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Ecosystem engineers can influence community and ecosystem dynamics by controlling resources, modifying the flow of energy or biomass, or changing physical characteristics of the habitat. Invasive hybrid cordgrass ( Spartina alterniflora × Spartina foliosa ) is an ecosystem engineer in salt marshes in San Francisco Bay, California, U.S.A. that raises intertidal elevations and may be either increasing C 4 plant carbon input into food webs or tying up carbon in a form that is not usable by consumers. A manipulative experiment compared abundance, growth, and stable isotope (δ 13 C and δ 15 N) composition of the clam Macoma petalum (= M. balthica ) among native marsh, hybrid Spartina , and mudflats in central San Francisco Bay. We found higher densities (individuals m −2 ) of M. petalum on mudflats compared to either native or hybrid Spartina ( p Macoma petalum shell growth was significantly greater in mudflats than in either vegetation type in 2002 ( p = 0.005) but not 2003. Differences in shell growth between native and hybrid Spartina were not significant. Stable isotope results showed differences between habitats in δ 13 C but not δ 15 N. Carbon signatures of M. petalum placed in Spartina were much more depleted than the isotopic signature of Spartina . Neither native nor hybrid Spartina appears to be a significant carbon source for M. petalum in San Francisco Bay, and we found no evidence that hybrid Spartina contributes carbon to M. petalum beyond what is provided by S. foliosa , despite the hybrid's much greater biomass. Our results show that loss of mudflat habitat, rather than increased input of C 4 carbon, is the greatest effect of the invasion of hybrid Spartina on M. petalum . |
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