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 Spartinaobserved in unmanipulated sediments <strong>of</strong> hybrid Spartinahabitat relative to tidal flats (Neira et al. 2005).Predation: The sediment surface in cages containingcrabs was disturbed with obvious depressions created byforaging crabs. The sediments also had lower organic matterand chlorophyll a content relative to those cages withoutcrabs. No differences were found in salinity, temperatureand light penetration between treatments. Total macr<strong>of</strong>aunaldensity in experimental enclosures containing crabs was 2.2times lower than those without crabs. Density <strong>of</strong> selectedsurface-feeding species such as G. japonica, Corophium spp.and G. gemma declined by 57%, 87% and 67%, respectivelyin enclosures with crabs. O<strong>the</strong>r taxa that also declined wereSphaerosyllis californiensis, turbellarians, a juvenilearenicolid species and juvenile capitellids (Neira et al.2006).DISCUSSION AND CONCLUSIONOur results show that composition and structure <strong>of</strong> tidalflat macr<strong>of</strong>aunal benthic communities are stronglyinfluenced by hybrid Spartina invasion. Specifically, wefound that Spartina reduces water flow. These, in turn,influence flux <strong>of</strong> recruiting larvae, transport <strong>of</strong> o<strong>the</strong>rbenthos, <strong>the</strong> input <strong>of</strong> organic matter, and sedimentdeposition.These physical changes interacted with chemicalchanges including increased porewater sulfideconcentrations and more negative redox potential levels(Neira et al. 2006, 2007). We also found changes in growthand survival via predation (Neira et al. 2006) and foodavailability (Levin et al. 2006), which can play key, possiblysynergistic, roles in structuring Spartina-invadedecosystems. The presence <strong>of</strong> hybrid Spartina in open tidalflats exerts a strong influence on <strong>the</strong> composition anddistribution <strong>of</strong> benthic invertebrates in Elsie Roemer (Neiraet al. 2005, 2006).The reduction <strong>of</strong> macr<strong>of</strong>aunal densities in hybridSpartina relative to <strong>the</strong> naturally unvegetated tidal flats isconsistent with results for invasive S. anglica (Jackson 1985)and S. alterniflora in o<strong>the</strong>r estuaries (Zipperer 1996), butcontrasts with existing paradigms about positive vegetationeffects on marine macrobenthos (e.g., Hedge and Kriwoken2000; Netto and Lana 1999). In addition, not all systemsrespond in <strong>the</strong> same way to <strong>the</strong> hybrid Spartina invasion(Neira et al. 2005). For example, at o<strong>the</strong>r sites such asRoberts Landing (15-year invasion), hybrid Spartina habitatdiffered from tidal flat sediments in composition but notabundance. At a third San Francisco Bay site, in San Mateo,where a Salicornia marsh is being invaded (8-10 years),sediment properties were similar, and no differences weredetected in densities or proportions <strong>of</strong> surface- orsubsurface-deposit feeders, but <strong>the</strong> proportion <strong>of</strong>carnivores/omnivores and grazers increased in <strong>the</strong> hybridSpartina habitat (Neira et al. 2005). At Elsie Roemer, most<strong>of</strong> <strong>the</strong> species shown to have reduced density in hybridSpartina-invaded tidal flats are surface-feeding animals,such as amphipods (Corophium spp., Grandidierellajaponica) bivalves (Gemma gemma) and polychaetes(Tharyx spp.). Thus, where hybrid Spartina has invaded tidalflats it is likely to cause not only changes in <strong>the</strong> habitatstructure, but also to shift macr<strong>of</strong>aunal feeding modes fromsurface-microalgae feeders to subsurface detritivores. Theshift from surface-feeding taxa can have negative ecologicalimplications for higher trophic levels. Birds and fishesdepend more on larger, surface-feeding species (Simenstadand Thom 1995), which decline in abundance with <strong>the</strong>invasion <strong>of</strong> Spartina. However, <strong>the</strong> full impact <strong>of</strong> <strong>the</strong>Spartina invasion on birds and fishes in San Francisco Bayhas yet to be measured.We can conclude that <strong>the</strong> invasion <strong>of</strong> hybrid Spartinahas resulted in substantial changes in benthic communitiesby modifying <strong>the</strong> physical and chemical environment.However, we find that not all ecosystems respond identicallyfollowing hybrid Spartina invasion in San Francisco Bay.Hybrid Spartina can have differing and complex effects on<strong>the</strong> sediment environment and associated fauna dependingon <strong>the</strong> location, type <strong>of</strong> habitat involved, age <strong>of</strong> invasion,and local hydrodynamics. The processes underlying <strong>the</strong>variable responses to Spartina invasion, and <strong>the</strong> differentrates <strong>of</strong> recovery following invasion, are factors that shouldbe considered in planning for Spartina eradication.ACKNOWLEDGMENTSWe acknowledge G. Mendoza, R. Blake, C. Tyler, S.Maezumi, C. Whitcraft, U. Mahl, E. Brusati, N. Rayl, N.Christensen, P. McMillan, P. Colombano, J. Gonzalez, S.Norton, and D. Chiang who kindly assisted in experimentsetup, sample collection in <strong>the</strong> field and lab analysis. We aregrateful to C. Tyler who provided sulfide data. We thank C.Nordby, E. Brusati and R. Blake for providing compliancewith California Clapper Rail permit requirements. Supportwas provided by National Science FoundationBiocomplexity Program (DEB 0083583) to L.A.L. andE.D.G.A full manuscript <strong>of</strong> this study was published inEcological Applications (Neira et al. 2006); see referencebelow.REFERENCESAyres, D.R., D.R. Strong and P. 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