Proceedings of the Third International Conference on Invasive ...

Chapter 3: Ecosystem Effects <strong>of</strong> Invasive Spartina<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaWe established ten study quadrats within Spartina ateach site, paired with ten on mudflats. These quadrats wereused for vegetation, sediment, and infauna sampling. Fordetails <strong>of</strong> methods, see Brusati and Grosholz (2006). Wemeasured stem heights and densities, aboveground biomass,and belowground biomass at each site, as well as sedimentcharacteristics such as organic matter content, bulk density,sediment porewater salinity, and oxidation-reductionpotential. Infauna cores (5 centimeters [cm] deep x 5 cmdiameter) were collected in winter and summer 2001-03.Cores were preserved in 8% formalin and organisms werecounted and weighed in <strong>the</strong> laboratory. We used t-tests ontransformed data to compare results between Spartina andmudflats.To analyze <strong>the</strong> effects <strong>of</strong> Spartina on invertebrate foodwebs, we collected species <strong>of</strong> infaunal and epifaunalinvertebrates in S. foliosa, hybrid Spartina, and mudflats.To understand how <strong>the</strong>se plants are incorporated into <strong>the</strong>invertebrate food webs we analyzed organisms for naturallyoccuring abundances <strong>of</strong> <strong>the</strong> stable isotope ratios 13 C and 15 N. The carbon signal reflects a weighted average <strong>of</strong> foodsources, with Spartina 13 C = -14 ‰, which is significantlymore enriched than o<strong>the</strong>r carbon sources in this habitat(Cloern et al. 2002). We predict that if hybrid Spartina isentering <strong>the</strong> food web, consumers collected within invadedareas will have a stronger Spartina signal than those frommudflats, with consumers from S. foliosa showing anintermediate signal.RESULTSHybrid Spartina differs from S. foliosa in its effects onhabitat structure, infaunal communities, and food webs.Hybrid Spartina produces more biomass, and <strong>the</strong>refore moredense structure on mudflats, than S. foliosa both above andbelow ground (Fig. 1). Alameda, where <strong>the</strong> invasion is 30years old, has greater belowground biomass than SanLorenzo, which was invaded in <strong>the</strong> 1990s. We found fewconsistent differences between sediment characteristics <strong>of</strong>hybrid or S. foliosa and mudflats. Spartina foliosa generallyhad significantly higher infaunal densities and biomass thanadjacent mudflats. Winter data are presented here, butsummer samples showed similar patterns. In contrast, hybridSpartina sediments never contained significantly greaterdensities or biomass than mudflats (Fig. 2, numbers abovebars are p-values, n = 10).Food web analysis also shows differences between S.foliosa and hybrid Spartina. Stable isotope results indicated thatsome species show evidence <strong>of</strong> a slight shift in 13 C towardSpartina for individuals collected within S. foliosa. (Fig. 3) Forexample, shore crabs (Hemigrapsus oregonensis) and Europeangreen crabs (Carcinus maenas) living within S. foliosa at ChinaCamp, and H. oregonensis in S. foliosa at Drakes Estero hadisotopic signatures more similar to S. foliosa than thosecollected on mudflats. At Alameda, <strong>the</strong>re was no significantdifference in isotopic signatures <strong>of</strong> C. maenas or Atlantic oysterdrills (Urosalpinx cinerea) from hybrid Spartina and mudflats.When results from all sites are compared, hybrid Spartina doesnot appear to produce a stronger shift in isotope signatures thanS. foliosa (Brusati and Grosholz 2009).DISCUSSIONOur results show that hybrid Spartina is not ecologicallyequivalent to <strong>the</strong> native cordgrass. While both S. foliosa andhybrid Spartina modify estuarine habitat, <strong>the</strong> magnitude <strong>of</strong><strong>the</strong> changes produced by hybrid Spartina are greater,resulting in qualitative differences in <strong>the</strong> infauna. Thestrongest differences were seen in <strong>the</strong> greater height andaboveground biomass <strong>of</strong> hybrid Spartina compared to S.foliosa. The greater aboveground biomass <strong>of</strong> hybrid Spartinais more than would be expected from differences in stemheight and density between <strong>the</strong> two species; it also reflects<strong>the</strong> fact that <strong>the</strong> hybrid’s stems are considerably thicker thanS. foliosa’s. Native S. foliosa faciliates infauna, possibly bystabilizing substrate or providing attachment sites for tubebuildingorganisms. In contrast, <strong>the</strong> dense roots andAboveground Biomass (g/m 2 )25002000150010005000Outer coastNodataSan FranciscoEstuaryBH BL DE SH TP CC SLF SLH AL---------------Spartina foliosa ----------------------- Hybrid2001 2002 2003Individuals/m 27000060000500004000030000200001000000.005Outer Coast0.2630.0020.000San FranciscoEstuary0.7240.114BL DE SM TP CC SL ALMudflat S. foliosa Hybrid0.001Fig. 1. Aboveground biomass <strong>of</strong> S. foliosa and hybrid Spartina 2001-03 (n= 10). Site abbreviations: BH = Bodega Harbor, BL = Bolinas Lagoon, DE= Drake’s Estero, SH = Shields March, TP = Tom’s Pt., CC = China Camp,SLF = Robert’s Landing S. foliosa, SLH = San Lorenzo hybrid, AL =Alameda.Fig. 2. Infaunal densities in S. foliosa exceed those in mudflats, whilehybrid Spartina never contained greater densities than mudflats (numbersabove bars are p-values, n = 10, winter 2001 data shown).- 162 -