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 SpartinaAno<strong>the</strong>r assumption <strong>of</strong> our habitat value models wasthat exposed mudflat areas are used evenly by shorebirds,and thus in direct proportion to <strong>the</strong>ir temporal availability.However, we know that individuals <strong>of</strong> many species tendto forage along <strong>the</strong> rising or receding tide line (Colwell andLandrum 1993; Durell et al. 1997), as invertebrates are moreabundant and more accessible in wet substrates and tendto burrow deeper as <strong>the</strong> tide recedes and <strong>the</strong> mud dries out(Goss-Custard 1984; White 1995). For shorebirds that onlyforaged along <strong>the</strong> tide line, any given full tidal range mudflatshould support a constant number <strong>of</strong> birds as long as someminimum mudflat area was exposed, and shorebird densities(with respect to exposed mudflat) should increase as <strong>the</strong> tiderises. In reality, it is likely that some intermediate conditionexists and mudflat use patterns vary by species. In SanFrancisco Bay we have observed that Semipalmated Plover,Least Sandpiper, and Black-bellied Plover tend to foragehigher along <strong>the</strong> tidal gradient, whereas Dunlin, dowitchers,Marbled Godwit, and o<strong>the</strong>r species forage closer to <strong>the</strong> tideline (PRBO unpubl. data; USGS unpubl. data).Shorebird densities are also known to vary accordingto <strong>the</strong> uneven distribution <strong>of</strong> sediments, prey densities, andprey availability across <strong>the</strong> intertidal zone (Burger et al.1977; Goss-Custard et al. 1977; Puttick 1977; Page et al.1979; Quammen 1982, Colwell and Landrum 1993; Yates etal. 1993). This highlights <strong>the</strong> need to study <strong>the</strong> spatial distribution<strong>of</strong> shorebirds and <strong>the</strong>ir invertebrate prey over SanFrancisco Bay mudflats, especially given <strong>the</strong> dynamic nature<strong>of</strong> <strong>the</strong> largely non-native invertebrate community (Nichols etal. 1986; Cohen and Carlton 1998).With respect to our estimates <strong>of</strong> mudflat elevation andtidal inundation, we used a simple approach that involvedassuming an unrealistic linear mudflat slope. While highresolutiontopographic/ bathymetric data were not availablefor <strong>the</strong> entire South Bay mudflat region at <strong>the</strong> time <strong>of</strong> thisstudy, we hope that recently-obtained Light Detection andRanging (LiDAR) data (Foxgrover and Jaffe 2005) may beused to improve our models in <strong>the</strong> future.Finally, our model predictions were based on a staticpicture <strong>of</strong> mudflat spatial extent and elevation. In reality,<strong>the</strong>re are several factors in addition to hybrid Spartina spreadthat may affect mudflat extent and quality, including sea levelrise (Donnelly and Bertness 2001; Galbraith et al. 2002),tidal marsh restoration, and natural geomorphic processes(Foxgrover et al. 2004). Thus it would be useful to apply ourmodels to future mudflat predictions developed by coastalgeomorphologists.CONCLUSIONThe above-described models <strong>of</strong> Spartina inundationtolerance and shorebird habitat value allowed us to generatepreliminary predictions about <strong>the</strong> potential impact <strong>of</strong>future Spartina spread on South Bay shorebird populations.Because mudflat habitat availability and Spartina spreadpotential are both determined by tidal inundation frequency,<strong>the</strong> areas <strong>of</strong> greatest value to shorebirds coincide with <strong>the</strong>areas <strong>of</strong> greatest Spartina invasion potential. In addition,<strong>the</strong> eastern shore mudflats, which had <strong>the</strong> highest recordedshorebird densities, are adjacent to one <strong>of</strong> <strong>the</strong> sites <strong>of</strong> initialSpartina invasion at Coyote Hills Slough, increasing <strong>the</strong>irsusceptibility to Spartina encroachment.Due to various sources <strong>of</strong> uncertainty, our preliminaryanalysis resulted in a wide range <strong>of</strong> predictions: a 14% to 54%loss in mudflat area, and a 27% to 80% loss in habitat valueacross <strong>the</strong> three Spartina spread scenarios examined. At <strong>the</strong>low end, this represents a significant loss <strong>of</strong> habitat for shorebirds,which may or may not result in population declines.At <strong>the</strong> high end, our predictions suggest an extreme changein habitat availability that would almost certainly reduce <strong>the</strong>number <strong>of</strong> foraging shorebirds in <strong>the</strong> South Bay.At <strong>the</strong> same time, o<strong>the</strong>r major changes are occurring in<strong>the</strong> South Bay, including <strong>the</strong> restoration <strong>of</strong> up to 5,000 hectares<strong>of</strong> commercial salt ponds to tidal, muted, and managedmarsh. The loss <strong>of</strong> <strong>the</strong>se salt ponds, which currently serveas valuable foraging and roosting habitat, may also reduceshorebird numbers (Stralberg et al. 2006). Thus we suspectthat South Bay shorebird populations may suffer from multiplenegative impacts within a relatively short timeframe ifSpartina spread is not arrested, reducing <strong>the</strong> value <strong>of</strong> SanFrancisco Bay as a major migratory stopover and winteringsite. Additional research and modeling will be needed toassess <strong>the</strong> cumulative effects <strong>of</strong> habitat change on shorebirds,while large-scale, integrated habitat conservation planningmay help to mitigate <strong>the</strong>se negative effects.AUTHORS’ NOTE:Due to extensive Spartina control efforts since <strong>the</strong>writing <strong>of</strong> this paper, Spartina spread now poses much less<strong>of</strong> an immediate threat to shorebirds in San Francisco Bay.ACKNOWLEDGMENTSThis project was requested by <strong>the</strong> Invasive SpartinaProject (ISP), and funded by <strong>the</strong> California CoastalConservancy, <strong>the</strong> State Resources Agency, and <strong>the</strong> CALFEDProgram, under contract #02-212. We would like to thankKaty Zaremba and Peggy Ol<strong>of</strong>son <strong>of</strong> <strong>the</strong> ISP, Debra Ayres,Don Strong, Janie Civille and Ted Grosholz <strong>of</strong> UC Davis,and Bruce Jaffe and John Takekawa <strong>of</strong> USGS for <strong>the</strong>ir valuableinput and support. We are also grateful to Josh Collinsand Stuart Siegel for advice on methods, to Hildie Spautz forassistance with <strong>the</strong> literature review, and to an anonymousreviewer for feedback on an earlier draft <strong>of</strong> this manuscript.Finally, we are very grateful for <strong>the</strong> assistance <strong>of</strong> hundreds <strong>of</strong>volunteers who made <strong>the</strong> shorebird surveys possible. This isPRBO contribution number 1221.REFERENCESAnttila, C.K., C.C. Daehler, N.E. Rank, and D.R. Strong. 1998.Greater male fitness <strong>of</strong> a rare invader (Spartina alterniflora,Poaceae) threatens a common native (Spartina foliosa) withhybridization. Am. J. Bot. 85:1597–1601.Ayres, D.R., D. Garcia-Rossi, H.G. Davis, and D.R. Strong. 1999.Extent and degree <strong>of</strong> hybridization between exotic (Spartina- 181 -