<str<strong>on</strong>g>Proceedings</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>Third</str<strong>on</strong>g> <str<strong>on</strong>g>Internati<strong>on</strong>al</str<strong>on</strong>g> <str<strong>on</strong>g>C<strong>on</strong>ference</str<strong>on</strong>g> <strong>on</strong> <strong>Invasive</strong> SpartinaChapter 2: Spartina Distributi<strong>on</strong> and SpreadATALE OF TWO INVADED ESTUARIES: SPARTINA IN SAN FRANCISCO BAY,CALIFORNIAAND WILLAPA BAY,WASHINGTOND.R. STRONGDepartment <str<strong>on</strong>g>of</str<strong>on</strong>g> Ecology and Evoluti<strong>on</strong>, University <str<strong>on</strong>g>of</str<strong>on</strong>g> California, Davis, Davis, CA 95616; drstr<strong>on</strong>g@ucdavis.eduMaritime Spartina species grow lower <strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> tidal plane than o<str<strong>on</strong>g>the</str<strong>on</strong>g>r vascular plants and maintain <str<strong>on</strong>g>the</str<strong>on</strong>g>shoreline <strong>on</strong> temperate coasts where <str<strong>on</strong>g>the</str<strong>on</strong>g>y are native. All but two <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> 14 known species are nativeto <str<strong>on</strong>g>the</str<strong>on</strong>g> Atlantic. Spartina alterniflora was introduced a century ago into Willapa Bay, Washingt<strong>on</strong>,far north <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> native limit <str<strong>on</strong>g>of</str<strong>on</strong>g> this genus. This Atlantic native spread exp<strong>on</strong>entially throughtidelands <str<strong>on</strong>g>the</str<strong>on</strong>g>re at a remarkably c<strong>on</strong>stant approximately 12% per year over <str<strong>on</strong>g>the</str<strong>on</strong>g> 55-year history <str<strong>on</strong>g>of</str<strong>on</strong>g>aerial photographs. In 2000, it covered approximately 1,670 <str<strong>on</strong>g>of</str<strong>on</strong>g> 6,000 hectares (ha) or 27% <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g>intertidal habitat <str<strong>on</strong>g>of</str<strong>on</strong>g> Willapa Bay. However, <str<strong>on</strong>g>the</str<strong>on</strong>g> rate <str<strong>on</strong>g>of</str<strong>on</strong>g> spread was slowed greatly by an Allee effectdue to poor pollen dispersal. Without <str<strong>on</strong>g>the</str<strong>on</strong>g> Allee effect, <str<strong>on</strong>g>the</str<strong>on</strong>g> invasi<strong>on</strong> would have covered <str<strong>on</strong>g>the</str<strong>on</strong>g> li<strong>on</strong>’sshare <str<strong>on</strong>g>of</str<strong>on</strong>g> Willapa Bay l<strong>on</strong>g ago. Large-scale chemical c<strong>on</strong>trol is now greatly reducing S. alterniflorain Willapa Bay. The San Francisco Bay regi<strong>on</strong> is <str<strong>on</strong>g>the</str<strong>on</strong>g> nor<str<strong>on</strong>g>the</str<strong>on</strong>g>rn limit <str<strong>on</strong>g>of</str<strong>on</strong>g> Spartina foliosa, <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> twoSpartina species native to <str<strong>on</strong>g>the</str<strong>on</strong>g> Pacific. Introduced Spartina played virtually no role in San FranciscoBay until 1975, when <str<strong>on</strong>g>the</str<strong>on</strong>g> U.S. Army Corps <str<strong>on</strong>g>of</str<strong>on</strong>g> Engineers planted S. alterniflora, which hybridizedwith <str<strong>on</strong>g>the</str<strong>on</strong>g> native S. foliosa so<strong>on</strong> afterwards. While S. alterniflora has become virtually extinct, <str<strong>on</strong>g>the</str<strong>on</strong>g>hybrids have had a truly phenomenal rate <str<strong>on</strong>g>of</str<strong>on</strong>g> spread. The few hybrids that formed in <str<strong>on</strong>g>the</str<strong>on</strong>g> late 1970sspread to about 1,500 ha when <str<strong>on</strong>g>the</str<strong>on</strong>g> San Francisco Estuary <strong>Invasive</strong> Spartina Project (ISP) began <str<strong>on</strong>g>the</str<strong>on</strong>g>irc<strong>on</strong>trol effort. The rapid spread <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrids is probably due to <str<strong>on</strong>g>the</str<strong>on</strong>g> evoluti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> self-pollinati<strong>on</strong>, thuseliminating <str<strong>on</strong>g>the</str<strong>on</strong>g> Allee effect. Ultimate success <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> ISP will depend up<strong>on</strong> a sophisticatedcombinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> biochemical systematics with ecological field research that determines dynamics <str<strong>on</strong>g>of</str<strong>on</strong>g>cryptic hybrids that could survive c<strong>on</strong>trol efforts.Key Words: Hybrid, rate <str<strong>on</strong>g>of</str<strong>on</strong>g> spreadSpartinas are cordgrasses (Str<strong>on</strong>g and Ayres 2009). Thespecies <str<strong>on</strong>g>of</str<strong>on</strong>g> estuarine cordgrasses that we have studied arewind-pollinated, largely self-incompatible and outbreeding.They are protogynous; <str<strong>on</strong>g>the</str<strong>on</strong>g> female flowers appear before <str<strong>on</strong>g>the</str<strong>on</strong>g>male flowers. In order to set much viable seed, each plantrequires pollen to be carried <strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> wind from a different,earlier-flowering plant that has progressed to <str<strong>on</strong>g>the</str<strong>on</strong>g> later stage<str<strong>on</strong>g>of</str<strong>on</strong>g> having male, pollen-bearing flowers.Cordgrasses are ecosystem engineers. Their tall densestems slow water movement and cause sediment to settle andbe bound by thick, fibrous roots. Roots grow upwardthrough <str<strong>on</strong>g>the</str<strong>on</strong>g> settling sediment to form thick peat that elevates<str<strong>on</strong>g>the</str<strong>on</strong>g> surface <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> marsh. Cordgrasses that invade areas withno emergent vegetati<strong>on</strong> greatly increase local photosyn<str<strong>on</strong>g>the</str<strong>on</strong>g>ticrates. Their roots greatly increase subsurface carb<strong>on</strong> whichremains in <str<strong>on</strong>g>the</str<strong>on</strong>g> anoxic sediment l<strong>on</strong>g after <str<strong>on</strong>g>the</str<strong>on</strong>g> plants havebeen removed. <strong>Invasive</strong> cordgrasses have transformed vastexpanses <str<strong>on</strong>g>of</str<strong>on</strong>g> open intertidal mudflat into meadows thatelevate with time. Estuarine cordgrasses disperse primarilyby floating seed that does not accumulate in soil. Noevidence indicates a seed bank more than <strong>on</strong>e year old forestuarine cordgrasses.All but <strong>on</strong>e <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> 14 nominal species <str<strong>on</strong>g>of</str<strong>on</strong>g> cordgrass arenative to <str<strong>on</strong>g>the</str<strong>on</strong>g> Americas: Spartina maritima is endemic to <str<strong>on</strong>g>the</str<strong>on</strong>g>south <str<strong>on</strong>g>of</str<strong>on</strong>g> England and France (Daehler and Str<strong>on</strong>g 1997).The Pacific has but two natives, Spartina densiflora in Chileand Spartina foliosa, California cordgrass, that thrives inBaja California and in San Francisco Bay. Peoplepurposefully and inadvertently spread cordgrasses, and fourn<strong>on</strong>-native Spartinas have been introduced to San FranciscoBay.Spartina patens is <str<strong>on</strong>g>the</str<strong>on</strong>g> least successful San Franciscoinvader and is known from <strong>on</strong>ly two plants in Suisun Bay(Ayres et al. 2004). Native to Atlantic marshes <str<strong>on</strong>g>of</str<strong>on</strong>g> NorthAmerica, S. patens spread rapidly at <strong>on</strong>e site in Oreg<strong>on</strong>during <str<strong>on</strong>g>the</str<strong>on</strong>g> mid-20th century and it has appeared recently inSpain. With somewhat greater success, S. densiflora spreadto several sites in San Francisco Bay after at least twointroducti<strong>on</strong>s in Marin County, where it was brought from<str<strong>on</strong>g>the</str<strong>on</strong>g> huge, century-old Humboldt Bay, California infestati<strong>on</strong>.This species has also been introduced into Spain.Spartina anglica arose in England in <str<strong>on</strong>g>the</str<strong>on</strong>g> 19th century asa hybrid <str<strong>on</strong>g>of</str<strong>on</strong>g> S. maritima and S. alterniflora, after <str<strong>on</strong>g>the</str<strong>on</strong>g> latterspecies was introduced from its native Atlantic shores <str<strong>on</strong>g>of</str<strong>on</strong>g>North America. Spartina anglica spread widely afterintroducti<strong>on</strong> to Puget Sound, Washingt<strong>on</strong>, <str<strong>on</strong>g>the</str<strong>on</strong>g> Ne<str<strong>on</strong>g>the</str<strong>on</strong>g>rlands,Tasmania, Australia, and New Zealand. However, S. anglicahas not spread to o<str<strong>on</strong>g>the</str<strong>on</strong>g>r places in San Francisco Bay postintroducti<strong>on</strong>in 1977 to Creekside Park in Greenbrae, MarinCounty.In misguided attempts at marsh restorati<strong>on</strong> in <str<strong>on</strong>g>the</str<strong>on</strong>g> mid-1970s, <str<strong>on</strong>g>the</str<strong>on</strong>g> fourth species, Spartina alterniflora, was-61-
Chapter 2: Spartina Distributi<strong>on</strong> and Spread<str<strong>on</strong>g>Proceedings</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>Third</str<strong>on</strong>g> <str<strong>on</strong>g>Internati<strong>on</strong>al</str<strong>on</strong>g> <str<strong>on</strong>g>C<strong>on</strong>ference</str<strong>on</strong>g> <strong>on</strong> <strong>Invasive</strong> Spartinaintroduced twice into south San Francisco Bay. Seed from aMaryland marsh was sown in New Alameda Creek, locatedbetween Frem<strong>on</strong>t and Uni<strong>on</strong> City (Faber 2000). The sec<strong>on</strong>dknown introducti<strong>on</strong>, a planting sp<strong>on</strong>sored by <str<strong>on</strong>g>the</str<strong>on</strong>g> ArmyCorps <str<strong>on</strong>g>of</str<strong>on</strong>g> Engineers at Alameda Island, occurred about 40kilometers (km) north <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> first (Ayres et al. 2003). As aresult <str<strong>on</strong>g>of</str<strong>on</strong>g> lack <str<strong>on</strong>g>of</str<strong>on</strong>g> pollen from c<strong>on</strong>specific plants and pollenswamping from S. alterniflora x S. foliosa hybrids, S.alterniflora has become quite rare in San Francisco Bay in<str<strong>on</strong>g>the</str<strong>on</strong>g> two or three decades since introducti<strong>on</strong>.One or both <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> S. alterniflora introducti<strong>on</strong>s resultedin hybridizati<strong>on</strong> with <str<strong>on</strong>g>the</str<strong>on</strong>g> native California cordgrass toproduce a backcrossing swarm <str<strong>on</strong>g>of</str<strong>on</strong>g> S. alterniflora x S. foliosahybrids in San Francisco Bay. The chloroplast DNA <str<strong>on</strong>g>of</str<strong>on</strong>g> bothparental species is found am<strong>on</strong>g <str<strong>on</strong>g>the</str<strong>on</strong>g> plants in <str<strong>on</strong>g>the</str<strong>on</strong>g> swarmindicating that both parental species have served as seedparents <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrids. While hybridizati<strong>on</strong> probably hasoccurred multiple times F1 hybrids are rare in <str<strong>on</strong>g>the</str<strong>on</strong>g> field. Withgreat effort in <str<strong>on</strong>g>the</str<strong>on</strong>g> greenhouse we have produced a few F1hybrids.A subset <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrid genotypes are extremely fit in <strong>on</strong>e ora combinati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> following traits: vegetative growth rate,numbers <str<strong>on</strong>g>of</str<strong>on</strong>g> viable seed, volume <str<strong>on</strong>g>of</str<strong>on</strong>g> pollen, and/or selfcompatibility.These hybrid traits can be transgressive,which means that hybrids exceed both parental species in <str<strong>on</strong>g>the</str<strong>on</strong>g>magnitude <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> trait. We entertain <str<strong>on</strong>g>the</str<strong>on</strong>g> hypo<str<strong>on</strong>g>the</str<strong>on</strong>g>sis that <str<strong>on</strong>g>the</str<strong>on</strong>g>most important transgressive trait <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrids is selfcompatibility,which allows a plant to pollinate itself and setseed at low density after invasi<strong>on</strong>.A great deal <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrid S. alterniflora x S. foliosa seed iscarried <strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> currents and tides around San Francisco Bayand new marshes are invaded every year. Hybrid seed floatsto open mud flats, germinates, and grows rapidly. This seedalso spreads into vegetated marshes comprised <str<strong>on</strong>g>of</str<strong>on</strong>g> S. foliosaand o<str<strong>on</strong>g>the</str<strong>on</strong>g>r native marsh plants. Hybrid invasi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> vegetativemarshes leads to severe ecological and genetic competiti<strong>on</strong>with native S. foliosa. Hybrid cordgrass is increasing atgreater than exp<strong>on</strong>ential rates in San Francisco Bay. In 2002,approximately 1,500 ha <str<strong>on</strong>g>of</str<strong>on</strong>g> salt marsh was dominated by <str<strong>on</strong>g>the</str<strong>on</strong>g>swarm in San Francisco Bay (Ayres et al. 2004). The mostrecent pair <str<strong>on</strong>g>of</str<strong>on</strong>g> censuses yields a doubling time <str<strong>on</strong>g>of</str<strong>on</strong>g> about threem<strong>on</strong>ths for coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> hybrid swarm in <str<strong>on</strong>g>the</str<strong>on</strong>g> Bay.The loss <str<strong>on</strong>g>of</str<strong>on</strong>g> native cordgrass due to competiti<strong>on</strong> andinterbreeding is accelerating. We can c<strong>on</strong>ceive <str<strong>on</strong>g>of</str<strong>on</strong>g> no naturallimitati<strong>on</strong> to this loss, which is a runaway, unregulatedprocess that could lead to <str<strong>on</strong>g>the</str<strong>on</strong>g> extincti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> S. foliosa in SanFrancisco Bay. Similarly, dispersal <str<strong>on</strong>g>of</str<strong>on</strong>g> hybrids to saltmarshes in Baja California could lead to <str<strong>on</strong>g>the</str<strong>on</strong>g> extincti<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> S.foliosa <str<strong>on</strong>g>the</str<strong>on</strong>g>re (Ayres et al. 2003). In c<strong>on</strong>trast, Willapa Bay,Washingt<strong>on</strong> has no native cordgrass and no potential forhybridizati<strong>on</strong>. Spartina alterniflora was introduced <str<strong>on</strong>g>the</str<strong>on</strong>g>remore than 100 years ago (Civille et al. 2005) and probablyarrived as a hitchhiker <strong>on</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> numerous trains from NewYork harbor that brought oysters for outplanting in WillapaBay at <str<strong>on</strong>g>the</str<strong>on</strong>g> end <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> 19th and beginning <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> 20thcenturies. The first hard historical evidence <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> invasi<strong>on</strong> isfrom a photo and publicati<strong>on</strong> in 1941. The large patch size<str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> plant implies that it had already been growing forseveral decades. The first aerial photos, from 1945, showseveral large col<strong>on</strong>ies, fur<str<strong>on</strong>g>the</str<strong>on</strong>g>r evidence that introducti<strong>on</strong>occurred decades earlier. The multiple, widely separatedcol<strong>on</strong>ies imply multiple introducti<strong>on</strong>s ra<str<strong>on</strong>g>the</str<strong>on</strong>g>r than spreadfrom a single focus. Coverage <str<strong>on</strong>g>of</str<strong>on</strong>g> S. alterniflora hasincreased at a rate very close to exp<strong>on</strong>ential between 1945and 2000, about 12% per year. This gives a doubling time <str<strong>on</strong>g>of</str<strong>on</strong>g>about six years. Approximately 1,670 ha (27%) <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> 6,000ha <str<strong>on</strong>g>of</str<strong>on</strong>g> intertidal habitat <str<strong>on</strong>g>of</str<strong>on</strong>g> Willapa Bay had been col<strong>on</strong>ized byS. alterniflora by 2000. Large-scale chemical c<strong>on</strong>trol is nowgreatly reducing S. alterniflora in Willapa Bay (Str<strong>on</strong>g andAyres 2009).In an echo <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> slow spread <str<strong>on</strong>g>of</str<strong>on</strong>g> S. alterniflora in SanFrancisco Bay, we have found that very little viable seed isset at <str<strong>on</strong>g>the</str<strong>on</strong>g> leading edge <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> invasi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> Willapa Bay (Daviset al. 2004a). Seed settles at low densities <strong>on</strong>to open mudand recruits are widely separated from <strong>on</strong>e ano<str<strong>on</strong>g>the</str<strong>on</strong>g>r. Decadeslater, when <str<strong>on</strong>g>the</str<strong>on</strong>g> circular cl<strong>on</strong>es coalesce to form c<strong>on</strong>tinuousmeadows, seed set increases by an order <str<strong>on</strong>g>of</str<strong>on</strong>g> magnitude. Thisis a weak Allee effect. A str<strong>on</strong>g Allee effect would result ifno seed is set by plants at low densities, indicating that S.alterniflora would have become extinct at <str<strong>on</strong>g>the</str<strong>on</strong>g> low densities<str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> initial invasi<strong>on</strong>. The weak Allee effect has slowed <str<strong>on</strong>g>the</str<strong>on</strong>g>rate <str<strong>on</strong>g>of</str<strong>on</strong>g> invasi<strong>on</strong>. Were <str<strong>on</strong>g>the</str<strong>on</strong>g>re no Allee effect, S. alterniflorawould have to be able to self-pollinate, and single plantswould set seed at <str<strong>on</strong>g>the</str<strong>on</strong>g> same rate as plants growing in highdensities. Without an Allee effect, <str<strong>on</strong>g>the</str<strong>on</strong>g> rate <str<strong>on</strong>g>of</str<strong>on</strong>g> increase incoverage <str<strong>on</strong>g>of</str<strong>on</strong>g> S. alterniflora would have been about 30% peryear, and <str<strong>on</strong>g>the</str<strong>on</strong>g> doubling time would have been as short as 2.5years instead <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> actual six years (Taylor et al. 2004).Thus, without <str<strong>on</strong>g>the</str<strong>on</strong>g> Allee effect, <str<strong>on</strong>g>the</str<strong>on</strong>g> approximately three ha <str<strong>on</strong>g>of</str<strong>on</strong>g>S. alterniflora shown in <str<strong>on</strong>g>the</str<strong>on</strong>g> 1945 aerial photos would havegrown to completely cover <str<strong>on</strong>g>the</str<strong>on</strong>g> entire 19,000 ha <str<strong>on</strong>g>of</str<strong>on</strong>g> intertidalarea <str<strong>on</strong>g>of</str<strong>on</strong>g> Willapa Bay by about 1977.The dearth <str<strong>on</strong>g>of</str<strong>on</strong>g> viable seed set at <str<strong>on</strong>g>the</str<strong>on</strong>g> leading edge <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g>invasi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> S. alterniflora in Willapa Bay and <str<strong>on</strong>g>the</str<strong>on</strong>g> weakAllee effect are a result <str<strong>on</strong>g>of</str<strong>on</strong>g> a lack <str<strong>on</strong>g>of</str<strong>on</strong>g> pollen produced by lowdensityplants (Davis et al. 2004b). We found ninefold morepollen <strong>on</strong> stigmas <str<strong>on</strong>g>of</str<strong>on</strong>g> high-density plants in old marshes than<strong>on</strong> those <str<strong>on</strong>g>of</str<strong>on</strong>g> low-density plants at <str<strong>on</strong>g>the</str<strong>on</strong>g> leading edge <str<strong>on</strong>g>of</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g>invasi<strong>on</strong>. Only in old marshes, where plants had growntoge<str<strong>on</strong>g>the</str<strong>on</strong>g>r to form dense meadows, was <str<strong>on</strong>g>the</str<strong>on</strong>g>re sufficient pollen<strong>on</strong> stigmas for much seed set. Experimental pollinati<strong>on</strong>augmentati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> low-density plants, but not <str<strong>on</strong>g>of</str<strong>on</strong>g> high-densityplants, increased seed set. Experimental pollen exclusi<strong>on</strong>from high-density plants, but not from low-density plants,decreased seed set.In summary, <str<strong>on</strong>g>the</str<strong>on</strong>g> cordgrass invasi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> San FranciscoBay is by hybrids <str<strong>on</strong>g>of</str<strong>on</strong>g> S. alterniflora and S. foliosa, not by S.alterniflora al<strong>on</strong>e. The hybrid has a truly phenomenal rate <str<strong>on</strong>g>of</str<strong>on</strong>g>spread, and this rate is accelerating. This is evidence that <str<strong>on</strong>g>the</str<strong>on</strong>g>hybrids are much more invasive than S. alterniflora al<strong>on</strong>e.-62-
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