of</strong> individual plants (along <strong>the</strong> x-axis) at Cogswell Marsh from 1998 to 2000 (complete data was only available for 52plants). Grey arrows are S. foliosa and black arrow is S. alterniflora plants (by genetic analyses); <strong>the</strong> rest are hybrids.10008006004002000Fertile seedInfertile seedFig. 3. Sexual reproduction <strong>of</strong> 48 cordgrass plants at Cogswell Marsh in 1998 (many plants did not flower); individual plants are along <strong>the</strong> x-axis.Each bar is <strong>the</strong> average number <strong>of</strong> flowers on an inflorescence; <strong>the</strong> black portion represents those florets that contained a fertile seed, <strong>the</strong> open portionrepresents those flowers without a seed. The S. foliosa plants are along <strong>the</strong> left side, with grey bars; <strong>the</strong> single S. alterniflora did not flower.-25-
Chapter 1: Spartina Biology<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive Spartina1.00.80.60.40.20.0S. foliosa Hybrids S. foliosa Hybrids S. foliosa HybridsPollen Viability Pollen/m 2 Pollen/PlantFig. 4. Relative pollen viability and production <strong>of</strong> S. foliosa and cordgrass hybrids; box-whisker plots show 25th, 50th and 75th per-centiles solidlines in <strong>the</strong> “box”, whiskers are <strong>the</strong> 5th and 95th percentile; solid circles are at <strong>the</strong> 2.5 and 95.6 percentile; dotted line is <strong>the</strong> mean. This figurefirst appeared in Ayres et al. 2008.San Francisco vs Willapa Invasions• S. alterniflora• “Open niche”• 25 YA• Hybridization• Evolution fueled bygenetic variation• Accelerating spread• S. altern iflora• “Open niche”• 100 YA• No native Spartina• Evolution constrainedby lack <strong>of</strong> variation• Constant spread rateFig. 5. Comparison <strong>of</strong> S. alterniflora introductions into 2 Pacific coast USAestuaries, San Francisco Bay, CA, and Willapa Bay, WA. We posit thatspread rates are a function <strong>of</strong> <strong>the</strong> amount <strong>of</strong> genetic variation.plants in <strong>the</strong> marsh were hybrids. The net result was that over95% <strong>of</strong> <strong>the</strong> seed produced from all 42 plants in our study (ca.40 million seeds in 1998) was hybrid.Hybrid pollen had 3.5 times <strong>the</strong> viability <strong>of</strong> S. foliosapollen (8.24% vs 2.38% viable grains, respectively), andhybrid plants produced twice <strong>the</strong> number <strong>of</strong> pollen grainsas native plants (9.5 million vs. 4.3 million grains per plant)(Fig. 4, Ayres et al. 2008). Viable pollen production rangedfrom a single hybrid individual (plant C1-5) that produced58% <strong>of</strong> <strong>the</strong> total viable pollen to zero. The cumulative effects<strong>of</strong> <strong>the</strong> scarcity <strong>of</strong> S. foliosa plants, and differences in pollenproduction and viability resulted in hybrid plants producingover 400 times more viable pollen than <strong>the</strong> native plants.Spread Rate. Combining <strong>the</strong> results <strong>of</strong> our 2000–2001cordgrass survey with literature-based estimates <strong>of</strong> populationssize, we found that spread rates <strong>of</strong> hybrid and S. alterniflorahave increased over time (Ayres et al. 2004), from10% (during 1973 to 1992) to 40% (from 1993 to 2001) peryear. This pattern is apparently not typical <strong>of</strong> S. alterniflorainvasion in general as analyses <strong>of</strong> spread in Willapa Bay,Washington found a constant growth rate <strong>of</strong> 12% over <strong>the</strong>past 100 years (Civille et al. 2005; Taylor et al. 2004, and inthis proceeding volume).DISCUSSION AND CONCLUSIONSWe have shown that hybrids are genetically variabledue to interbreeding among hybrids and backcrossing to <strong>the</strong>native (Ayres et al. 1999; Anttila et al. 2000). In <strong>the</strong> presentstudy, we demonstrate that hybrids are also phenotypicallyhighly variable in features <strong>of</strong> vegetative and sexual reproduction,with a subset <strong>of</strong> hybrid individuals having superiorgrowth and contributing disproportionately to seed andpollen production.We <strong>the</strong>orize that <strong>the</strong> open niche <strong>of</strong> native marshes,intertidal mud flats, and restoration sites provides strongselection for vigorous vegetative growth and seed production.Hybridization has provided large amounts <strong>of</strong> geneticand phenotypic variation for <strong>the</strong>se traits. We suggest that <strong>the</strong>combination <strong>of</strong> natural selection and variability in fitnesstraits sets up a scenario <strong>of</strong> positive feedback whereby strongnatural selection favors a subset <strong>of</strong> highly fit hybrids thatdrives growth rate, followed by fur<strong>the</strong>r selection on hybridplants for invasion ability and enhanced growth rates (Fig. 5).The accelerating growth rate we have observed in cordgrasspopulations in San Francisco Bay is indirect evidence thatthis has already occurred.This outcome contrasts with that in Willapa Bay, whereS. alterniflora was introduced into <strong>the</strong> same open niche <strong>of</strong>intertidal mudflat habitat 100 years ago. There is no nativeSpartina at Willapa, and <strong>the</strong>refore hybridization has notoccurred. We propose that evolution <strong>the</strong>re has been constrainedby a relative lack <strong>of</strong> genetic diversity in <strong>the</strong> popula--26-
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CHAPTER THREEEcosystem Effects <str
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