A Gradient of Wildlife Subsidies Alters Riverine Food Web Structure

ABSTRACT Terrestrial subsidies play a key role in freshwater food web structure and ecosystem function. Animal‐mediated subsidies in particular provide high quality resources that shape aquatic food webs. We know little about how the magnitude of wildlife subsidies affects food web structure and stability, and how this relationship is affected by abiotic factors such as river flow. We use a wildlife subsidy gradient (hippo dung and wildebeest carcasses) in the Mara River, Kenya, to quantify the effect of subsidy magnitude on consumer use of allochthonous resources and degree of omnivory. Furthermore, we examined this relationship across seasons and flow rates to determine whether discharge affected the relationship between wildlife subsidy magnitude and allochthonous resource use. We selected three sites along the Mara River wildlife gradient with no, medium, and high wildlife input and obtained stable isotopes from dominant basal resources and consumers across invertebrate functional feeding groups and fish feeding guilds. Samples were collected across varying flows and seasons. We used Bayesian mixing models, with consumer gut contents as priors, to assess changes in the consumption and assimilation of dominant food sources by invertebrate and fish consumers across the subsidy gradient and flow rates. The Mara River food web shifted from being driven by autochthonous basal resources to being increasingly driven by allochthonous resources along the wildlife subsidy gradient. Aquatic insects and fish increased their allochthonous resource use from 27% to 52% and from 2% to 73.5%, respectively, mostly through increased consumption of hippo dung and vertebrate material during the wildebeest migration season. This diet shift led to a higher level of omnivory and a decrease in mean trophic position for invertebrates. River flow affected isotope values of consumers but not their resources at the no and medium wildlife input sites. Flow had little impact on stable isotopes at the high input site. Our results suggest that wildlife input provides an important resource for aquatic consumers and that its importance in the aquatic food web increases with its relative magnitude. Higher levels of animal‐mediated subsidies also increased consumer reliance on multiple resource pathways, suggesting higher wildlife input may increase food web stability. Abiotic factors like river flow can affect the relationship between allochthonous resource use and consumers by altering the quantity of wildlife input, but our results suggest that this is only the case when there is medium to no wildlife input. We propose that in areas with high wildlife input, subsidies are sufficiently abundant that flow has little effect on the ability of organisms to access the resource. Our results show that the magnitude of wildlife subsidies helps shape aquatic food web structure and provide the basis for the ecological theory that increasing magnitude of resource subsidies increases food web stability.