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 3: Ecosystem Effects <strong>of</strong> Invasive SpartinaCONTRASTING EFFECTS OF SPARTINA FOLIOSA AND HYBRID SPARTINA ON BENTHICINVERTEBRATESE.D. BRUSATI 1,2 AND E.D. GROSHOLZ 11 Department <strong>of</strong> Environmental Science and Policy, University <strong>of</strong> California, Davis, One Shields Ave., Davis, CA, USA 956162 Present address: California Invasive Plant Council, 1442-A Walnut St. #462, Berkeley, CA USA 94709;edbrusati@cal-ipc.orgIn San Francisco Bay, California, mudflats and native California cordgrass (Spartina foliosa)marshes are being invaded by a hybrid cordgrass formed by hybridization between S. foliosa andintroduced S. alterniflora. We investigated differences in vegetation and sediment structure, benthicinfauna, and food webs within native and invaded Spartina marshes between San Francisco Bay andBodega Bay, California. The greatest impact <strong>of</strong> hybrid Spartina in San Francisco Bay appears to beits alteration <strong>of</strong> habitat structure ra<strong>the</strong>r than food webs. Habitat structure differed significantlybetween native and hybrid Spartina. Hybrid Spartina produced greater biomass both above andbelow ground, and taller stem heights. Spartina foliosa contained significantly higher densities andbiomass <strong>of</strong> infaunal organisms in benthic cores than did mudflats, while densities and biomass <strong>of</strong>infauna in hybrid Spartina were lower than, or did not differ from, mudflats. Stable isotopes <strong>of</strong>carbon and nitrogen were used to examine whe<strong>the</strong>r macr<strong>of</strong>aunal food webs differ between native orhybrid Spartina and mudflats. Some consumers collected within Spartina showed evidence <strong>of</strong> a shiftin carbon isotope ratios indicating a possible increase in Spartina consumption within vegetation;however, <strong>the</strong> pattern was not consistent across species and sites. Due to <strong>the</strong> differences in <strong>the</strong>ireffects on infauna, hybrid Spartina and S. foliosa should not be considered equivalent for marshrestoration projects.Keywords: hybrid Spartina, Spartina foliosa, infauna, food webs, CaliforniaINTRODUCTIONIn San Francisco Bay, hybrids formed between nativeCalifornia cordgrass (S. foliosa) and introduced S.alterniflora accrete sediment, increase elevation compared tosurrounding mudflats, and significantly reduce light levelsunder <strong>the</strong>ir canopies (Neira et al. 2005). Based on tidallevels, hybrid Spartina would also be able to fill large areas<strong>of</strong> shallow outer coast bays if it establishes populations <strong>the</strong>re(Daehler and Strong 1997). The spread <strong>of</strong> hybrid Spartinathreatens flood control channels and habitat for migratingshorebirds, and may impact invertebrates that are food forbirds and fishes.Spartina cordgrasses may ei<strong>the</strong>r facilitate or inhibitinfauna depending on location (e.g., Capehart and Hackney1989, Netto and Lana 1999). The invasion <strong>of</strong> hybridSpartina provides an opportunity to examine howdifferences in structure between two closely-relatedecosystem engineers affect infaunal communities.Understanding differences between S. foliosa and hybridSpartina may help managers predict <strong>the</strong> impacts <strong>of</strong> <strong>the</strong>continued spread <strong>of</strong> hybrid Spartina.This study examined whe<strong>the</strong>r hybrid Spartina isecologically equivalent to native S. foliosa in its impacts oninfaunal and epifaunal invertebrates. We investigated twohypo<strong>the</strong>ses. First, we predicted that differences in structurebetween S. foliosa and hybrid Spartina will be reflected indifferences in infaunal density, biomass, and taxonomiccomposition. Second, we predicted that, due to <strong>the</strong> greateraboveground biomass produced by hybrid Spartina,organisms living within <strong>the</strong> hybrid will show greater use <strong>of</strong>hybrid as a food source than those in S. foliosa or mudflats,based on stable isotopes <strong>of</strong> carbon and nitrogen.METHODSStudy sites included five S. foliosa marshes and twohybrid Spartina marshes in nor<strong>the</strong>rn California. Spartinafoliosa sites included: China Camp State Park (38° 0.37’N,122° 28.66’W) on San Pablo Bay; Bolinas Lagoon (38°19.22’N, 122° 41.73’W); Shields Marsh (38° 5.35’N, 122°50.44’W) and Tom’s Point (38° 13.21’N, 122° 56.86’W) onTomales Bay; Drakes Estero (38° 5.36’N, 122° 55.86’W).Hybrid marshes were located on <strong>the</strong> eastern side <strong>of</strong> SanFrancisco Bay in San Lorenzo (Roberts Landing, 37°40.22’N 122° 09.70’W) and Alameda (Elsie Roemer BirdSanctuary, 37° 45.58’N 122° 28.80’W). The San Lorenzosite is unique in that it contains discrete patches <strong>of</strong> both S.foliosa and hybrid Spartina (genotypes confirmed by D.Ayres, University <strong>of</strong> California, Davis, pers. comm.).- 161 -