Proceedings of the Third International Conference on Invasive ...

Chapter 2: Spartina Distribution and Spread<strong>Proceedings</strong> <strong>of</strong> <strong>the</strong> <strong>Third</strong> <strong>International</strong> <strong>Conference</strong> on Invasive SpartinaTable 3. Change in Area Covered by Non-native Spartina between 2001 and 2003 for Five Sites Comparing Aerial Photo Interpretation Measurements(APIM), Field Estimate and Field Measurement Measurements.Site NameBunker MarshSiteTypeISpeciesArea in 2001(square meters)APIMFieldEstimate*Area in 2003(square meters)APIMFieldMeasurementAPIM 2001vsAPIM 2003Change in Area2001 to 2003Field Estimate2001 vs FieldMeasurement2003Difference BetweenMethods <strong>of</strong>Estimating AreasField Measurement2003 vsAPIM 2003Hybrids 5,100.86 1,580.11 8,501.62 5,665.59 67% 259% -33%Citation MarshCogswell Marsh (No. Quad.)Alameda Island - North ElsieRoemerIdeal MarshIIIIIIHybrids 244.83 2,059.735 2,448.35 7,803.41 900% 279% 219%Hybrids 700.12 1,503.16 700.12 5,334.08 0% 255% 662%Hybrids 2,882.37 1,171.89 3,610.10 3,948.94 25% 237% 9%Hybrids 2,229.01 1,070.72 3,863.62 3,541.00 73% 231% -8%Average Avg Avg 2,231.43 1,477.12 3,824.74 5,258.60 213% 252% 170%interpretation and field measurement methods. The aerialphoto interpretation measurements <strong>of</strong> area covered byhybrids were on average 170% less than <strong>the</strong> fieldmeasurements (Table 3). However, both methods recorded atleast a doubling <strong>of</strong> area invaded in two years. The aerialphoto interpretation measurements indicated that <strong>the</strong> hybridsincreased 322% at Site Type I and 49% at Site Type II.This discrepancy affects <strong>the</strong> estimates <strong>of</strong> total acreageinvolved but does not substantially change <strong>the</strong> estimates <strong>of</strong>rate <strong>of</strong> increase <strong>of</strong> <strong>the</strong> invasion between 2001 and 2003,which is approximately doubled or more, whe<strong>the</strong>r averagedby region or by site type. The relationship <strong>of</strong> aerial photomeasurements <strong>of</strong> area to field measurements <strong>of</strong> area wastested statistically using a t-test and linear regression (onlog-transformed data) <strong>of</strong> field estimates on photo estimatesfor five sites in 2003. No difference in mean area coveredwas found between methods (t-test, p = 0.616). However, noregression relationship was found (p = 0.797). This waslikely due to high variability among <strong>the</strong> small sample size <strong>of</strong>aerial photo measurements. Thus, aerial photo interpretationmeasurements should be used with caution to estimate cover.Accuracy <strong>of</strong> Field IdentificationOver <strong>the</strong> course <strong>of</strong> <strong>the</strong> 2003 monitoring season, 12landowners, managers or concerned stakeholders requestedsurveys for <strong>the</strong> presence <strong>of</strong> non-native Spartina (Fig. 2). Inaddition, genetic tests were performed to confirm speciesidentification by field staff at each <strong>of</strong> <strong>the</strong> inventorymonitoring sites (Table 1). A total <strong>of</strong> 68 plant samples wereidentified to species in <strong>the</strong> field using plant morphology andanalyzed using genetic tools (Daehler and Strong 1997;Ayres et al. 1999). Fifty-five <strong>of</strong> <strong>the</strong> plants were fieldidentified as S. alterniflora hybrids, and 13 as S. foliosa.Forty-nine <strong>of</strong> <strong>the</strong> plants were confirmed genetically to be S.alterniflora hybrids, and 18 were confirmed to be S. foliosa.There was not a statistically significant difference betweenfield classification and subsequent genetic (or true)identification at <strong>the</strong> 0.05 level (Chi Square). More native S.foliosa were called hybrid than vice versa. 89% <strong>of</strong> <strong>the</strong>hybrids and 100% <strong>of</strong> <strong>the</strong> S. foliosa were correctly identified.EfficacySites treated in 2002 were monitored in 2003 as well aslimited monitoring for <strong>the</strong> few sites that were treated in 2003(Table 4.) Manual Methods such as digging <strong>of</strong> S. densifloraat Piper Park or trampling and covering in Point ReyesNational Seashore (PRNS) were effective on a small scale.Return visits were required to make sure no new shoots hademerged.DISCUSSIONSpreadBased on <strong>the</strong> ISP’s 2003 survey results, <strong>the</strong> averageincrease in area (calculated by site) <strong>of</strong> all non-nativeSpartina species was 244% over a period <strong>of</strong> two years. ForS. alterniflora and hybrids, <strong>the</strong> increase was 317%. Applyingthis rate <strong>of</strong> increase, which represents an average across allsite types with differing and variable environmentalconditions and stages <strong>of</strong> invasion, to <strong>the</strong> 190 ha (470 ac)recorded in <strong>the</strong> 2001 survey suggests that hybrids could havespread to as high as 793 ha (1,960 ac). In contrast, S.alterniflora spread only 25% in 11 years in Willapa Bay,*No field measurement collected in 2001. 2001 field estimate was calculated using a regression curve based on a correlation between <strong>the</strong> existing 2001 and 2003 field measurements.-78-