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
C4 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
13C and δ
15N) 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 < 0.001).
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
13C but not δ
15N. 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
C4 carbon, is the greatest effect of the invasion of hybrid
Spartina on
M. petalum.
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