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 SpartinaPOTENTIAL IMPACTS OF SPARTINA SPREAD ON SHOREBIRD POPULATIONS IN SOUTH SANFRANCISCO BAYD. STRALBERG 1 , V. TONIOLO 1,2 , G.W. PAGE 1 , AND L.E. STENZEL 11 PRBO Conservation Science, 3820 Cypress Drive #11, Petaluma, CA 94954, dstralberg@prbo.org2 Current address: Ocean Biogeochemistry Lab, Dept. <strong>of</strong> Environmental Earth System Science, Mitchell Bldg, A67, StanfordUniversity Stanford, CA 94305-2215San Francisco Bay holds 70% <strong>of</strong> California’s mudflats and provides habitat to more wintering andmigratory shorebirds than any o<strong>the</strong>r wetland along <strong>the</strong> Pacific coast <strong>of</strong> <strong>the</strong> contiguous U.S. The bay’smudflats are currently threatened by <strong>the</strong> spread <strong>of</strong> a non-native cordgrass, Spartina alterniflora, andassociated hybrids, which grow at lower elevations than <strong>the</strong> native S. foliosa and can render largemudflat areas effectively unavailable to shorebirds for foraging. Using shorebird survey data, tidestation data, and GIS-based habitat data, we analyzed <strong>the</strong> potential effect <strong>of</strong> S. alterniflora x foliosahybrids on shorebird habitat in <strong>the</strong> South Bay by creating grid-based spatial models <strong>of</strong> shorebirdhabitat value and potential Spartina spread. We developed three scenarios <strong>of</strong> potential habitat valueloss for shorebirds based on assumptions about <strong>the</strong> inundation tolerance <strong>of</strong> S. alterniflora and temporalavailability <strong>of</strong> mudflat resources. Predictions <strong>of</strong> habitat value loss across seasons ranged from27% to 80%. We identified <strong>the</strong> upper mudflats, due to <strong>the</strong>ir greater exposure time, and <strong>the</strong> east andsouth shore mudflats, due to <strong>the</strong> high numbers <strong>of</strong> birds detected <strong>the</strong>re, as <strong>the</strong> areas <strong>of</strong> highest valueto shorebirds in <strong>the</strong> South Bay. These areas also coincide with <strong>the</strong> areas <strong>of</strong> greatest Spartina invasionpotential.Keywords: Spartina alterniflora, mudflats, inundation tolerance, Charadrii, GISINTRODUCTIONThe San Francisco Bay estuary holds 70% <strong>of</strong> <strong>the</strong> mudflatsin California (Josselyn et al. 1990), providing habitatto over 350,000 migrating shorebirds (Charadrii) in <strong>the</strong>fall, over 325,000 in winter, and over 900,000 in <strong>the</strong> spring(based on single-day counts) (Stenzel et al. 2002). Along <strong>the</strong>Pacific coast <strong>of</strong> <strong>the</strong> contiguous United States alone (excludingAlaska), San Francisco Bay holds more shorebirds thanany o<strong>the</strong>r wetland in all seasons (Page et al. 1999). Surveydata suggest that San Francisco Bay holds over 50% <strong>of</strong> <strong>the</strong>Pacific coast populations <strong>of</strong> several shorebird species—11<strong>of</strong> <strong>the</strong> 12 most abundant species in fall, six <strong>of</strong> 13 in winter,and seven <strong>of</strong> 13 in spring (Page et al. 1999). Within <strong>the</strong> Bay,mudflats are <strong>the</strong> most important habitats for <strong>the</strong> majority <strong>of</strong><strong>the</strong>se species (Stenzel et al. 2002).San Francisco Bay’s mudflats are now threatened by<strong>the</strong> spread <strong>of</strong> cordgrass hybrids (Spartina alterniflora xfoliosa) (Ayres et al. 2008). The non-native S. alterniflorawas originally introduced to <strong>the</strong> South San Francisco Bay(South Bay) in <strong>the</strong> 1970s (Callaway and Josselyn 1992) andhybridized with native S. foliosa (Ayres et al. 1999). Spartinaalterniflora x foliosa hybrids exhibit higher tolerance totidal submersion and salinity, as well as higher growth andgermination rates than <strong>the</strong> native S. foliosa (Callaway andJosselyn 1992; Daehler and Strong 1997; Anttila et al. 1998;Collins 2002). Projections from sampling sites suggested thatnearly 800 hectares (ha) <strong>of</strong> South Bay marshes, channels,and mudflats had been invaded by 2003, an increase <strong>of</strong> morethan 300% since 2001 (Zaremba et al., this volume). Fur<strong>the</strong>rspread <strong>of</strong> hybrid Spartina on <strong>the</strong> Bay’s mudflats poses a greatthreat to shorebirds, which cannot forage in areas <strong>of</strong> densegrowth (Josselyn 1983, Evans 1986, Goss-Custard and Moser1988, White 1995).In Willapa Bay, Washington, which was initiallyinvaded in <strong>the</strong> late 19th century, S. alterniflora recentlyexperienced explosive growth, tripling its areal extentbetween 1994 and 2002 and converting many mudflats toSpartina marshes (Buchanan 2003; Civille 2005). Surveys<strong>of</strong> Willapa Bay birds suggested a reduction in shorebirdnumbers by as much as 67% and foraging time by as muchas 50% (Jaques 2002). Unlike San Francisco Bay, WillapaBay is outside <strong>the</strong> range <strong>of</strong> native Spartina and <strong>the</strong>reforehas no hybrid Spartina, which has been found to exhibitmuch more rapid rates <strong>of</strong> lateral expansion than its parentalspecies (Ayres et al. 2008). Thus <strong>the</strong> invasion potential inSan Francisco Bay is thought to be substantially greaterthan in Willapa Bay.While San Francisco Bay and Willapa Bay also differin <strong>the</strong>ir bathymetry and tidal inundation regimes, relationshipsthat have been identified between tidal inundationparameters and S. alterniflora growth tolerance (McKeeand Patrick 1988; Collins 2002) may be used to estimate <strong>the</strong>potential for hybrid Spartina spread in San Francisco Bay.Here we present a preliminary GIS-based analysis <strong>of</strong> <strong>the</strong>potential effects <strong>of</strong> hybrid Spartina on shorebird habitat inSouth San Francisco Bay (<strong>the</strong> South Bay), using grid-basedspatial models <strong>of</strong> (a) shorebird habitat value and (b) potentialSpartina spread.- 175 -