A specialist detritivore links Spartina alterniflora to salt marsh food webs

Because most plant production is subject to senescence and is eventually consumed by detritivores, the factors that drive detritivore diet choice are pivotal to the flow of energy and materials through food webs. Here, we investigated the common salt marsh amphipod Gammarus palustris, which is a habitat specialist that feeds specifically on the dead leaves of its living host plant, salt marsh cordgrass Spartina alterniflora. Restricted use and consumption of dead S. alterniflora was reinforced by superior amphipod performance (survival, size, and sexual development) on dead S. alterniflora relative to other diets, and was driven at least in part by amphipods being physically able to feed on soft, decaying plant tissues but not live, turgid tissues. Stable isotopes from field surveys and laboratory assimilation assays suggest that amphipods also feed on S. alterniflora in the field, and that the important marsh fish Fundulus hetroclitus feeds on amphipods. Thus, consumption of G. palustris by F. heteroclitus may be an important trophic pathway linking cordgrass production to nearshore food webs. Importantly, direct isotopic analyses of amphipods and their known food sources demonstrated substantial deviation of observed fractionation factors from idealized standards. This suggests caution when using idealized trophic shifts to describe food web linkages, and a renewed focus on assimilation assays to determine the realized fractionation of dietary isotopes.