Fine-scale variability in the dietary sources of grazing invertebrates in a temperate Australian saltmarsh

Saltmarsh floristic diversity declines with increasing latitude on the Australian east coast, with the dominant tropical C4 grass Sporobolus virginicus being replaced progressively by a suite of mostly succulent C3 species. The temperate Towra Point saltmarsh consists of a mosaic of vegetation communities, including stands of the C4 saltmarsh grass Sporobolus virginicus, and the C3 succulents Suaeda australis and Sarcocornia quinqueflora. The contrasting stable isotope signatures of these plants provide an opportunity to determine the extent to which plant material is contributing to the diet of grazing invertebrates inhabiting these communities. The grazing crabs Parasesarma erythrodactyla and Helograpsus haswellianus, and the snail Littoraria luteola, were sampled for their carbon and nitrogen isotope signatures. In the Sarcocornia communities, crab and snail δ13C signatures could not be matched to the signature of dominant plants, but were close to the fine benthic material on the marsh surface. In the Sporobolus community, the δ13C signatures of the same species were enriched and closer to that of the dominant plant. Results suggest that grazing herbivores feed over very small spatial ranges within mosaics of vegetation on locally sourced benthic material, with S. virginicus plant material making a contribution to dietary carbon where present.