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 SpartinaRemaining Plants (%)100806040200StemsLeavesHybridS. foliosaFig. 3. Results <strong>of</strong> feeding preference experiments showing means (+ 1s.e.) <strong>of</strong> remaining stems and leaves <strong>of</strong> hybrid (black bars) and nativeSpartina (white bars) after being grazed by captive geese.Grazing Trials with Clipped StemsThe results <strong>of</strong> <strong>the</strong> second set <strong>of</strong> feeding trials withclipped stems produced very different results than <strong>the</strong> trialsinvolving intact clones. The captive geese showed nosignificant preference for clipped stems <strong>of</strong> native Spartinacompared with stems <strong>of</strong> hybrid Spartina. Geese grazing wasvigorous overall and geese spent similar amounts <strong>of</strong> timegrazing as <strong>the</strong>y did in <strong>the</strong> trials with <strong>the</strong> clones (p>0.05).However, nei<strong>the</strong>r <strong>the</strong> amount <strong>of</strong> biomass consumed bygrazing geese nor <strong>the</strong> amount discarded was significantlydifferent for hybrid stems relative to native stems (p>0.25for all).DISCUSSIONOur findings from <strong>the</strong> field surveys suggest that grazingby western Canada geese on native S. foliosa is widespreadthroughout <strong>the</strong> central portion <strong>of</strong> San Francisco Bay. Datafrom at least five sites showed a large reduction in <strong>the</strong>aboveground biomass <strong>of</strong> native Spartina. By contrast,adjacent areas where hybrid Spartina was present showedlittle sign <strong>of</strong> grazing. Geese appeared to ignore hybridSpartina entirely when it was present in our surveys. Thereis <strong>the</strong> possibility that geese may consume a higherproportion <strong>of</strong> hybrid genotypes that are morphologicallymore similar to native S. foliosa, however, we have not yetseen evidence for this.Our results from <strong>the</strong> goose exclusion studies indicatethat lateral spread <strong>of</strong> <strong>the</strong> hybrid into areas occupied by nativeS. foliosa is more rapid in areas where goose grazing hasbeen excluded for two years. The rate <strong>of</strong> lateral spread is25% greater than in adjacent control areas where goosegrazing continued. This 25% increase in lateral spread mayhave significant consequences for <strong>the</strong> overall rate <strong>of</strong>invasion <strong>of</strong> hybrid Spartina in San Francisco Bay. Although<strong>the</strong> rate <strong>of</strong> sexual reproduction <strong>of</strong> hybrid Spartina isoverwhelming native reproduction due to higher seed set andpollen swamping, plant clonal growth is also important formaintaining its presence in <strong>the</strong> bay. The degree to whichhybrid plants can overgrow and outcompete native plantswill have a significant effect on <strong>the</strong> speed that hybridSpartina may be able to drive S. foliosa locally extinct incentral San Francisco Bay (Ayres et al. 2004).The eventual replacement <strong>of</strong> native with hybrid mayalso have some negative consequences for <strong>the</strong> nesting geese.Native Spartina is not likely to be <strong>the</strong> only food source fornesting geese in an urbanized estuary like San FranciscoBay, where lawns and golf courses <strong>of</strong>fer easy forage.However, <strong>the</strong> high levels <strong>of</strong> grazing on native Spartina at ourstudy sites suggest that this is still an important source <strong>of</strong>forage for <strong>the</strong>se birds. O<strong>the</strong>r sources <strong>of</strong> forage are not likelyas proximate to nesting sites as native Spartina and,<strong>the</strong>refore may be costly in terms <strong>of</strong> foraging time and timespent away from <strong>the</strong> nest. This ultimately may affect nestingsuccess. Unfortunately, <strong>the</strong>re are no data with which to test<strong>the</strong>se possibilities.The results from our experiments with captive geesestrongly support our observations in <strong>the</strong> field. The data from<strong>the</strong> first set <strong>of</strong> aviary trials show a strong ability <strong>of</strong> geese todiscriminate between native S. foliosa and hybrid Spartina inside-by-side choice trials. This supports <strong>the</strong> idea that geeseare also discriminating among <strong>the</strong>se same plant types in <strong>the</strong>field. It is possible that geese may also discriminate amongdifferent hybrids, possibly preferring hybrids that moreclosely resemble <strong>the</strong> native S. folioa. However, we cannotdetermine that from <strong>the</strong> current study.Our data from <strong>the</strong> second set <strong>of</strong> aviary trials isconsistent with <strong>the</strong> idea that geese are able to distinguishhybrid from native Spartina based on physicalcharacteristics ra<strong>the</strong>r than chemical characteristics. Incontrast to <strong>the</strong> first trials (plants upright and intact), resultsfrom <strong>the</strong>se trials (cut stems laid sideways in trays) showedthat geese exhibited no ability to distinguish hybrid fromnative.Physical characteristics <strong>of</strong> intact plants may allow geeseto assess palatability <strong>of</strong> potential forage by tugging on <strong>the</strong>plant and gauging its resistance (Lang and Black 2001).Physical differences between native and hybrid Spartina areimmediately obvious to any human investigator as hybridstems and leaves are much thicker, tougher and morefibrous. Results from <strong>the</strong> second experiment, designed toeliminate physical differences experienced by geese tuggingon leaves and stems, showed that geese failed todiscriminate between native and hybrid Spartina. Thegeneral conclusion from studies <strong>of</strong> secondary defensive plantcompounds is that if by removing physical differences, <strong>the</strong>reis no subsequent preference shown by <strong>the</strong> herbivore, <strong>the</strong>nplant chemistry can generally be ruled out as important. If- 194 -