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 Spartinadescription <strong>of</strong> <strong>the</strong> study site is provided in Neira et al.(2005). Using a paired sampling design, we established 10blocks (2x2 meters (m)), approximately 10 m inside <strong>the</strong>hybrid Spartina meadow, and 10 similar blocks on <strong>the</strong>adjacent open tidal flat approximately 5-10 m from <strong>the</strong>meadow edge. We performed in situ manipulativeexperiments to identify what mechanisms are responsible forobserved shifts in faunal community structure followinghybrid Spartina invasion.Water motion: To evaluate <strong>the</strong> influence <strong>of</strong> <strong>the</strong> hybridSpartina canopy on relative water motion, we used <strong>the</strong>gypsum dissolution technique (Doty 1971). Pre-weighedgypsum cards were deployed about 10 cm above <strong>the</strong> bottomfor 6, 3 and 4 days during April and June 2002 in vegetatedand unvegetated tidal flat habitats. It is assumed thatdissolution rate <strong>of</strong> <strong>the</strong> gypsum, grams per day (g d -1 ), isproportional to water velocity (Porter et al. 2000). Watervelocity in both habitats was measured on several days nearmaximum ebb and flood tides just below <strong>the</strong> water surfacewith a Marsh-McBirney Flow Meter 2000.Larval flux: The influence <strong>of</strong> <strong>the</strong> hybrid Spartinacanopy on larval flux was measured in vegetated andunvegetated habitats using Geukensia demissa (mussel)shells as a hard substrate for settlement. Barnacle larvalabundance over time was used as a proxy for larval flux.Three dowels, each with one attached Geukensia shell, wereinserted into <strong>the</strong> sediment (1 m 2 plot) in each block <strong>of</strong> bothhybrid Spartina habitat and tidal flat (i.e., 60 in total).Mussels remained 10-20 cm above <strong>the</strong> sediment.Sediment deposition: We investigated <strong>the</strong> effects <strong>of</strong>Spartina plant canopy on short-term sediment depositionrates by deploying petri-dish sediment traps (Reed 1992) inboth vegetated and unvegetated habitats (10 replicates each)during low tide. Sediment traps were composed <strong>of</strong> GF/FWhatman filters (9-cm diameter) supported on <strong>the</strong> petridishes affixed on <strong>the</strong> sediment surface. Filters were removedand replaced 24 hours (h) later (one tidal cycle) during twosuccessive days in July 2002. Filters were gently rinsed withdistilled water to remove salts on pre-weighed aluminumdishes, oven-dried at 60 o C, and re-weighed. Sedimentdeposition rate was calculated as mass deposited per trap andexpressed in milligrams per centimeter squared per day (mgcm -2 d -1) . Percent organic matter was determined by massloss after ignition at 500 o C for 4 h. Mud content wasestimated after wet sieving (63 micrometers(μm)) <strong>the</strong>sediment deposited on traps and weighing both fractions (>63 μm and < 63 μm) after drying at 60°C.Animal transport: The influence <strong>of</strong> hybrid Spartinacanopy on animal-transport dynamics was evaluated usingpassive tube traps made <strong>of</strong> polypropylene (22 cm tall x 2.7cm diameter). To prevent escape <strong>of</strong> animals from <strong>the</strong> tube,15 milliliters (ml) <strong>of</strong> a dense brine solution (>90 parts perthousand (ppt)) was added; <strong>the</strong> remained volume was filledwith filtered seawater. Ten replicate tube traps weredeployed on <strong>the</strong> unvegetated tidal flat about 10 cm above <strong>the</strong>sediment surface, supported on PVC pipe inserted into <strong>the</strong>sediment. In <strong>the</strong> hybrid Spartina habitat, tube traps wereinserted into <strong>the</strong> sediment leaving <strong>the</strong>ir opening about 5 cmabove <strong>the</strong> bottom. Sediment was collected from traps inplastic jars and preserved in 8% buffered formalin.Sediment properties: In order to explore how hybridSpartina ultimately affects <strong>the</strong> sediment ecosystem, weexamined some key sediment properties in vegetated andunvegetated habitats (10 replicates each). Porewater salinity(top 3 cm) was measured with a hand-held refractometer,sediment redox potential (top 1 cm) was measured with aportable Mettler Toledo redox meter. Plexi-glass cores (18.1cm 2 , 0-6 cm depth) were taken for total organic content(determined after combustion at 500°C x 4h); syringe coreswere taken for chlorophyll a (an estimate <strong>of</strong> sedimentmicroalgal biomass) and determined according to Plante-Cuny (1973) after extraction with 90% acetone. Sedimentporosity was determined according to Buchanan (1984).Porewater sulfide was measured concurrently by A.C. Tyler(UC Davis) based on Cline (1969) with a few modifications.Macr<strong>of</strong>auna were collected from both habitats with cores(18.1 cm 2 , 0-6 cm depth) and sieved through a 0.3-mm meshsieve.Plant removal experiment: We also performed anexperiment to determine if <strong>the</strong> invaded habitats would returnto <strong>the</strong> original unvegetated condition and resembleunvegetated tidal flats. We clipped all above-groundSpartina plant material to <strong>the</strong> sediment level in ten replicate2x2 m areas with adjacent controls. In both removal andcontrols, we compared sediment properties and macr<strong>of</strong>aunalcommunities among habitats after 90 days.Sediment transplant experiment: We conducted atransplant experiment to determine whe<strong>the</strong>r unvegetatedsediment and fauna from <strong>the</strong> tidal flat would begin to looklike those in <strong>the</strong> invaded areas when moved into <strong>the</strong> areainvaded by hybrid Spartina. In summer 2002, intactsediment (625 cm 2 x 10 cm depth) was moved from <strong>the</strong> tidalflat (about 7 m from <strong>the</strong> edge) to Spartina-invaded habitat(10 m inside <strong>the</strong> meadow). Tidal flat sediment removed andreplaced served as a control treatment. Each treatmentcontained ten replicate blocks (20 in total).Predation: In order to assess if <strong>the</strong> shifts observed inmacr<strong>of</strong>aunal composition in <strong>the</strong> initial transplant experimentwere <strong>the</strong> result <strong>of</strong> changes in predation pressure followingSpartina-hybrid invasion, we performed controlled andreplicated in situ predator exclusion experiments beginningon September 10, 2003. We used European green crabs(Carcinus maenas), which are common (introduced) marshpredators in San Francisco Bay (Cohen et al. 1995). Intactuninvaded tidal flat sediment and associated fauna weretransplanted to hybrid Spartina habitat as in <strong>the</strong> transplantexperiment. However, this time we enclosed one individualC. maenas (carapace width 40 mm) in each <strong>of</strong> eightreplicated enclosures (0.3 x 0.3 m x 0.5 m height) made <strong>of</strong>galvanized hardware cloth (0.63-cm mesh). Crabs were- 142 -