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 SpartinaQUANTIFYING THE POTENTIAL IMPACT OF THE SPARTINA INVASION ON INVERTEBRATEFOOD RESOURCES FOR FORAGING SHOREBIRDS IN SAN FRANCISCO BAYN. CHRISTIANSEN 1 , E.D. GROSHOLZ 2 AND P. ROSSODepartment <strong>of</strong> Environmental Science and Policy, University <strong>of</strong> California, Davis, One Shields Ave., Davis, CA 956161 n.a.christiansen@gmail.com; 2 tedgrosholz@ucdavis.eduThe extensive mudflats <strong>of</strong> <strong>the</strong> San Francisco Bay estuary are critical habitats and foraging groundsfor migratory shorebirds. However, San Francisco Bay has experienced invasion by hundreds <strong>of</strong>non-native introduced species, and its productive mudflats are now threatened by one <strong>of</strong> <strong>the</strong>se —hybrid Spartina alterniflora. To assess <strong>the</strong> potential effects <strong>of</strong> <strong>the</strong> invasive cordgrass on <strong>the</strong>shorebird foraging habitat, we quantified invertebrate abundance and biomass along transectsextending from <strong>the</strong> edge <strong>of</strong> <strong>the</strong> Spartina meadow to <strong>the</strong> lowest tide line at two locations in <strong>the</strong> bay,Robert’s Landing and Elsie Roemer. We also estimated <strong>the</strong> size <strong>of</strong> areas potentially invaded bySpartina using tidal elevation data based on LIDAR images and tidal inundation data taken fromcontinuous tidal height monitoring stations. We found that invertebrate biomass was greatest athigher tidal elevations nearer to <strong>the</strong> marsh, while abundance was greatest at intermediate distancesfrom <strong>the</strong> marsh. Current estimates <strong>of</strong> tolerance to tidal inundation indicate that <strong>the</strong> area colonized bySpartina could increase by as much as four to eight times its present size, depending on <strong>the</strong>individual site characteristics. These data suggest that <strong>the</strong>re is a high potential for losing significantamounts <strong>of</strong> valuable foraging habitat for shorebirds if Spartina extends its distribution to predictedtidal elevations in San Francisco Bay.Keywords: shorebirds, San Francisco Bay, Hybrid Spartina, infaunaINTRODUCTIONSan Francisco Bay is now home to more than 250species <strong>of</strong> non-native plants and animals (Cohen and Carlton1998). One <strong>of</strong> <strong>the</strong> most serious recent invasions has been <strong>the</strong>smooth cordgrass Spartina alterniflora, which is native to<strong>the</strong> eastern coast <strong>of</strong> <strong>the</strong> North America. Since itsintroduction, S. alterniflora has hybridized with <strong>the</strong> nativecordgrass S. foliosa creating a hybrid that has rapidlycolonized many areas <strong>of</strong> central and south San FranciscoBay (Daelher and Strong 1997, Ayres et al. 2004). Thehybrid can colonize open mudflats as well as out-competenative vegetation at higher tidal heights (Ayres et al. 2004).The ecological repercussions <strong>of</strong> this invasion are farreaching with widespread impacts on community structureand ecosystem function (Neira et al. 2005; 2006, Levin et al.2006).One <strong>of</strong> <strong>the</strong> most serious consequences <strong>of</strong> <strong>the</strong> hybrid Spartinainvasion is <strong>the</strong> loss <strong>of</strong> open mudflat habitat for shorebirds.San Francisco Bay, with its extensive mudflats, containscritical habitat for more than one million migratory shorebirdsand is one <strong>of</strong> <strong>the</strong> most important estuaries along <strong>the</strong>Pacific Flyway (Page et al. 1999). This important habitat isnow threatened by <strong>the</strong> smooth cordgrass. Shorebirds requireunvegetated mudflat habitats for foraging (Goss-Custard andMoser 1988), and invasive hybrid Spartina threatens tocolonize critical upper levels <strong>of</strong> this important habitat. As aresult <strong>of</strong> <strong>the</strong> rapid spread <strong>of</strong> hybrid Spartina, which increasedby 317% at sampled sites between 2001 and 2003(Ayres et al. 2004), by <strong>the</strong> end <strong>of</strong> 2003 it was estimated tooccupy nearly 800 hectares <strong>of</strong> bay habitat, up from less than200 acres in 2001 (Zaremba and McGowen 2004). Thisrapid spread suggests that <strong>the</strong> risk for shorebird habitat lossmay be substantial.To better estimate <strong>the</strong> potential value and extent <strong>of</strong> lostforaging habitat, we measured <strong>the</strong> biomass and abundance <strong>of</strong>invertebrates at different tidal elevations at two sites in SanFrancisco Bay. To accurately measure tidal elevation acrossbroad areas <strong>of</strong> <strong>the</strong> two study sites, we used LIDAR (LightDetection and Ranging), a flight-based radar system that canaccurately measure elevation with high spatial resolution.We used <strong>the</strong>se elevation data toge<strong>the</strong>r with tidal height datafrom continuous recording stations to estimate <strong>the</strong> arealikely to be invaded by Spartina based on current estimates<strong>of</strong> its inundation tolerance. Therefore, we were able toestimate <strong>the</strong> value and extent <strong>of</strong> habitat lost to shorebirdsunder different scenarios <strong>of</strong> Spartina spread. Thisinformation, when combined with <strong>the</strong> data on patterns <strong>of</strong>shorebird usage generated by PRBO Conservation Science(Stralberg et al. 2010) can help guide future eradicationefforts by highlighting areas <strong>of</strong> particular importance forshorebirds.- 171 -