Ecosystem Structure Emerges as a Strong Determinant of Food-Chain Length in Linked Stream–Riparian Ecosystems

Environmental determinants of fluvial food-chain length (FCL) remain unresolved, with predominant hypotheses pointing to productivity, disturbance, and/or ecosystem size. However, drainage configuration (for example, drainage density, and stream length)—in spite of recent advances demonstrating the significance of catchment structure to habitat and biodiversity of fluvial systems—has yet to be explored in relation to FCL. In this study, we quantified the relative influences of ecosystem size and structure on FCL for linked stream–riparian food webs. At 19 stream reaches distributed within three mountain catchments of northern Idaho, USA, we sampled aquatic and riparian consumers and determined FCL using the naturally abundant stable isotopes 13C and 15N. Food-chain length was then related to reach measures of size and structure using an information-theoretic model selection approach. Model selection was followed by exploratory linear regression of FCL with purported mechanistic factors (that is, resource availability and disturbance regime). FCL ranged from 2.6 to 4.4 across study reaches and was best explained by catchment structure such as number of tributary junctions and distance to nearest downstream confluence. Regression analyses suggested that disturbance regime may mechanistically link number of tributary junctions and FCL, as well as drainage area and FCL. Our results introduce novel evidence that ecosystem structure may integrate the effects of several mechanistic factors and thus be an important predictor of food-web structure.