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 SpartinaMECHANISTIC PROCESSES DRIVING SHIFTS IN BENTHIC INFAUNAL COMMUNITIESFOLLOWING HYBRID SPARTINA TIDAL FLAT INVASIONC. NEIRA 1 , E.D. GROSHOLZ 2 AND L.A. LEVIN 31 Integrative Oceanography Division, Scripps Institution <strong>of</strong> Oceanography, La Jolla, California 92093-0218, USA;cneira@coast.ucsd.edu2 Department <strong>of</strong> Environmental Science and Policy, University <strong>of</strong> California, Davis, One Shields Ave., Davis, CA 95616,USA; tedgrosholz@ucdavis.edu3 Integrative Oceanography Division, Scripps Institution <strong>of</strong> Oceanography, La Jolla, California 92093-0218, USA;llevin@ucsd.eduSpartina alterniflora x foliosa hybrids are perennial cordgrasses that have rapidly invaded mudflatsand marshes in central and sou<strong>the</strong>rn San Francisco Bay. Recent studies conducted by <strong>the</strong> authors at<strong>the</strong> Elsie Roemer Bird Sanctuary in Alameda (San Francisco Bay) showed a 75% reduction inmacr<strong>of</strong>aunal densities and shift in macr<strong>of</strong>aunal composition. Here we identify <strong>the</strong> mechanisms thatunderlie such observed changes in macr<strong>of</strong>aunal community structure following tidal flat invasion by<strong>the</strong> hybrid Spartina. Specifically we performed a series <strong>of</strong> in situ manipulative experiments toexamine hybrid Spartina canopy influence on water motion and water flow speed, larval flux,animal transport, and predation, as mediators <strong>of</strong> change for sediments and macrobenthos. Overall,hybrid Spartina exerted a strong influence on <strong>the</strong> hydrodynamic regime, reducing water flow that, inturn, influences flux <strong>of</strong> recruiting larvae, transport <strong>of</strong> o<strong>the</strong>r benthos, and <strong>the</strong> input <strong>of</strong> organic matterand sediment deposition. Habitat modification results in poor survivorship <strong>of</strong> surface-feeding taxavia sulfide toxicity, altered predation pressure and changed food availability. All <strong>the</strong>se mechanismscan play key, possibly synergistic roles in structuring Spartina-invaded ecosystems.Keywords: tidal flat invasion, Spartina alterniflora, hybrid Spartina, benthos, macr<strong>of</strong>auna,mechanistic processesINTRODUCTIONSan Francisco Bay is one <strong>of</strong> <strong>the</strong> most heavily invadedestuaries in <strong>the</strong> world with nearly 250 non-native orintroduced species (Cohen and Carlton 1998). One <strong>of</strong> <strong>the</strong>most serious invasions has been that <strong>of</strong> <strong>the</strong> Atlanticcordgrass Spartina alterniflora and its hybrids (hereafter“hybrid Spartina”). The genetic background <strong>of</strong> <strong>the</strong> plantswas confirmed by molecular genetic analysis by D. Ayres(unpublished results). Hybrid Spartina has invaded morethan 800 hectares (ha) <strong>of</strong> mudflats and marshes in centraland south San Francisco Bay (Ayres et al. 2004). Thisinvasive cordgrass is converting mudflats at low tidal levelsinto dense, nearly monotypic meadows (Ayres et al. 2003,2004). Of extreme concern is that this plant’s invasion intounvegetated tidal flats will result in a loss <strong>of</strong> open foragingarea for shorebirds and fishes, flow reductions, highersedimentation rates, changes in light penetration, andreduction <strong>of</strong> benthic algal production (Zipperer 1996;Daehler and Strong 1996; Stenzel et al. 2002; Grosholz et al.2009).To date very little is known about <strong>the</strong> impact <strong>of</strong> <strong>the</strong>hybrid Spartina invasion on sediment properties,macr<strong>of</strong>aunal communities, and ecosystem functioning. Whatwe do know results from recent studies <strong>of</strong> three sites in SanFrancisco Bay, including <strong>the</strong> Elsie Roemer Bird Sanctuary inAlameda (San Francisco Bay, CA, USA) (Neira et al. 2005),which has experienced hybrid Spartina invasion for <strong>the</strong> past30 years. At this site macr<strong>of</strong>aunal densities were 75% lowerin hybrid Spartina-invaded sediments than on adjacent,uninvaded tidal flats (Neira et al. 2005). Biomass was 57%lower in invaded sediments (Levin et al. 2006). We alsoobserved important shifts in species composition in <strong>the</strong>hybrid-invaded patches relative to tidal flats (Neira et al.2005). Surface feeders such as Gemma gemma (Bivalvia),Corophium spp. and Grandidierella japonica (Amphipoda),and Tharyx sp. and Eteone sp. (Polychaeta) were negativelyaffected by Spartina invasion, exhibiting reduced densities.Subsurface-deposit feeders such as capitellid polychaetesand tubificid oligochaetes were less affected or unaffected(Neira et al. 2005). Because surface-feeding taxa are moreaccessible to epibenthic consumers than capitellidpolychaetes and oligochaetes that live deeper in <strong>the</strong>sediment, <strong>the</strong> loss <strong>of</strong> surface feeding taxa could havepr<strong>of</strong>ound implications for higher trophic levels and henceaffect <strong>the</strong> whole ecosystem. Therefore, <strong>the</strong> aim <strong>of</strong> <strong>the</strong>following work was to experimentally document <strong>the</strong> causes<strong>of</strong> <strong>the</strong> benthic changes identified in <strong>the</strong> initial mensurativestudy.METHODSThe study site was located on Alameda Island (SanFrancisco Bay) along <strong>the</strong> shoreline adjacent to Elsie RoemerBird Sanctuary (37 o 45’35”N; 122 o 28’48”W). Detailed- 141 -