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 Spartinaintroduced twice into south San Francisco Bay. Seed from aMaryland marsh was sown in New Alameda Creek, locatedbetween Fremont and Union City (Faber 2000). The secondknown introduction, a planting sponsored by <strong>the</strong> ArmyCorps <strong>of</strong> Engineers at Alameda Island, occurred about 40kilometers (km) north <strong>of</strong> <strong>the</strong> first (Ayres et al. 2003). As aresult <strong>of</strong> lack <strong>of</strong> pollen from conspecific plants and pollenswamping from S. alterniflora x S. foliosa hybrids, S.alterniflora has become quite rare in San Francisco Bay in<strong>the</strong> two or three decades since introduction.One or both <strong>of</strong> <strong>the</strong> S. alterniflora introductions resultedin hybridization with <strong>the</strong> native California cordgrass toproduce a backcrossing swarm <strong>of</strong> S. alterniflora x S. foliosahybrids in San Francisco Bay. The chloroplast DNA <strong>of</strong> bothparental species is found among <strong>the</strong> plants in <strong>the</strong> swarmindicating that both parental species have served as seedparents <strong>of</strong> hybrids. While hybridization probably hasoccurred multiple times F1 hybrids are rare in <strong>the</strong> field. Withgreat effort in <strong>the</strong> greenhouse we have produced a few F1hybrids.A subset <strong>of</strong> hybrid genotypes are extremely fit in one ora combination <strong>of</strong> <strong>the</strong> following traits: vegetative growth rate,numbers <strong>of</strong> viable seed, volume <strong>of</strong> pollen, and/or selfcompatibility.These hybrid traits can be transgressive,which means that hybrids exceed both parental species in <strong>the</strong>magnitude <strong>of</strong> <strong>the</strong> trait. We entertain <strong>the</strong> hypo<strong>the</strong>sis that <strong>the</strong>most important transgressive trait <strong>of</strong> hybrids is selfcompatibility,which allows a plant to pollinate itself and setseed at low density after invasion.A great deal <strong>of</strong> hybrid S. alterniflora x S. foliosa seed iscarried on <strong>the</strong> 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 <strong>of</strong> S. foliosaand o<strong>the</strong>r native marsh plants. Hybrid invasion <strong>of</strong> vegetativemarshes leads to severe ecological and genetic competitionwith native S. foliosa. Hybrid cordgrass is increasing atgreater than exponential rates in San Francisco Bay. In 2002,approximately 1,500 ha <strong>of</strong> salt marsh was dominated by <strong>the</strong>swarm in San Francisco Bay (Ayres et al. 2004). The mostrecent pair <strong>of</strong> censuses yields a doubling time <strong>of</strong> about threemonths for coverage <strong>of</strong> <strong>the</strong> hybrid swarm in <strong>the</strong> Bay.The loss <strong>of</strong> native cordgrass due to competition andinterbreeding is accelerating. We can conceive <strong>of</strong> no naturallimitation to this loss, which is a runaway, unregulatedprocess that could lead to <strong>the</strong> extinction <strong>of</strong> S. foliosa in SanFrancisco Bay. Similarly, dispersal <strong>of</strong> hybrids to saltmarshes in Baja California could lead to <strong>the</strong> extinction <strong>of</strong> S.foliosa <strong>the</strong>re (Ayres et al. 2003). In contrast, Willapa Bay,Washington has no native cordgrass and no potential forhybridization. Spartina alterniflora was introduced <strong>the</strong>remore than 100 years ago (Civille et al. 2005) and probablyarrived as a hitchhiker on <strong>the</strong> numerous trains from NewYork harbor that brought oysters for outplanting in WillapaBay at <strong>the</strong> end <strong>of</strong> <strong>the</strong> 19th and beginning <strong>of</strong> <strong>the</strong> 20thcenturies. The first hard historical evidence <strong>of</strong> <strong>the</strong> invasion isfrom a photo and publication in 1941. The large patch size<strong>of</strong> <strong>the</strong> plant implies that it had already been growing forseveral decades. The first aerial photos, from 1945, showseveral large colonies, fur<strong>the</strong>r evidence that introductionoccurred decades earlier. The multiple, widely separatedcolonies imply multiple introductions ra<strong>the</strong>r than spreadfrom a single focus. Coverage <strong>of</strong> S. alterniflora hasincreased at a rate very close to exponential between 1945and 2000, about 12% per year. This gives a doubling time <strong>of</strong>about six years. Approximately 1,670 ha (27%) <strong>of</strong> <strong>the</strong> 6,000ha <strong>of</strong> intertidal habitat <strong>of</strong> Willapa Bay had been colonized byS. alterniflora by 2000. Large-scale chemical control is nowgreatly reducing S. alterniflora in Willapa Bay (Strong andAyres 2009).In an echo <strong>of</strong> <strong>the</strong> slow spread <strong>of</strong> S. alterniflora in SanFrancisco Bay, we have found that very little viable seed isset at <strong>the</strong> leading edge <strong>of</strong> <strong>the</strong> invasion <strong>of</strong> Willapa Bay (Daviset al. 2004a). Seed settles at low densities onto open mudand recruits are widely separated from one ano<strong>the</strong>r. Decadeslater, when <strong>the</strong> circular clones coalesce to form continuousmeadows, seed set increases by an order <strong>of</strong> magnitude. Thisis a weak Allee effect. A strong Allee effect would result ifno seed is set by plants at low densities, indicating that S.alterniflora would have become extinct at <strong>the</strong> low densities<strong>of</strong> <strong>the</strong> initial invasion. The weak Allee effect has slowed <strong>the</strong>rate <strong>of</strong> invasion. Were <strong>the</strong>re no Allee effect, S. alterniflorawould have to be able to self-pollinate, and single plantswould set seed at <strong>the</strong> same rate as plants growing in highdensities. Without an Allee effect, <strong>the</strong> rate <strong>of</strong> increase incoverage <strong>of</strong> S. alterniflora would have been about 30% peryear, and <strong>the</strong> doubling time would have been as short as 2.5years instead <strong>of</strong> <strong>the</strong> actual six years (Taylor et al. 2004).Thus, without <strong>the</strong> Allee effect, <strong>the</strong> approximately three ha <strong>of</strong>S. alterniflora shown in <strong>the</strong> 1945 aerial photos would havegrown to completely cover <strong>the</strong> entire 19,000 ha <strong>of</strong> intertidalarea <strong>of</strong> Willapa Bay by about 1977.The dearth <strong>of</strong> viable seed set at <strong>the</strong> leading edge <strong>of</strong> <strong>the</strong>invasion <strong>of</strong> S. alterniflora in Willapa Bay and <strong>the</strong> weakAllee effect are a result <strong>of</strong> a lack <strong>of</strong> pollen produced by lowdensityplants (Davis et al. 2004b). We found ninefold morepollen on stigmas <strong>of</strong> high-density plants in old marshes thanon those <strong>of</strong> low-density plants at <strong>the</strong> leading edge <strong>of</strong> <strong>the</strong>invasion. Only in old marshes, where plants had growntoge<strong>the</strong>r to form dense meadows, was <strong>the</strong>re sufficient pollenon stigmas for much seed set. Experimental pollinationaugmentation <strong>of</strong> low-density plants, but not <strong>of</strong> high-densityplants, increased seed set. Experimental pollen exclusionfrom high-density plants, but not from low-density plants,decreased seed set.In summary, <strong>the</strong> cordgrass invasion <strong>of</strong> San FranciscoBay is by hybrids <strong>of</strong> S. alterniflora and S. foliosa, not by S.alterniflora alone. The hybrid has a truly phenomenal rate <strong>of</strong>spread, and this rate is accelerating. This is evidence that <strong>the</strong>hybrids are much more invasive than S. alterniflora alone.-62-