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 Spartinagrazing might have on <strong>the</strong> rate <strong>of</strong> invasion <strong>of</strong> hybridSpartina into areas occupied by native Spartina.METHODSGrazing Intensity in San Francisco BayTo estimate <strong>the</strong> intensity <strong>of</strong> grazing by western Canadageese, we measured grazing at three sites in <strong>the</strong> centralportion <strong>of</strong> San Francisco Bay: Point Isabel (37º53’30’’N;122º20’55’’W), Robert’s Landing (37º40’13’’N;122º28’48’’W), and Oro Loma (37º37’45’’N;122º09’08’’W). These sites were distributed along <strong>the</strong>eastern margin <strong>of</strong> San Francisco Bay and each containedareas <strong>of</strong> native S. foliosa with varying degrees <strong>of</strong> cover byhybrid Spartina. At each site, we laid 30 meter (m) transectsalong which 10 0.25 m x 0.25 m quadrats were established atrandomly chosen points. Plant samples to verify geneticidentification (hybrid vs. native) were taken within severalquadrats and kept refrigerated until genetic assays usingrandom amplified polymorphic DNA (RAPD) markers couldbe conducted at <strong>the</strong> Spartina genetics lab at UC Davis(Ayres, unpublished results), typically less than a week aftercollection. Genetic determination followed previouslypublished protocols (Ayres et al. 1999). In some cases, <strong>the</strong>area covered by native S. foliosa was smaller so shortertransects with fewer quadrats were used. Within eachquadrat, we measured <strong>the</strong> number <strong>of</strong> stems that had beengrazed or not by geese and <strong>the</strong> heights <strong>of</strong> 15 stems to <strong>the</strong>nearest one centimeter (cm). Grazing produces characteristicdamage that is easily identified. At sites where we also haddata for hybrid Spartina, <strong>the</strong> same measurements were madealong similar transects. We tested <strong>the</strong> proportion <strong>of</strong> stemsgrazed by geese between native and hybrid transects.Field Grazing ExclosuresTo experimentally measure <strong>the</strong> impact <strong>of</strong> grazingCanada geese on both native and hybrid Spartina in <strong>the</strong>field, we conducted exclosure experiments at <strong>the</strong> RobertsLanding site. At this site, we established goose exclosuresin three areas (six replicates per treatment). The first areawas in a continuous meadow <strong>of</strong> native S. foliosa. Thesecond was in a continuous meadow <strong>of</strong> hybrid Spartina. Thethird was located where hybrid Spartina was overgrowingnative Spartina. In single species stands we constructed a 1m x 1 m exclosure cage <strong>of</strong> plastic mesh (4 cm x 4 cmopenings) attached by electrical cable ties to polyvinylchloride (PVC) posts. At border areas with both native andhybrid Spartina, we established exclosure cages 1 m x 4 mwith <strong>the</strong> long axis <strong>of</strong> <strong>the</strong> exclosure perpendicular to <strong>the</strong>hybrid-native border. The center <strong>of</strong> <strong>the</strong> exclosure (at 2 m)was positioned to be over <strong>the</strong> approximate border edge at <strong>the</strong>time <strong>of</strong> construction. Adjacent control areas for both singlespecies exclosures and border exclosures were establishedwithin 2 m <strong>of</strong> <strong>the</strong> exclosure cages. The control areas weredelineated with PVC posts at <strong>the</strong> corners. We estimated <strong>the</strong>linear distance in spread for control and exclosure treatmentsby measuring <strong>the</strong> distance (to nearest 0.1 m) from <strong>the</strong> hybridborder at <strong>the</strong> start <strong>of</strong> <strong>the</strong> experiment to <strong>the</strong> leading edge <strong>of</strong><strong>the</strong> spread after two years. We tested differences withAnalysis <strong>of</strong> Variance (ANOVA) using log transformedvalues for distance and treatment (exclosure vs. control) asfixed factors using SAS version 9.1 (SAS Institute, Cary,North Carolina).Grazing Trials with Intact ClonesIn order to determine <strong>the</strong> extent that observed grazingwas <strong>the</strong> result <strong>of</strong> preference for native Spartina ra<strong>the</strong>r thanavailability or o<strong>the</strong>r factors, we conducted grazing trialsduring February-March 2003 using a small group (six) <strong>of</strong>captive western Canada geese maintained by <strong>the</strong> WildlifeDepartment at Humboldt State University (HSU), Arcata,California. This group <strong>of</strong> geese had been maintainedtoge<strong>the</strong>r for several years on processed diets. Theexperiment involved using intact segments (0.5 m x 0.5 m x0.5 m) <strong>of</strong> two clones <strong>of</strong> ei<strong>the</strong>r hybrid or native Spartina<strong>of</strong>fered in brown plastic bins. Each plant segment had beenexcavated from field sites in San Francisco Bay (hybridplants from Elsie Roemer Bird Sanctuary, City <strong>of</strong> Alamedaand S. foliosa from Bothin Marsh, City <strong>of</strong> Mill Valley) anddriven to <strong>the</strong> Humboldt aviary <strong>the</strong> next day. All stems andleaves were counted for each clone prior to <strong>the</strong> start <strong>of</strong> <strong>the</strong>experiment. At <strong>the</strong> beginning <strong>of</strong> <strong>the</strong> experiment, <strong>the</strong> tubswith <strong>the</strong> clone segments were established in a two by twoarray approximately 2 m apart. The geese were <strong>the</strong>n allowedto enter <strong>the</strong> aviary and encounter <strong>the</strong> clones. We made focalobservations on each clone at one minute intervals noting <strong>the</strong>number <strong>of</strong> geese grazing on each. After two hours, wecollected <strong>the</strong> clones and measured <strong>the</strong> number <strong>of</strong> stems andleaves that had been grazed by geese. The entire experimentwas repeated three times over <strong>the</strong> following three weeks.We estimated <strong>the</strong> intensity <strong>of</strong> grazing by counting <strong>the</strong>percentage <strong>of</strong> leaves and stems that had been grazed during<strong>the</strong> trial for both hybrid and native clones. We testeddifferences in grazing with ANOVA using arc-sintranformed percentage <strong>of</strong> leaves or stems with treatment as afixed factor (as above).Grazing Trials with Clipped StemsTo determine whe<strong>the</strong>r <strong>the</strong> preferences <strong>of</strong> experimentalgeese were determined by differences in <strong>the</strong> physicalproperties <strong>of</strong> <strong>the</strong> plants versus differences in plant chemistry,we used <strong>the</strong> same group <strong>of</strong> geese during experiments fromFebruary-March 2004. We collected stems <strong>of</strong> both nativeand hybrid Spartina at <strong>the</strong> same sites used for collecting <strong>the</strong>clones <strong>the</strong> previous year. We cut <strong>the</strong> stems at <strong>the</strong> base <strong>of</strong> <strong>the</strong>plant and wrapped <strong>the</strong>m in large plastic bags for shipment.Stems were kept refrigerated from <strong>the</strong> time <strong>of</strong> clipping andsent to <strong>the</strong> aviary at HSU within 24 hours and were usedimmediately <strong>the</strong>reafter in feeding trials. Geese showed <strong>the</strong>- 192 -