Effects of native and hybrid cordgrass on benthic invertebrate communities and food webs

Spartina cordgrasses are ecosystem engineers that modify habitat structure and can be important contributors to secondary production in estuaries. In San Francisco Bay, California, USA, introduced hybrid cordgrass (S. alterniflora x S. foliosa) is invading marshes of native Pacific cordgrass (Spartina foliosa). This study investigated differences in vegetation and sediment structure, benthic macrofauna, and food webs in S. foliosa and hybrid Spartina marshes from San Francisco Bay to Bodega Bay, California.; Habitat structure and infauna differed significantly between S. foliosa and hybrid Spartina marshes. Hybrid Spartina produced greater stem densities, taller mean and mode stem heights, and greater aboveground and belowground biomass. Both species significantly reduce light levels and water flow. S. foliosa contained significantly higher densities and biomass of infaunal organisms in benthic cores compared to mudflats. Densities and biomass of infauna in hybrid Spartina were lower than, or not significantly different from, mudflats.; A manipulative experiment found higher densities and greater shell growth of the clam Macoma petalum on mudflats than in either Spartina species; stable isotope ratios of carbon and nitrogen differed significantly between seasons but not habitats, implying that neither native nor hybrid Spartina is a significant carbon source for M. petalum in San Francisco Bay.; Stable isotopes were used to examine if macrofaunal food webs differ between habitats. I hypothesized that consumers collected within hybrid Spartina would show isotope signatures more similar to Spartina than those in mudflats or Spartina foliosa. Differences occurred in carbon sources between habitats for some species, while others showed differences in nitrogen ratios that indicate a shift in the trophic level of prey between habitats. Patterns were not consistent across species or sites. Overall, my results indicate that S. foliosa provides habitat for infauna while hybrid Spartina does not. However, despite the changes it produces to habitat structure, hybrid Spartina does not appear to contribute carbon to macrofaunal food webs.