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 SpartinaBay marshes are also larger than marshes in <strong>the</strong> sou<strong>the</strong>rnreach, higher in elevation, have a lower rate <strong>of</strong> subsidencerelative to mean tidal level, and have more emergent marshvegetation (Atwater et al. 1979; Josselyn 1983; GoalsProject 1999; Baye et al. 2000).ONGOING THREATS TO THE CLAPPER RAIL POPULATIONExtensive conversion <strong>of</strong> tidal lands resulting fromhistoric and ongoing pressures <strong>of</strong> agricultural production,urbanization, and salt production has drastically reducedCalifornia clapper rail habitat in <strong>the</strong> San Francisco Estuary(Goals Project 1999; Albertson and Evens 2000). Theremnant tidal marshlands <strong>of</strong> <strong>the</strong> estuary, <strong>the</strong> largest and lastrefuge <strong>of</strong> R.l. obsoletus, occupy only about 12-15% <strong>of</strong> <strong>the</strong>irhistoric extent, yet even in such diminished capacitycomprise more than 90% <strong>of</strong> all remaining California tidalmarshlands (Dedrick 1989; Goals Project 1999). In additionto, and exacerbated by, habitat loss and fragmentation, <strong>the</strong>rail population is vulnerable to a redundancy <strong>of</strong> threats—predation by introduced red fox (Vulpes vulpes); depredation<strong>of</strong> nests and eggs by non-native rodents (Rattus spp.) andinflated populations <strong>of</strong> native mammals (e.g. striped skunk,raccoon); contamination (Lonzarich et al. 1992;Schwarzbach et al. 2006); diminution <strong>of</strong> peripheral refugialvegetation; and sea-level rise. The ecological alteration <strong>of</strong><strong>the</strong> estuary’s tidal marshes by non-native vegetation,especially Spartina alterniflora and its hybrids, and possiblyLepidium (Spautz and Nur 2004), poses yet ano<strong>the</strong>r potentialthreat to an already beleaguered species.Ecologists have found that non-native hybrid cordgrassexceeds <strong>the</strong> narrower niche <strong>of</strong> <strong>the</strong> native tidal marsh plantand predicted that as <strong>the</strong> invasion progresses, hybridSpartina will likely spread to higher and lower elevations(Cohen 2001; Callaway and Josselyn 1992; Daehler andStrong 1996; Collins 2002; Baye 2004). As cordgrass bedsexpand, <strong>the</strong>y are expected to “constrain <strong>the</strong> tidal network”(Collins 2002) and, as <strong>the</strong> culms reach mature densities, <strong>the</strong>stands are apt to become effective sediment traps, promotinginfilling (Baye 2004). Additionally, <strong>the</strong> lower elevationlimits <strong>of</strong> non-native cordgrass “correspond to <strong>the</strong> slumpblocks and lower banks <strong>of</strong> large channels” (Collins 2002),prime clapper rail foraging areas.One <strong>of</strong> <strong>the</strong> forecasts <strong>of</strong> a study on <strong>the</strong> geomorphiceffects <strong>of</strong> non-native cordgrass invasion was that it will tend“to isolate <strong>the</strong> headward reaches <strong>of</strong> first order channelsfrom <strong>the</strong>ir networks,” [in effect] “shortening and simplifyingintertidal channel networks and <strong>the</strong> shoreline <strong>of</strong> <strong>the</strong> Estuaryas a whole” (Collins 2002). This narrow elevational nicheoverlaps almost precisely with habitat that is critical for <strong>the</strong>California clapper rail (Collins et al. 1994; Albertson andEvens 2000). Given <strong>the</strong> importance <strong>of</strong> channel complexity toclapper rails, a reduction in channel density and complexitypredicted for marshes invaded by non-native Spartina(Collins 2002; Ayres et al. 2003; Baye 2004) is likely toreduce <strong>the</strong> availability <strong>of</strong> habitat for clapper rails.RAIL DENSITY AND HABITAT QUALITYThe population estimates reported above suggest thatclapper rails may occur in dramatically higher densities insome invaded marshes than in non-invaded marshes. Thisapparent selection <strong>of</strong> non-native over native habitats by railspredicates a question: Do increased densities <strong>of</strong> rails meanincreased viability <strong>of</strong> rail populations?Some have interpreted <strong>the</strong> density <strong>of</strong> rails detectedwithin non-native Spartina patches as a positive trend forthis highly endangered taxon. However, <strong>the</strong> literaturecautions that density is a misleading indicator <strong>of</strong> habitatquality or reproductive success (VanHorne 1983; Vickery etal. 1992; Pulliam 1996). Given <strong>the</strong> biogeography <strong>of</strong> tidalmarshlands within <strong>the</strong> estuary, and <strong>the</strong> disparity in habitattypes that exists between marshes in its nor<strong>the</strong>rn andsou<strong>the</strong>rn reaches (Josselyn 1983; Goals Project 1999; Bayeet al. 2000) land managers and biologists should consider<strong>the</strong> possibility that non-native Spartina pastures might beecological traps, or perhaps “attractive sinks,” phenomenathat occur most commonly in anthropogenically modifiedhabitats (Delibes et al. 2001; Schlaepfer et al. 2002; Battin2004; Robertson and Hutto 2006). Indeed, San FranciscoEstuary and <strong>the</strong> California clapper rail fulfill most if not all<strong>of</strong> <strong>the</strong> criteria that increase vulnerability <strong>of</strong> animalpopulations to ecological traps summarized by Battin(2004). It is also worth considering <strong>the</strong> possibility that <strong>the</strong>increasing trends in abundance associated with invadedmarshes (east bayshore) and coincident negative trends inless invaded sites (west bayshore) reported in Broom (2008),suggest <strong>the</strong> possibility that rails are being lured away fromhigher quality habitat (larger patch size, age heterogeneity,native Spartina beds ) to occupy falsely attractive habitat.However, <strong>the</strong> supposition that ecological traps account for<strong>the</strong>se apparent trends is open to o<strong>the</strong>r interpretations (Battin2004; Robertson and Hutto 2006; Gilroy and Su<strong>the</strong>rland2007).To determine <strong>the</strong> impact <strong>of</strong> non-native Spartina beds toclapper rail long-term population viability, studiesdetermining reproductive success, survivorship, andpredation rates in both habitat types will be needed. Futurestudies aimed at determining reproductive success ininvaded and non-invaded habitat may help address thisimportant question <strong>of</strong> conservation biology.SUMMARY AND CONCLUSIONSNon-native cordgrass has invaded intertidal marshlandsin <strong>the</strong> San Francisco Estuary, crucial habitat for <strong>the</strong> endemicand endangered California clapper rail. The ecologicaltransformation <strong>of</strong> <strong>the</strong> intertidal marsh zone is apparentlyunderway and future changes are anticipated. Densities <strong>of</strong>rails in invaded habitat appear to be somewhat higher than at- 187 -