Effects of a habitat-altering invader on nesting sparrows: An ecological trap?
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Many invading species impact native species through predation, parasitism or competition, while others affect natives indirectly by restructuring their habitat. How invasive plants affect native animals, and to what extent native animals respond to changes in their habitat and the novel selection pressures that follow, is not well known. We investigated the impacts of a habitat-altering invader, the Atlantic cordgrass Spartina alterniflora, on the nesting success of Alameda song sparrows (Melospiza melodia pusillula), a California Species of Special Concern, in tidal marshes in three sites in San Francisco Bay. Date of laying was the most influential factor in determining daily survival rate of nests, but whether the nest was placed in exotic Spartina was the most important ecological variable. Nests placed in exotic Spartina had a success rate that was 30% lower than those placed in native vegetation. Nests in exotic Spartina were significantly more likely to fail due to tidal flooding than were nests placed in native vegetation, because the densest stands of exotic Spartina occurred at significantly lower elevations relative to the tides. Our results suggest that exotic Spartina may be an ecological trap for song sparrows in San Francisco Bay, attracting birds to nest sites that are often destroyed by tidal flooding.
KeywordsAlameda song sparrow Behavior Ecological trap Exotic species invasions Invasive cordgrass Melospiza melodia Nest success Spartina alterniflora Tidal salt marsh
We thank Letitia Grenier, Jules Evens, Joy Albertson, Katy Zaremba and Hildie Spautz for assistance with initiating this research. We also thank Jen McBroom, Becky Ducore, Lisa Eigner and April Robinson for their exceptional assistance in the field. The California Coastal Conservancy’s Invasive Spartina Project and Pablo Rosso graciously provided the geo-referenced aerial photograph GIS files. We thank the U.S. Fish and Wildlife Service, CA Department of Fish and Game, East Bay Regional Park District and the City of San Leandro for research permits and access to marshes. This research was conducted under an approved University of California, Berkeley animal care protocol, a U.S. Fish and Wildlife Service Federal Fish and Wildlife Endangered Species permit, a U.S. Fish and Wildlife Special Use Permit, a U.S. Geological Survey Bird Banding Permit, a California State Scientific Collecting Permit and an East Bay Regional Park District research permit. Funding was provided by a David H. Smith Conservation Research Fellowship from The Nature Conservancy, the National Science Foundation Biocomplexity grant DEB 0083583, and the San Francisco Estuary Institute.
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