Associations in ephemeral systems: the lack of trophic relationships between sandhoppers and beach wrack

摘要

In ephemeral systems, material subsidies can play a key role in the persistence and connectivity of populations, especially if the organisms living within them are trophically dependent on imported resources. Sandy beaches are heavily subsidized by organic material of both terrestrial and marine origin. For highly mobile supratidal fringe species, such as amphipods, which are marine but with a high tolerance of aerial conditions, such material potentially provides both food and shelter. We investigated the relationship between beach wrack and amphipods by examining the trophic contribution of allochthonous food sources to sandhopper diets using stable isotope analysis. Replicate samples of the sandhopper Talorchestia capensis and several types of beach wrack (including seagrass, wood and different macrophytes) colonized by these amphipods were collected from 11 sites within one biogeographical region along the south coast of South Africa. Analysis of variance showed that the δ~(13)C and δ~(15)N signatures of sandhoppers were significantly affected by the type of wrack with which they were associated, but stable isotope mixing models showed that there was no close relationship between T. capensis diet and the material under which they were found. Thus sandhopper isotope signatures differed with wrack type, but were not clearly derived from wrack. This was interpreted as an indication that amphipod feeding was extremely opportunistic, with different sandhopper populations feeding at different trophic levels (based on δ~(15)N signatures) and obtaining their δ~(13)C signatures from material associated with specific wrack types, rather than directly from the wrack itself or from imported suspended particulate material. These results suggest alternative links in sandy beach food webs, with amphipods obtaining their carbon indirectly from wrack via bacterial communities that are specific to different types of imported material.