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 SpartinaTable 3. Predicted loss <strong>of</strong> fall shorebird numbers by species, based on arange <strong>of</strong> Spartina spread scenarios. All predictions assume that mudflatsare at carrying capacity, and that mudflat habitat value is proportional totidal inundation.Table 4. Predicted loss <strong>of</strong> spring shorebird numbers by species, based on arange <strong>of</strong> Spartina spread scenarios. All predictions assume that mudflatsare at carrying capacity, and that mudflat habitat value is proportional totidal inundation.CurrentScenario 1:60%inundationtoleranceScenario 2:70%inundationtoleranceScenario 3:80%inundationtoleranceCurrentScenario 1:60%inundationtoleranceScenario 2:70%inundationtoleranceScenario 3:80%inundationtoleranceMudflatHectares6,062 -904 -1,988 -3,269MudflatHectares6,062 -904 -1,988 -3,269Bird Numbers:AmericanAvocetBlack-belliedPlover5,023 -1,701 -3,034 -4,1018,138 -2,756 -4,916 -6,645Dowitcher 13,377 -4,530 -8,081 -10,923Long-billedCurlewMarbledGodwit371 -126 -224 -30314,251 -4,826 -8,609 -11,636Red Knot 1,678 -568 -1,014 -1,370SemipalmatedPlover1,501 -508 -907 -1,225Willet 15,612 -5,286 -9,431 -12,747Westernand LeastSandpiper,160,374 -54,305 -96,880 -130,948DunlinTotal 220,325 -48,615 -70,055 -94,175Bird Numbers:AmericanAvocetBlack-belliedPlover844 -263 -476 -6564,595 -1,432 -2,590 -3,570Dowitchers 33,008 -10,289 -18,608 -25,644Long-billedCurlewMarbledGodwit218 -68 -123 -16913,437 -4,188 -7,575 -10,439Red Knot 503 -157 -284 -391SemipalmatedPlover725 -226 -409 -563Willet 2,112 -658 -1,191 -1,641Westernand LeastSandpiper,Dunlin450,817 -140,528 -254,137 -350,241Total 506,259 -104,793 -156,097 -212,813shorebird surveys, Spartina spread would have <strong>the</strong> biggestnumerical impact on small shorebirds, dowitchers, andMarbled Godwits (Tables 3, 4). Willets and Black-belliedPlovers would be most affected during <strong>the</strong> fall, when <strong>the</strong>irnumbers are highest. In terms <strong>of</strong> total bird biomass (seeStenzel et al. 2002), <strong>the</strong> largest predicted losses were in <strong>the</strong>spring, due to higher overall biomass densities (Table 5).DISCUSSIONThe results presented herein are based on several basicassumptions, all <strong>of</strong> which should be examined in fur<strong>the</strong>rdetail in order to restrict <strong>the</strong> wide range <strong>of</strong> predicted shorebirdlosses. Our most fundamental assumption was that <strong>the</strong>mudflat habitats were at carrying capacity during <strong>the</strong> falland spring survey periods. However, it is possible that onlypreferred areas are functioning at carrying capacity (Goss-Custard 1979). If individuals could switch to lower-qualitymudflat areas without significantly affecting <strong>the</strong>ir overallfitness, <strong>the</strong>n <strong>the</strong> potential loss <strong>of</strong> birds may have been overestimated(Goss-Custard 2003). Anecdotal evidence suggeststhat San Francisco Bay mudflats may reach carrying capacityduring <strong>the</strong> winter, when storm-related flooding may promptsome species to move inland to <strong>the</strong> Central Valley (Warnocket al. 1995; Takekawa et al. 2002) but we do not know if mudflatsand neighboring tidal and salt pond habitats are at carryingcapacity during migration. Currently, we lack <strong>the</strong> data onmudflat food resources, shorebird energetics, and individualforaging behavior (especially prey preference) that would benecessary to obtain an estimate <strong>of</strong> carrying capacity.A potential bias in <strong>the</strong> o<strong>the</strong>r direction, however, was thatour models examined only <strong>the</strong> lower limits <strong>of</strong> Spartina spreadand <strong>the</strong> subsequent impacts on shorebird habitat value. Inreality, upward Spartina spread may pose an equally seriousthreat to shorebirds as mudflats along tidal channels andopen areas within <strong>the</strong> marsh plain are colonized by invasiveSpartina and become unavailable to foraging shorebirds.Table 5. Predicted loss <strong>of</strong> fall and spring shorebird biomass, based on a range<strong>of</strong> Spartina spread scenarios (1-3). All predictions assume that muflats areat carrying capacity, and that mudflat habitat value is proportional to tidalinundation.Fallbiomass(kg)Springbiomass(kg)CurrentScenario 1:60%inundationtoleranceScenario 2:70%inundationtoleranceScenario 3:80%inundationtolerance21,416 -6,245 -12,263 -17,06725,289 -6,884 -13,560 -19,202- 180 -