Food Web Structure in an Austral River Highlights Elephant Dung as a Potential Subsidy to Consumers in Riverine Food Webs

ABSTRACT Elephant dung is characterized by its high organic content, and through defecation in aquatic systems, dung may function as a resource that fuels primary production within these systems. However, how elephant dung directly or indirectly supports the secondary production of aquatic consumers in these ecosystems has not been adequately assessed. Here, we examined the role of elephant dung as an allochthonous subsidy in a riverine ecosystem by measuring the stable isotope values of basal resources and consumers across upstream and downstream sites of an austral river in Kruger National Park, South Africa. Stable isotope analysis showed that upstream, C3 macrophytes and dung contributed significantly to soil organic matter (SOM) and suspended particulate matter (SPM). However, autochthonous production remained the primary source of carbon for consumers. Downstream, dung played a more significant role in SPM and SOM, with isotopic analyses showing increased reliance on terrestrial inputs. Overall, stable isotope mixing models indicated that dung contributed up to 51% of the dietary carbon for certain fish species and was a dominant source for freshwater invertebrates, including invasive aquatic gastropods such as Tarebia granifera . These findings provide further insights into the ecological function of large terrestrial herbivores in sustaining aquatic productivity and highlight the potential consequences of their decline on riverine food web dynamics. Understanding these trophic interactions in rivers could provide valuable insights into the flow of resources in aquatic food webs, particularly in response to allochthonous subsidies such as elephant dung.