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 SpartinaIMPACT OF INVASIVE HYBRID CORDGRASS (SPARTINA ALTERNIFLORA X FOLIOSA) ONSONG SPARROW AND MARSH WREN POPULATIONS IN SAN FRANCISCO BAY SALTMARSHESJ.C. NORDBY 1,4 , J.T. MCBROOM 2 ,A.N.COHEN 3 AND S.R. BEISSINGER 11Department <strong>of</strong> Environmental Science, Policy and Management, University <strong>of</strong> California, Berkeley, 151 Hilgard Hall #3110Berkeley, CA, 947202San Francisco Estuary Invasive Spartina Project, 2612-A 8 th Street, Berkeley, CA 947103 San Francisco Estuary Institute, 7770 Pardee Lane, 2 nd Floor, Oakland, CA 946214Current address: Institute <strong>of</strong> <strong>the</strong> Environment and Department <strong>of</strong> Ecology and Evolutionary Biology, University <strong>of</strong>California, Los Angeles, Los Angeles, CA 90095; nordby@ucla.eduExotic hybrid cordgrass, Spartina alterniflora x foliosa, is altering <strong>the</strong> vegetative structure andcomposition <strong>of</strong> <strong>the</strong> San Francisco Bay tidal marsh ecosystem, which has multiple impacts on <strong>the</strong>Alameda song sparrow (Melospiza melodia pusillula), a resident passerine species that is aCalifornia Species <strong>of</strong> Special Concern. These sparrows are affected not only by <strong>the</strong> altered habitat,but also by <strong>the</strong> occupation <strong>of</strong> this habitat by a potential competitor, <strong>the</strong> marsh wren (Cistothoruspalustris). To assess <strong>the</strong> impact <strong>of</strong> <strong>the</strong> S. alterniflora invasion on song sparrow and marsh wrenpopulations we: 1) located song sparrow nests to observe nesting habitat preferences and nestsuccess and 2) used focal observations <strong>of</strong> color-banded birds to assess <strong>the</strong> vegetation composition <strong>of</strong>each territory and <strong>the</strong> amount <strong>of</strong> territory overlap between <strong>the</strong> two species. Our findings suggest that<strong>the</strong> changes in salt marsh habitat associated with <strong>the</strong> invasion <strong>of</strong> cordgrass hybrids may favor marshwrens over song sparrows and could eventually result in a decrease in salt marsh song sparrowpopulations.Keywords: Spartina alterniflora, song sparrow, marsh wren, nest success, territory, competition,San Francisco BayINTRODUCTIONNative San Francisco Bay tidal salt marshes arecharacterized by broad expanses <strong>of</strong> open tidal mudflats, anarrow mid-marsh zone where Spartina foliosa occurs, and ahigh-marsh zone composed mainly <strong>of</strong> low-growingSarcocornia spp. (formerly Salicornia spp.) with narrowareas <strong>of</strong> Grindelia that line <strong>the</strong> meandering tidal channels.Spartina alterniflora, a cordgrass native to <strong>the</strong> Atlantic andGulf coasts <strong>of</strong> North America, was introduced to SanFrancisco Bay in <strong>the</strong> early 1970s (Ayres et al. 2004). Theexotic cordgrass subsequently hybridized with <strong>the</strong> nativecordgrass, S. foliosa, and <strong>the</strong>se hybrids have spread and nowcover more than 600 hectares (ha) <strong>of</strong> tidal flat and tidalmarsh habitat (Zaremba et al., this volume).The tall, dense exotic Spartina (S. alterniflora and/or<strong>the</strong> hybrid S. alterniflora x foliosa) can grow fur<strong>the</strong>r down<strong>the</strong> tidal gradient than any native tidal marsh plant speciesand so is able to colonize open tidal flats. This exoticcordgrass can also grow much fur<strong>the</strong>r up <strong>the</strong> tidal gradientthan native S. foliosa and thus displaces o<strong>the</strong>r native plantspecies in <strong>the</strong> mid- to high-marsh zones as well (Ayres et al.1999; Nordby pers. obs.).The pr<strong>of</strong>ound changes in habitat structure andcomposition that accompany <strong>the</strong> exotic Spartina invasion(Callaway and Josselyn 1992) will likely have <strong>the</strong> greatestimpact on species, such as birds, that are wholly dependenton <strong>the</strong> tidal salt marsh system. The Alameda song sparrowFig. 1. Alameda song sparrow (Melospiza melodia pusillula). Photo byJen McBroom.- 197 -