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 Spartinaaffinities <strong>of</strong> clapper rails in non-invaded marshes aresummarized as follows:• Distribution limited to fully tidally, saline to brackishmarshlands and <strong>the</strong>ir foreshores within <strong>the</strong> estuary. Clapperrails do not occur upstream in <strong>the</strong> Sacramento-San JoaquinDelta and fresher portions <strong>of</strong> <strong>the</strong> system.• Well-developed channel and slough systems that extendthrough or into patches <strong>of</strong> tall monocot vegetation.Channels function as important areas for foraging and asmovement corridors.• Monocot vegetation is used for nesting material;nests are built at or about Mean Higher High Water(MHHW), <strong>of</strong>ten at <strong>the</strong> headward reach <strong>of</strong> a tidal channel(ARA 1992). Non-native Spartina extends upward to nearlyMHHW (Collins 2002.)• Habitat patches typically comprise some mature andsome youthful marsh. 1 The highest densities in <strong>the</strong> NorthBay marshes are generally associated with <strong>the</strong> largestcontiguous areas <strong>of</strong> youthful, saline marshland adjoining atleast moderate amounts <strong>of</strong> historical, mature marshland(Evens and Collins 1992; Collins et al. 1994).• Broad marshes on <strong>the</strong> bayshore, or near <strong>the</strong> mouths<strong>of</strong> tidal tributaries are favored; narrow, linear, strip marshessupport much lower densities, but serve as important corridorsbetween habitat patches and as foraging areas.• Densities <strong>of</strong> rails are highest where patches <strong>of</strong> habitatare at least 100 hectares (ha) in size (Collins et al. 1994).• Densities <strong>of</strong> rails tend to increase with channel density(length <strong>of</strong> channel per unit <strong>of</strong> marshland.)• Density is positively correlated (R 2 = 0.74) to arealextent <strong>of</strong> contiguous marshland.• Small parcels <strong>of</strong> marsh along <strong>the</strong> immediate margin<strong>of</strong> a tributary are more likely to support California clapperrails than small parcels that are more isolated.• Refugial vegetation, or even man-made structures(docks, duck blinds, etc.) may provide some protection forrails, particularly during high tides and flood events. Populations<strong>of</strong> mesopredators (raccoons, skunks, feral cats, etc.),and especially <strong>the</strong> non-native red fox (Vulpes vulpes),threaten rail survival; aerial predators also pose a threat.• Boardwalks, fence lines, towers, and stakes may increasepredation pressure by providing perch sites for avianpredators (Barn Owl, Great Horned Owl, Nor<strong>the</strong>rn Harrier, etc.)Numerous protocol-level population surveys have beenconducted in <strong>the</strong> nor<strong>the</strong>rn reaches <strong>of</strong> <strong>the</strong> estuary since <strong>the</strong>early 1980s (e.g. Evens and Page 1987; Collins et al. 1994;ARA 2004; Herzog et al. 2005).At San Pablo Bay sites where rails have been detected,densities typically varied between 0.1 and 2.8 birds perhectare (Collins et al. 1992; ARA 2004; Herzog et al. 2005)with highest densities at sites where youthful marsh hasrecently developed, e.g. Bahia Lagoon with 1.7 to 2.8birds/ha (ARA 2004; Herzog et al. 2005).In large maturemarshes, with well-developed channel systems, but wherenative cordgrass is limited to a narrow fringing edge, densityestimates are more typically on <strong>the</strong> order <strong>of</strong> about 0.5birds/ha but, in <strong>the</strong> largest marshes may range up as high as1.8 birds/ha (e.g Gallinas Creek mouth—ARA 2004; Herzoget al. 2005).NON-NATIVE SPARTINA DISTRIBUTION AND ABUNDANCEAND EXTENT OF OVERLAP WITH RAILSWithin <strong>the</strong> estuary, non-native cordgrass (Spartinaalterniflora and hybrids) is limited primarily to <strong>the</strong>marshlands and tidal flats south <strong>of</strong> Point San Pedro andPoint San Pablo (Hogle 2006). Therefore, <strong>the</strong> San Pablo Bayrail subpopulations exist essentially independent <strong>of</strong> invadedhabitat while <strong>the</strong> Central and South Bay rail subpopulationsoverlap extensively with actively colonizing non-nativecordgrass (Broom 2008).Protocol-level surveys over <strong>the</strong> three years 2005-2007that covered 60 sites and were limited to marshes invaded bynon-native cordgrass found mean baywide densities in <strong>the</strong>range <strong>of</strong> 0.58 rails/ha to 0.68 rails/ha (Broom 2008). Thesevalues are not highly disparate from those reported in <strong>the</strong>nor<strong>the</strong>rn reaches <strong>of</strong> <strong>the</strong> estuary (Collins et al. 1994; ARA2004; Herzog et al. 2005) Ano<strong>the</strong>r study <strong>of</strong> 45 Central andSouth bay sites conducted in 2005-2006 derived an overallmean abundance 0f 0.84 (±0.163) rails/ha and found nostatistically significant differences among categories (0 to>50%) <strong>of</strong> Spartina hybrid cover (Spautz et al. 2006).However, at several locations where Spartina clones haveaggresively invaded <strong>the</strong> foreshore, clapper rail densitiesgreatly exceeded those reported in San Pablo Bay, in <strong>the</strong>Central and South bays prior to <strong>the</strong> invasion by non-nativecordgrass, and <strong>the</strong> mean density reported in <strong>the</strong> twoaforementioned surveys. The most obvious examples:San Bruno Marsh (Colma Creek mouth): 2.4 to 3.8birds/ha (J. Evens, unpublished data., Spautz et al. 2006)Arrowhead Marsh: 3.8 to 4.2 birds/ha (Spautz et al.2006; J. Didonato, EBRPD, pers. comm.).This positive response to youthful, recently established,intertidal marsh vegetation by obsoletus is not unexpected.As North Bay studies in <strong>the</strong> early 1990s reported: “. . .highest densities <strong>of</strong> clapper rails were generally associatedwith <strong>the</strong> largest contiguous areas <strong>of</strong> youthful, salinemarshland adjoining at least moderate areas <strong>of</strong> historical,mature marshland . . .” (Collins et al. 1994). The colonization<strong>of</strong> marsh edge and foreshore by non-native cordgrass isapparently mimicking conditions in “native marshes” thatprograde as ecological conditions change (e.g. depositionalerosionpatterns). However, <strong>the</strong> Central and South Baymarshes invaded by non-native cordgrass differ frommarshlands <strong>of</strong> San Pablo Bay, in that few adjoin largemature marshlands, few support a heterogeneity <strong>of</strong> marshage classes, few exhibit a natural transitional grade from <strong>the</strong>low marsh to <strong>the</strong> high marsh plain, and vegetative cover at<strong>the</strong> marsh/upland ecotone is <strong>of</strong>ten sparse or absent. North- 186 -