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 SpartinaCriterion) revealed that overall nesting success wasestimated to be 30% lower in areas <strong>of</strong> exotic Spartina than inareas <strong>of</strong> native vegetation. We also found strong evidence <strong>of</strong>marsh wren destruction <strong>of</strong> sparrrow eggs, particularly inareas <strong>of</strong> high marsh wren density (Nordby et al. 2009).Fig. 2. Marsh wren (Cistothorus palustris). Photo by Jen McBroom.(Melospiza melodia pusillula) (Fig. 1), a California Species<strong>of</strong> Special Concern, resides entirely within salt marshes inSouth San Francisco Bay. In a native marsh, this sparrow is<strong>the</strong> main resident passerine species and occupies territoriesand nests in Sarcocornia and Grindelia near tidal channels.Ano<strong>the</strong>r passerine species, <strong>the</strong> marsh wren (Cistothoruspalustris) (Fig. 2), that normally occurs in fresh or brackishwater marshes on <strong>the</strong> Pacific Coast, has started to occupy <strong>the</strong>newly available exotic Spartina habitat (Nordby and Cohen,pers. obs.). Marsh wrens are very aggressive and will defend<strong>the</strong>ir territories against o<strong>the</strong>r birds, even o<strong>the</strong>r species, bybreaking <strong>the</strong> eggs in nests that are close to <strong>the</strong>ir ownterritories (Picman 1977).To assess <strong>the</strong> impact <strong>of</strong> <strong>the</strong> exotic Spartina invasion onsong sparrow and marsh wren populations in San FranciscoBay we 1) studied sparrow nesting habitat preferences andnest success and also looked for evidence <strong>of</strong> <strong>the</strong> destruction<strong>of</strong> song sparrow eggs by marsh wrens, and 2) assessed <strong>the</strong>vegetation composition <strong>of</strong> sparrow and wren territories aswell as <strong>the</strong> amount <strong>of</strong> territory overlap between <strong>the</strong> twospecies.SONG SPARROW NEST SUCCESSDuring <strong>the</strong> 2002 and 2003 breeding seasons wefollowed and observed <strong>the</strong> fates <strong>of</strong> 351 nests in 45+territories across three study sites (Newark, San Leandro andAlameda). Once <strong>the</strong> fate <strong>of</strong> a nest had been determined(failed due to predation, failed due to tidal flooding, failedfor o<strong>the</strong>r reasons, or successful), we recorded <strong>the</strong> locationand vegetation composition <strong>of</strong> <strong>the</strong> nest site.We found that song sparrows did use exotic Spartina asnesting habitat, but <strong>the</strong>se nests were much more likely to faildue to tidal flooding than nests placed in native vegetation.As a result, our model <strong>of</strong> daily nest survival (a generalizedlinear modeling approach using Akaike’s InformationSONG SPARROW AND MARSH WREN TERRITORIESDuring <strong>the</strong> 2003 breeding season (from March toAugust), we conducted two to four focal observations oneach color-banded male to map <strong>the</strong> territory boundaries for<strong>the</strong> 32 song sparrows and 16 marsh wrens in <strong>the</strong> twoSpartina-invaded study sites (San Leandro and Alameda).Observers used binoculars, a compass, and a range-finder tomap bird locations during each one-hour observation period.Observation locations were marked using a GPS unit, birdlocations were <strong>the</strong>n calculated and <strong>the</strong>se data were added toan ArcView GIS database. The points were used to construct100% minimum convex polygons <strong>of</strong> each territory. Usingcolor-infrared aerial photographs, we identified areas <strong>of</strong>invasive Spartina throughout <strong>the</strong> sites. In ArcView, wecombined <strong>the</strong> layer <strong>of</strong> Spartina vegetation with <strong>the</strong> layers <strong>of</strong>marsh wren and song sparrow territory polygons. We <strong>the</strong>ndetermined <strong>the</strong> percentage <strong>of</strong> each territory that wascomposed <strong>of</strong> Spartina habitat, as well as <strong>the</strong> amount <strong>of</strong>overlap between <strong>the</strong> two species.Although song sparrows did include some exoticSpartina habitat in <strong>the</strong>ir territories, all but one song sparrowterritory included some areas <strong>of</strong> native salt marsh habitat.The one territory that was determined from aerialphotographs to be entirely covered by Spartina, actually hada large portion <strong>of</strong> native pickleweed vegetation underlying<strong>the</strong> Spartina stand. In contrast, marsh wren territories weremore highly correlated with exotic Spartina habitat andmany territories were exclusively composed <strong>of</strong> <strong>the</strong> exoticcordgrass. We also found that <strong>the</strong>re was little overlapbetween <strong>the</strong> territories <strong>of</strong> <strong>the</strong> two species (Nordby et al., inprep).DISCUSSIONThese results suggest that <strong>the</strong> changes in salt marshhabitat associated with <strong>the</strong> invasion <strong>of</strong> exotic Spartina mayfavor marsh wrens over song sparrows. While song sparrowsare occupying and nesting in <strong>the</strong> exotic Spartina, those thatdo so may be at a disadvantage. It is possible that songsparrows are being drawn to nesting sites in exotic Spartinathat are inappropriate because <strong>the</strong>y are too low in elevationrelative to <strong>the</strong> tides. This increase in nest failure due t<strong>of</strong>looding coupled with an apparent increase in interferencecompetition from marsh wrens may serve to negativelyimpact salt marsh song sparrow populations in SanFrancisco Bay.However, we do not yet know whe<strong>the</strong>r exotic Spartinais acting as an ‘ecological trap’ for song sparrows, where- 198 -