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 Spartinasame interest in <strong>the</strong> clipped stems as <strong>the</strong>y did in <strong>the</strong> intactclones, <strong>the</strong>refore we assume that <strong>the</strong> condition <strong>of</strong> <strong>the</strong> stemsand leaves were similar to that in <strong>the</strong> previous experiment.In this second experiment, we <strong>of</strong>fered <strong>the</strong> geese a choice<strong>of</strong> cut stems <strong>of</strong> ei<strong>the</strong>r hybrid Spartina or native S. foliosausing <strong>the</strong> same bins <strong>the</strong> geese are normally <strong>of</strong>fered <strong>the</strong>irpelleted feed. We arranged three bins <strong>of</strong> each <strong>of</strong> <strong>the</strong> twoplant types in a 2 x 3 array with each bin approximately 2 mfrom <strong>the</strong> o<strong>the</strong>rs. After setting up <strong>the</strong> feeding array, <strong>the</strong> geesewere allowed to graze for at least one hour after which all<strong>the</strong> plants remaining in <strong>the</strong> bins were collected. Also, plantsthat had been removed from <strong>the</strong> bins and discarded werealso collected and stored separately. We repeated thisexperiment two times over <strong>the</strong> following eight weeks. Weestimated <strong>the</strong> intensity <strong>of</strong> grazing by weighing <strong>the</strong> dryweight <strong>of</strong> <strong>the</strong> plants in each bin remaining at <strong>the</strong> end <strong>of</strong> <strong>the</strong>experiment as well as <strong>the</strong> discarded plants adjacent to thatbin. We tested differences in grazing with ANOVA usinglog-transformed weights for ei<strong>the</strong>r bins or discards withtreatment as a fixed factor (as above).RESULTSGrazing Intensity in San Francisco BayOur data from several sites in San Francisco Bay showintense grazing by western Canada geese on native S.foliosa. Our most extensive data from Robert’s Landing(Fig. 1) show greater than 90% loss <strong>of</strong> aboveground biomasswith nearly all stems grazed to within a few centimeters <strong>of</strong><strong>the</strong> ground on all three transects (Fig. 1). By contrast, geesevirtually ignored hybrid Spartina with no measurablegrazing on any plants along <strong>the</strong> transect. Similarly intensegrazing was measured at Oro Loma (greater than 85% <strong>of</strong>stems grazed) and Pt. Isabel (greater than 95% <strong>of</strong> stemsgrazed).Field Grazing ExclosuresHybrid Spartina grew laterally into <strong>the</strong> native Spartinazones in nearly all <strong>of</strong> <strong>the</strong> experimental areas independent <strong>of</strong>treatment. However, <strong>the</strong> rate <strong>of</strong> lateral spread <strong>of</strong>aboveground plant biomass was significantly greater(p