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 SpartinaFig. 6. Predicted extent <strong>of</strong> Spartina spread based on monthly inundationtolerances ranging from 60% to 80% (from Collins 2002). Sharp breaks inpredictions are due to breaks in nearest tide station locations.*tial were <strong>the</strong> upper mudflats, due to <strong>the</strong>ir higher elevation andlower tidal inundation frequency (Fig. 6).Weighting <strong>the</strong> areas <strong>of</strong> potential Spartina spread byrelative shorebird value, <strong>the</strong> predicted loss to shorebirdsranged from 27% to 80% (Table 2). The upper mudflats had<strong>the</strong> highest relative value due to <strong>the</strong>ir lower tidal inundationfrequency (Figs. 7, 8). The east and south shore mudflats had<strong>the</strong> highest value during <strong>the</strong> fall (Fig. 7), and <strong>the</strong> east shoremudflats had <strong>the</strong> highest value during <strong>the</strong> spring (Fig. 8),based on shorebird survey numbers.Multiplying <strong>the</strong> predicted mudflat habitat loss in eachcensus tract by estimated current shorebird numbers yieldedloss projections <strong>of</strong> 48,615 to 94,175 birds in <strong>the</strong> fall (Table 3)and 104,793 to 212,813 birds in <strong>the</strong> spring (Table 4).Species that concentrated in <strong>the</strong> South Bay were BlackbelliedPlover, Willet, Marbled Godwit, small sandpipers, anddowitchers (Stenzel et al. 2002). Overall shorebird numberswere higher in spring than in fall, driven primarily by <strong>the</strong>large number <strong>of</strong> Western Sandpipers that use San FranciscoBay as a staging area during spring migration. BecauseWestern Sandpipers are small-bodied shorebirds, <strong>the</strong> differencebetween fall and spring biomass was much smaller than<strong>the</strong> difference between fall and spring numbers.Because our models did not incorporate any species-specificdifferences in shorebird foraging habits, <strong>the</strong> predictedproportional change in numbers was <strong>the</strong> same for all groups.Due to migration timing and overall numbers detected on* Current extent <strong>of</strong> invasive Spartina is based on pre-control mapping effortsFig. 7. Predicted extent <strong>of</strong> Spartina spread, based on a 70% inundationtolerance, and overlap with tidal mudflats, classified according to <strong>the</strong>irpotential fall season value for shorebirds. Shorebird habitat value wasbased on: (a) PRBO Pacific Flyway shorebird survey data (1988-1993) and(b) length <strong>of</strong> mudflat inundation during September.*Fig. 8. Predicted extent <strong>of</strong> Spartina spread, based on a 70% inundationtolerance, and overlap with tidal mudflats, classified according to <strong>the</strong>irpotential spring season value for shorebirds. Shorebird habitat value wasbased on: (a) PRBO Pacific Flyway shorebird survey data (1988-1993) and(b) length <strong>of</strong> mudflat inundation during April.*- 179 -