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 Spartinageese preference in <strong>the</strong> first experiment was based ondifferences in defensive chemistry between native S. foliosaand hybrid Spartina, <strong>the</strong>y would presumably still be able touse those cues in <strong>the</strong> second trial. We also measureddifferences in <strong>the</strong> amount <strong>of</strong> material that geese discarded toexplore <strong>the</strong> idea that geese might discard a greaterproportion <strong>of</strong> <strong>the</strong> hybrid if defensive chemistry was a cue.However, we measured no significant differences indiscarded material between native and hybrid Spartina.Our results parallel o<strong>the</strong>r studies <strong>of</strong> factors influencinggoose grazing. Earlier work by Buchsbaum et al. (1981)with eastern Canada geese (Branta canadensis canadensis)suggested that geese can detect defensive compounds suchas ferulic acid in lawn grass. Their results suggest thatphenolics at relatively high concentrations could beimportant in grazing preferences. O<strong>the</strong>r studies havegenerally found less support for <strong>the</strong> role <strong>of</strong> phenolicsinfluencing goose grazing. A study with greater snow geese(Anser caerulescens atlantica) tested <strong>the</strong> influence <strong>of</strong> water,fiber, phenolic and protein content among grass species andfound that only water content was important (Gauthier andBedard 1991). Ano<strong>the</strong>r study using barnacle geese (Brantaleucopsis) found that feeding preferences <strong>of</strong> geese were bestcorrelated with water and nitrogen content, although <strong>the</strong>relationship with water was stronger (Owen et al. 1977).Coleman and Boag (1987) found that for western Canadageese non-structural carbohydrate and not fiber or proteincontent was <strong>the</strong> best predictor <strong>of</strong> preferences among foragespecies. Structural compounds such as cellulose have alsobeen found to be poorly digested by geese relative toproteins and soluble carbohydrates (Buchsbaum et al. 1986).The conclusions from our study are generally consistent with<strong>the</strong>se o<strong>the</strong>r studies that found little evidence for a strong rolefor defensive chemicals such as phenolics in determining <strong>the</strong>observed preferences and stronger influences <strong>of</strong> structuralcompounds, nitrogen and water content. Interestingly,native S. foliosa has a lower carbon to nitrogen (C:N) ratio(more protein, less cellulose) relative to hybrid Spartina(Tyler et al. 2007), and also has a less coarse physicalstructure and presumably higher water content.In summary, our results suggest that Canada geese maybe influencing <strong>the</strong> rate <strong>of</strong> spread <strong>of</strong> hybrid Spartina in <strong>the</strong>central portion <strong>of</strong> San Francisco Bay. The eventual loss <strong>of</strong>native S. foliosa due to <strong>the</strong> hybrid invasion, at least in someportion <strong>of</strong> San Francisco Bay, could also have negativeconsequences for foraging by removing a potentiallyimportant food source. The intense and highly selectivegrazing on native Spartina and almost total avoidance <strong>of</strong> <strong>the</strong>hybrid results from a strong preference for <strong>the</strong> native that isapparently based on structural ra<strong>the</strong>r than chemicaldifferences.ACKNOWLEDGMENTSOur greatest thanks to J. Black, Humboldt StateUniversity, for many informative discussions <strong>of</strong> geese andgrazing, his very generous assistance and collaboration with<strong>the</strong> HSU captive goose facility and his help with permitting,logistics, and procedures for behavioral studies. We alsothank HSU students E. Bjerre and K. Spragens for <strong>the</strong>irgenerous assistance with followup experiments, aviarylogistics and Spartina care. We also thank S. Norton, N.Rayl, U. Mahl, N. Christiansen and C. Love for help withfield grazing studies, plant collections, sample processingand data entry.REFERENCESAyres, D.R., D. Garcia-Rossi, H.G. Davis, and D.R. Strong1999. 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