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 SpartinaTHE EFFECTS OF GRAZING GEESE ON HYBRID AND NATIVE SPARTINA IN SANFRANCISCO BAYE.D. GROSHOLZ 1 AND R.E. BLAKE 2Department <strong>of</strong> Environmental Science and Policy, University <strong>of</strong> California, Davis, One Shields Ave., Davis, CA 95616tedgrosholz@ucdavis.edu 1 ; reblake@vims.edu 2The invasion <strong>of</strong> Spartina alterniflora has become one <strong>of</strong> <strong>the</strong> most significant invasions in <strong>the</strong>already heavily invaded San Francisco Bay. The Spartina invasion has resulted in many changes tocommunity and ecosystem processes at lower trophic levels, however, we have so far failed todocument equivalent changes at higher trophic levels that are likely occurring as well. In this study,we report <strong>the</strong> consequences <strong>of</strong> <strong>the</strong> hybrid invasion with respect to an important vertebrate grazer,western Canada geese (Branta canadensis m<strong>of</strong>fitti). Canada geese regularly nest during wintermonths in <strong>the</strong> vicinity <strong>of</strong> <strong>the</strong> invaded central and sou<strong>the</strong>rn areas <strong>of</strong> San Francisco Bay. We foundthat Canada geese intensively graze native Spartina foliosa at several study sites in <strong>the</strong> central bayregion removing up to 90% <strong>of</strong> <strong>the</strong> aboveground vegetation. However, geese completely ignorehybrid Spartina with virtually no evidence <strong>of</strong> grazing in hybrid areas adjacent to heavily grazednative Spartina areas. Experiments with captive geese demonstrated that geese repeatedly preferrednative Spartina when presented intact clones <strong>of</strong> hybrid and native clones in side-by-side preferencetrials. However, when cut stems were presented in similar trials, geese showed no preference for <strong>the</strong>native over <strong>the</strong> hybrid. We conclude that <strong>the</strong> preference demonstrated in <strong>the</strong> first experiment wasnot <strong>the</strong> result <strong>of</strong> plant defensive chemistry, but <strong>the</strong> result <strong>of</strong> physical differences between intactclones and cut stems that geese could assess. Field exclosure experiments conducted for two years inareas where hybrid Spartina was overgrowing native Spartina showed that grazing <strong>of</strong> nativeSpartina by geese resulted in 25% greater rates <strong>of</strong> lateral spread <strong>of</strong> <strong>the</strong> hybrid into native areas. Thissuggests that grazing by geese may be accelerating <strong>the</strong> rate <strong>of</strong> invasion and ultimate replacement <strong>of</strong>native Spartina by hybrid Spartina.Keywords: Canada geese, San Francisco Bay, hybrid Spartina, grazing, invasion rateINTRODUCTIONThe introduction <strong>of</strong> <strong>the</strong> smooth cordgrass (Spartinaalterniflora) has been among <strong>the</strong> most significant <strong>of</strong> <strong>the</strong>nearly 250 non-native species invasions recorded in SanFrancisco Bay (Cohen and Carlton 1998). Spartinaalterniflora became established in 1976 as <strong>the</strong> result <strong>of</strong> anintentional introduction by <strong>the</strong> Army Corp <strong>of</strong> Engineers formitigation purposes (Faber 2000). Following establishment,S. alterniflora hybridized with <strong>the</strong> native cordgrass S.foliosa. Hybrid Spartina has rapidly colonized many areas<strong>of</strong> central and south San Francisco Bay (Daehler and Strong1997; Ayres et al. 2004). Hybrid Spartina can colonize openmudflats as well as out-compete native vegetation at highertidal heights (Ayres et al. 2004).Recent studies have shown that <strong>the</strong> ecologicalrepercussions <strong>of</strong> <strong>the</strong> hybrid Spartina invasion are farreaching with widespread impacts on community structureand ecosystem function (Neira et al. 2005, 2006, 2007;Levin et al. 2006). However, work to date has focused onlyon changes at lower trophic levels, despite <strong>the</strong> fact thatvertebrate herbivores are known to consume S. alterniflorain its native region (Buchsbaum et al. 1981). Little is knownregarding <strong>the</strong> consequences <strong>of</strong> hybridization forsusceptibility to herbivores <strong>of</strong> any kind (Daehler and Strong1997).Among <strong>the</strong> most common and potentially importantvertebrate herbivores <strong>of</strong> Spartina are western Canada geese,Branta canadensis m<strong>of</strong>fitti (Banks et al. 2004). The westernsubspecies is <strong>the</strong> only one regularly found in San FranciscoBay (Mowbray et al. 2002) and <strong>the</strong>y typically nest fromNovember to April. While nesting, <strong>the</strong>ir foraging isgenerally restricted to nearby areas that typically includemudflats with native Spartina. Our initial observationssuggested that geese were grazing intensively on areasoccupied by native Spartina. One <strong>of</strong> <strong>the</strong> key questions <strong>of</strong>this study was to determine whe<strong>the</strong>r grazing by westernCanada geese could influence <strong>the</strong> rate <strong>of</strong> invasion <strong>of</strong> hybridSpartina in San Francisco Bay. Therefore, our goal was tomeasure grazing by Canada geese on native Spartina andcompare this with grazing on hybrid Spartina. If we foundevidence <strong>of</strong> selective grazing, <strong>the</strong> next goal would be todetermine whe<strong>the</strong>r selective grazing was due to ameasurable preference for one plant over <strong>the</strong> o<strong>the</strong>r. Wewould also determine <strong>the</strong> basis for any preference that mightexist. Finally, we would quantify any influence that geese- 191 -