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 SpartinaFig. 4. Cumulative duration <strong>of</strong> inundation curves for June based on waterlevel data from Alameda, Redwood City, and Dumbarton Bridge tide stations.Elevations corresponding to 60% inundation were 0.864 m, 1.101m, and 1.113 m, respectively. Elevations corresponding to 70% inundationwere 0.674 m, 0.882 m, and 0.874 m, respectively. Elevations correspondingto 80% inundation were 0.413 m, 0.575 m, and 0.646 m, respectively.Table 2. Projected percent <strong>of</strong> mudflat area and shorebird habitat value lost,based on a range <strong>of</strong> Spartina spread scenarios (1-3). All predictions assumethat mudflats are at carrying capacity, and that mudflat habitat value isinversely proportional to tidal inundation.Fall1 (60% inundationtolerance)2 (70% inundationtolerance)3 (80% inundationtolerance)Spring1 (60% inundationtolerance)2 (70% inundationtolerance)3 (80% inundationtolerance)Percent <strong>of</strong> mudflatinvaded bySpartinaPercent<strong>of</strong> habitatvalue lost toshorebirds14% 29%33% 57%54% 80%14% 27%33% 54%54% 76%Fig. 5. Shorebird biomass density (kg/ha) based on overall means fromcomprehensive spring shorebird survey data (1988-1993). Species-specificbiomass estimates were taken from <strong>the</strong> shorebird literature.tion <strong>of</strong> inundation curves were used to identify <strong>the</strong> elevationsat which 60%, 70%, and 80% cumulative monthly inundationwere achieved (Fig. 4). Each elevation was considered apotential threshold below which S. alterniflora and associatedhybrids would not grow (i.e., <strong>the</strong>ir inundation tolerance). We<strong>the</strong>n calculated potential Spartina spread for each inundationtolerance, selecting all mudflat pixels with modeled elevationsabove that particular elevation threshold.Shorebird NumbersWe used PRBO’s shorebird survey data to estimate falland spring shorebird numbers and overall biomass in kilograms(kg) for each <strong>of</strong> six South Bay mudflat census tracts(Stenzel et al. 2002). This resulted in fall and spring shorebirddensity grids, with densities uniformly distributed over eachcensus tract (Fig. 5).Potential Effects <strong>of</strong> Spartina Spread on Shorebird NumbersUsing <strong>the</strong> GIS grid layers described above, we estimated<strong>the</strong> potential effects <strong>of</strong> non-native Spartina spread on shorebirdnumbers. For each <strong>of</strong> <strong>the</strong> six South Bay census tractsand for each Spartina spread scenario (60%, 70%, and80% inundation tolerance), we first calculated <strong>the</strong> percent<strong>of</strong> mudflat area that would be lost. Then we calculated <strong>the</strong>proportion <strong>of</strong> habitat value that would be lost to shorebirds ifmudflat value were inversely related to <strong>the</strong> percent <strong>of</strong> mudflatinundation time.For each <strong>of</strong> <strong>the</strong> three Spartina spread scenarios, we calculateda predicted loss <strong>of</strong> spring and fall shorebird biomassby multiplying <strong>the</strong> proportion <strong>of</strong> mudflat value lost in eachcensus tract with <strong>the</strong> total estimated shorebird biomass supportedby that census tract. The same was done for individualspecies’ numbers.RESULTSOur Spartina spread model predicted that between14% and 54% <strong>of</strong> <strong>the</strong> total South Bay mudflat area could beencroached upon by S. alterniflora and associated hybrids(Fig. 6, Table 2). The areas <strong>of</strong> greater Spartina spread poten-- 178 -